Oleh:
Ieke W. Ayu, S. Priyono dan Soemarno
psl-ppsub nopember 2012
LENGAS TANAH
Definisi:
(1) Air yang disimpan dalam tanah.
(2) Salah satu faktor yang sangat penting dalam
proses pedologis dan pertumbuhan tanaman.
Ada tiga macam bentuk lengas tanah:
1. Water adhering in thin films by molecular
attraction to the surface of soil particles and not
available for plants is termed hygroscopic water.
2. Water forming thicker films and occupying the
smaller pore spaces is termed capillary water.
Since it is held against the force of gravity it is
permanently available for plant growth and it is
this type of soil water which contains plant
nutrients in solution.
3. Water in excess of hygroscopic and capillary
water is termed gravitational water, which is of a
transitory nature because it flows away under
the influence of gravity. When the excess has
drained away the amount of water retained in the
soil is termed its field capacity, when some of its
pore spaces are still free of water.
(Source: LANDY / DUNSTE)
Diunduh dari:
….. 15/11/2012
SOIL WATER BALANCE
Plant production involves CO2 intake through stomatal openings in the
epidermis. Most water that plants take up from the soil is again lost to the
atmosphere by transpiration through the same openings. The daily
turnover can be considerable: transpiration from 0.4 cm of water from a
crop surface on a clear sunny day corresponds with a water loss from the
root zone of than 40.000 kg ha-1 d-1. If soil moisture uptake by the roots is
not replenished, the soil will dry out to such an extent that the plants wilt
and - ultimately- die.
The tenacity with which the soil retains its water is equalled by the suction
which roots must exert to be able to take up soil moisture. This suction
known as the soil moisture potential or 'matrix suction', can be measured.
In hydrology, the potential is usually used and is expressed as energy unit
per weight of soil water, with the dimension of length (van Bakel, 1981). An
optimum range exists within which the plant takes up water freely. Above
or below this level the plant senses stress; it reacts by actively curbing its
daily water consumption through partial or complete closure of the
stomata. The consequence is evident: this stomatal closure interferes with
CO2 intake and reduces assimilation and dry matter production
consequently.
A crop growth simulation model must therefore keep track of the soil
moisture potential to determine when and to what degree a crop is
exposed to water stress. This is commonly done with the aid of a water
balance equation, which compares for a given period of time, incoming
water in the rooted soil with outgoing water and quantifies the difference
between the two as a change in the soil moisture amount stored.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
PERSAMAAN NERACA LENGAS
TANAH
Soil water balance, like a financial statement of income and
expenditure, is an account of all quantities of water added,
removed or stored in a given volume of soil during a given
period of time.
The soil water balance equation thus helps in making
estimates of parameters, which influence the amount of soil
water.
Using the soil water balance equation, one can identify
periods of water stress/excesses which may have adverse
affect on crop performance. This identification will help in
adopting appropriate management practices to alleviate the
constraint and increase the crop yields.
The amount of water in a soil layer is determined by those
factors that add water to the soil and those factors that
remove water from it.
The soil water balance equation in its simplest form of
expression is:
Perubahan air dalam tanah =
Input air – Kehilangan air
Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsupdfs/Soil%20Water%20Balance%20for%20Crop%20Land.pdf….. 12/11/2012
PENAMBAHAN AIR KE TANAH
Water is usually added to the soil in three measurable ways precipitation (P), irrigation (I), and contribution from the
ground-water table (C). The contribution from the ground
water will be significant only if the ground-water table is near
the surface.
So, the inputs of water can be presented as:
Input Air = P + I + C
Kehilangan Air dari tanah:
Water is removed from the soil through evaporation from soil
surface or transpiration through plant together known as
evapotranspiration (ET), and deep drainage (D). Further, a part
of the rain water received at the soil surface may be lost as
surface run-off (RO).
The above three factors are negative factors in the equation.
The losses of water from soil can then be represented by the
following equation.
Kehilangan Air = ET + D + RO
Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-
NERACA LENGAS TANAH
The change in the soil water content which is the difference
between the water added and water withdrawn will now read:
Change in Soil water = (P + I + C) - (ET + D + RO)
Soil water refers to the amount of water held in the root zone
at a given time. This amount can be measured. The change in
soil water from one measurement to another depends on the
contribution of components in the equation. Suppose the
amount of water in the root zone at the beginning is M1 mm
and at the end of a given period is M2 mm, thus the equation is
expressed as :
M1 - M2 = P + I + C - ET - D - RO or M1 + P + I + C
= ET + D + RO + M2
With the help of this equation one can compute any one
unknown parameter in the equation if all others are known.
The quantitative data on rainfall (P) evapotranspiration (ET),
deep drainage (D) and soil moisture at a given time (M1 or M2)
for different locations and for different practices are useful for
selecting appropriate water-management strategies.
Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-
PENGHITUNGAN NERACA LENGAS TANAH
Let us work a few examples using the Soil Water Balance Equation
to appreciate the usefulness of this model.
Contoh 1:
Soil = Vertisol Crop = Sorghum Period = 01 to 31 Aug Area = 2
ha
Given:
Soil moisture in the profile # on Aug 01 (M1) = 300 mm
Precipitation or Rainfall (P) = 70 mm
Irrigation (I) = Nil
Contribution from ground water (C) = Nil
Run-off of 200 cubic m from 2 ha field (R) = 10 mm
Deep drainage (D) = Nil
Soil moisture in the profile on Aug 31 (M2) = 250 mm
Estimate evapotranspiration (ET) from the field during 01 to 31 Aug.
Equation:
M1 + P + I + C = ET + D + RO +M2 300 +70 + 0 + 0
= ET + 0 + 10 + 250
ET = 370 mm - 260 mm = 110 mm
Thus, evapotranspiration which is difficult to be measured could be
estimated using the Soil Water Balance Equation.
Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-
PENGHITUNGAN NERACA LENGAS TANAH
Contoh 2:
Soil = Alfisol Crop = Millet Area=1 ha Period = 10 June (sowing
date) to 30 Sept (harvesting) given:
Soil moisture in the profile on Jun 10 (M1) = 150 mm
Precipitation or Rainfall (P) = 600 mm
Irrigation (I) = Nil
Contribution from ground water (C) = Nil
Evapotranspiration (estimated) (ET) = 530 mm
Run-off of 200 cubic m from 1 ha field (RO) = 70 mm
Soil moisture in the profile on Sep 30 (M2) = 60mm
Estimate:
Deep drainage (D) losses from the field during crop period.
Equation:
M1 + P + I + C = ET + D + RO + M2
150 + 600+ 0 + 0 = 530 + D + 70 + 60
D = 750 mm - 660 mm = 90 mm Thus, deep drainage (D) losses
in the field which is not easy to measure could be estimated
using the Soil Water Balance Equation.
We hope that this lesson and the examples have helped you in
understanding and computing the various components of the
Soil Water Balance Equation. For more detailed treatment
please refer any standard textbook on soil physics.
Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-
NERACA LENGAS TANAH
Evapotranspiration can also be determined by measuring the various
components of the soil water balance. The method consists of
assessing the incoming and outgoing water flux into the crop root
zone over some time period (Figure 6). Irrigation (I) and rainfall (P)
add water to the root zone. Part of I and P might be lost by surface
runoff (RO) and by deep percolation (DP) that will eventually recharge
the water table. Water might also be transported upward by capillary
rise (CR) from a shallow water table towards the root zone or even
transferred horizontally by subsurface flow in (SFin) or out of (SFout)
the root zone. In many situations, however, except under conditions
with large slopes, SFin and SFout are minor and can be ignored. Soil
evaporation and crop transpiration deplete water from the root zone.
If all fluxes other than evapotranspiration (ET) can be assessed, the
evapotranspiration can be deduced from the change in soil water
content (D SW) over the time period:
ET = I + P - RO - DP + CR ± D SF ± D SW
Diunduh dari: http://www.fao.org/docrep/X0490E/x0490e04.htm….. 13/11/2012
NERACA LENGAS TANAH
The estimation of Ks requires a daily water balance computation for
the root zone. The root zone can be presented by means of a
container in which the water content may fluctuate.
The daily water balance, expressed in terms of depletion at the end of
the day is:
Dr, i = Dr, i-1 - (P - RO)i - Ii - CRi + ETc, i + DPi
where :
1. Dr, i root zone depletion at the end of day i [mm],
2. Dr, i-1 water content in the root zone at the end of the previous day, i-1
[mm],
3. Pi precipitation on day i [mm],
4. ROi runoff from the soil surface on day i [mm],
5. Ii net irrigation depth on day i that infiltrates the soil [mm],
6. CRi capillary rise from the groundwater table on day i [mm],
7. ETc, i crop evapotranspiration on day i [mm],
8. DPi water loss out of the root zone by deep percolation on day i [mm].
Diunduh dari: http://www.fao.org/docrep/x0490e/x0490e0e.htm….. 13/11/2012
…NERACA LENGAS TANAH
Transpor air dalam profil tanah
Diunduh dari: http://www.sckcen.be/fr/Media/Images/Our-Research/RD-disposal-ofwaste/%28offset%29/40….. 13/11/2012
MODEL NERACA LENGAS TANAH
Soil-water balance is an accounting procedure for nearsurface soil-moisture.
It is included in a class of models known as “bucket”
models :
1. Properties are averaged for each drainage basin , for example,
a watershed has a single water holding capacity
2. Changes in soil moisture are calculated using a mass balance
3. Historical precipitation is known
4. Evaporation can be estimated using the temperature-based
(Hargreaves) equation .
5. Runoff is dependent upon soil moisture and precipitation
Diunduh dari: http://www.ce.utexas.edu/prof/maidment/grad/martinez/seminar/sld008.htm
MODEL GROUNDWATER
The groundwater model is a lumped-parameter type
1. Portions of the formation fed by different catchment areas are assumed
to have uniform storage and transmissivity parameters
2. Properties are also averaged over depth
3. This is different from finite element models like MODFLOW or GMS
where there is an analysis grid consisting of a large number of cells
Recharge functions :
1. Simulate interaction between surface water and groundwater
2. Recharge to aquifer is mostly from channel losses in streams that
flow over outcrop
3. Empirical functions were developed by plotting inflows against known
recharge determined by USGS
4. Inflow is the stream flow at a gauge above the recharge zone plus
intervening runoff; intervening runoff is calculated using an areal
scaling ratio assuming that conditions are the same for catchment
and intervening regions
Used historical pumping data
Diunduh dari: http://www.ce.utexas.edu/prof/maidment/grad/martinez/seminar/sld010.htm
WATERBANK: MODEL NERACA LENGAS TANAH
Diunduh dari: http://www.dpi.nsw.gov.au/archive/agriculture-today-stories/december2009/more-time-at-or-below-wilting-point/waterbank ….. 13/11/2012
SOIL WATER BALANCE (SWB)
The components of the soil water balance (SWB) are depicted in
Figure . The water balance can be summarised as in equation :
ÄSm = Sm + P – T – E – I – R – D
The change in the soil water content (ÄSm) over a period of time
depends on the original water content (Sm) plus precipitation (rain
and irrigation, P); minus transpiration (loss of water by plants, T);
minus evaporation (loss of water from the soil surface, E); minus
interception (water held in the plant canopy, I); minus runoff
(surface water not penetrating the soil and running away, R) ;
minus drainage (water draining away below root zone, D).
Diunduh dari: http://www.senwes.co.za/Files/main_Scenario/archive_articles/2006/200612-01_effective_irrigation_soil_water_balance.htm ….. 13/11/2012
. HASIL-HASIL NERACA AIR
The combined results from the water balances have been used to
assess how much water systems have; how much is stored; what
the variability factors are; and what the connections between
resources are.
A discussion of the relative contribution of the main components of
the balances is provided.
Diunduh dari:
http://www.water.gov.au/WaterAvailability/Waterbalanceassessments/Waterbalanceresults
/index.aspx?Menu=Level1_3_2_3….. 13/11/2012
MODEL NERACA LENGAS TANAH
In large canal irrigation project areas, integrated management of
surface and groundwater resources can improve water use
efficiencies and agricultural productivity and also control water
logging. Such integrated management requires an estimation of
spatial distribution of recharge and ground water flow in the
underlying aquifer. Recharge occurs both as percolation losses from
fields and seepage losses from the water distribution network.
Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0378377403001446…..
13/11/2012
NERACA LENGAS TANAH
A soil water balance model, if designed to adequately represent the
physical processes involved, and if carried out with a short enough (daily)
time step, can provide realistic estimates of deep drainage (potential
recharge) over long periods.
The single store (single layer) mass water balance model applicable to
semi-arid areas, which recognises the wetting of the near surface during
rainfall, with subsequent availability of water for evaporation and
transpiration in the days following rainfall. The model allows for the major
hydrological processes taking place at or near the soil-vegetation surface
including runoff.
Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0016706107000924…..
13/11/2012
Schematization of the soil profile in the AWC calculation of the
CERU32 program (Le Bas et al., 1997)
The soil profile is schematized in three layers (Le Bas et al., 1997):
1. A worked surface layer, defined by ploughing depth. The available water
for this layer corresponds to the total available water (water volume
between –1500 kPa suction (wilting point) and –5 kPa (field capacity))
estimated by rules using the topsoil input variables.
2.
Variabel Topsoil
3.
Variabel Subsoil
Kedalaman Akar
A subsurface layer, between
the ploughing depth and the
EAW (Easily Available Water)
depth. The latter is user
defined. The available water
for this layer corresponds to
the total available water (water
volume between –1500 kPa
suction (wilting point) and –5
kPa (field capacity)) estimated
by rules valid for the subsoil,
using the subsoil input
variables. But if the soil layer
is less than the depth to
textural change the rules for
subsoil uses topsoil input
variables.
A deeper layer, between the
EAW depth and the maximum
rooting depth. The available
water for this layer
corresponds to the easily
available water (water volume
between –200 kPa suction and
–5 kPa (field capacity))
estimated by rules valid for
the subsoil, using the subsoil
input variables. But if the soil
layer is less than the depth to
textural change the rules for
subsoil uses topsoil input
variables.
Diunduh dari: http://eusoils.jrc.ec.europa.eu/projects/sinfo/5_2_1_en.htm….. 13/11/2012
PERHITUNGAN NERACA LENGAS TANAH
Therefore the purpose of soil water balance
calculations is to estimate daily value of the actual
soil moisture content, which influences soil
moisture uptake and crop transpiration.
Schematic representation of the different
components of a soil water balance
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…KANDUNGAN LENGAS TANAH AKTUAL
Actual soil moisture content can be established
according to (Driessen, 1986):
Where:
where :
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
qt : Actual moisture content of the root zone at time step t [cm3 cm-3]
Nup : Rate of net influx through the upper root zone boundary [cm d-1]
INlow : Rate of net influx through the lower root zone boundary [cm d-1]
Ta : Actual transpiration rate of crop [cm d-1]
RD : Actual rooting depth [cm]
P : Precipitation intensity [cm d-1]
Ie : Effective daily irrigation [cm d-1]
Es : Soil evaporation rate [cm d-1]
SSt : Surface storage [cm]
SR : Rate of surface runoff [cm d-1]
CR : Rate of capillary rise [cm d-1]
Perc : Percolation rate [cm d-1]
Dt : Time step [d]
Zt : Depth of groundwater table [cm]
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
LENGAS TANAH ZONE PERAKARAN
The processes directly affecting the root zone soil moisture content
can be defined as:
1.
2.
3.
4.
Infiltration: i.e. transport from the soil surface into the root zone;
Evaporation: i.e. the loss of soil moisture to the atmosphere;
Plant transpiration: i.e. loss of water from the interior root zone;
Percolation: i.e. downward transport of water from the root
zone to the layer below the root zone;
5. Capillary rise: i.e. upward transport into the rooted zone.
The textural profile of the soil is conceived
homogeneous. Initially the soil profile consists
of three layers (zones):
1. The rooted zone between soil surface and
actual rooting depth
2. The lower zone between actual rooting
depth and maximum rooting depth
3. The subsoil below maximum rooting depth
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
SISTEM SWB TANPA EFEK GROUNDWATER
The variables of the soil water balance in the actual water-limited
production situation are calculated for freely draining soil. No
influence from groundwater is assumed and crop water
requirements for continuous growth with either drought stress or
water excess and a possible reduction of the crop transpiration
rate, leading to a reduced growth are quantified.
Submodel Lengas Tanah
For the rooted zone the water balance equation is solved every
daily time step. The water balance is driven by rainfall, possibly
buffered as surface storage, and evapotranspiration.
The processes considered are infiltration, soil water retention,
percolation and the loss of water beyond the maximum root zone.
At the upper boundary, processes comprise infiltration of water
from precipitation or irrigation, evaporation from the soil surface
and crop transpiration. If the rainfall intensity exceeds the infiltration
and surface storage capacity of the soil, water runs off.
Water can be stored in the soil till the field capacity is reached.
Additional water percolates beyond the lower boundary of the
rooting zone.
Flow rates are limited by the maximum percolation rate of the root
zone and the maximum percolation rate to the subsoil.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
SISTEM SWB TANPA EFEK GROUNDWATER
The variables of the soil water balance in the actual water-limited
production situation are calculated for freely draining soil. No
influence from groundwater is assumed and crop water
requirements for continuous growth with either drought stress or
water excess and a possible reduction of the crop transpiration
rate, leading to a reduced growth are quantified.
Submodel Lengas Tanah
The textural profile of the soil is conceived homogeneous. Initially the soil
profile consists of three layers (zones):
1. the rooted zone between soil surface and actual rooting depth
2. the lower zone between actual rooting depth and maximum rooting
depth
3. the subsoil below maximum rooting depth
Root zone extension from initial rooting depth to maximum rooting depth.
Its effect on the soil moisture content is accounted for in this soil water
balance calculation. From the moment that the maximum rooting depth is
reached the soil profile is described as a two layer system (Driessen,
1986).
The lower zone no longer exists. As mentioned earlier, no groundwater
influence is assumed and capillary rise is not accounted for.
Only downward flow, evaporation from the soil surface and transpiration
are estimated.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…Kandungan Lengas Tanah Initial
The initial value of the actual soil moisture content in the root zone
can be calculated as:
Where qt : Actual soil moisture content in rooted zone [cm3 cm-3]
qwp : Soil moisture content at wilting point [cm3 cm-3] Wav : Initial
available soil moisture amount in excess of qwp [cm] RD : Actual
rooting depth (see Section 5.6.) [cm]
The initial actual soil moisture content, qt, cannot be
lower than the soil moisture content at wilting point. In
case the crop cannot develop airducts, the initial soil
moisture content cannot be higher than the soil
moisture content at field capacity. If the crop can
develop airducts the initial soil moisture content cannot
exceed the soil porosity. Wav, the initial available soil
moisture amount in excess of qwp should be provided by
the user. Multiplying the actual soil moisture content
with the rooting depth yields the initial water amount in
the rooted zone.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…LENGAS TANAH DI BAGIAN ZONE
BAWAH
. The initial amount of soil moisture in the zone between
the actual rooting depth and the maximum rooting depth
(i.e. lower zone), can be calculated as:
Where
1.
2.
3.
4.
5.
6.
Wlz : Soil moisture amount in the lower zone [cm]
Wav : Initial available soil moisture amount in excess of qwp [cm]
RDmax : Maximum rooting depth [cm]
RD : Actual rooting depth [cm]
qt : Actual soil moisture content in rooted zone [cm3 cm-3]
qwp : Soil moisture content at wilting point [cm3 cm-3]
The soil moisture content of the lower zone is also
limited by the field capacity in case the crop cannot
develop airducts, else the soil moisture content is
limited by the soil porosity.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…EVAPORASI
.
Evaporation depends on the available soil water and the infiltration
capacity of the soil. If the water layer on the surface, the so called
surface storage, exceeds 1 cm, the actual evaporation rate from
the soil is set to zero and the actual evaporation rate from the
surface water is equal to the maximum evaporation from a shaded
water surface.
If the surface storage is less than 1 cm and the infiltration rate of the
previous day exceeds 1 cm d-1, the actual evaporation rate from the
surface water is set to zero and the actual evaporation rate from the soil is
equal to the maximum evaporation from a shaded soil surface. All water on
the surface can infiltrate within one day. The value of the variable days
since last rain, Dslr, is reset to unity. If the infiltration rate is less than 1 cm
d-1, the amount of infiltrated water is considered too small to justify a reset
of the parameter Dslr and the evaporation rate decreases as the top soil
starts drying. The reduction of the evaporation is thought to be proportional
to the square root of time (Stroosnijder, 1987, 1982).
The evaporation can be calculated as:
where Es : Evaporation rate from a shaded soil surface [cm d-1] Es,max :
Maximum evaporation rate from a shaded soil surface (see eq. 6.7) [cm d1] D : Days since last rain [d] When a small water amount has infiltrated,
slr
or rather wetted the soil surface, this amount can be evaporated the same
day, irrespective of Dslr. Therefore, the actual evaporation from the soil
surface, as calculated according to equation 6.20, should be corrected for
this water amount infiltrating the soil. This amount should be added to the
actual evaporation rate. However, it should be noted that the actual
evaporation can never exceed the maximum evaporation rate.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…PRESIPITASI
Not all precipitation will reach the surface. A fraction will be
intercepted by leaves, stems, etc. From the amount of precipitation
which reaches the soil surface, a part runs off. Runoff from a field
can be 0-20 percent, and even higher on unfavorable surfaces
(Stroosnijder & Koné, 1982).
It can be assumed that a fixed fraction of the precipitation will not
infiltrate during that particular day. This fraction can be reduced in
situations with relatively small amounts of rainfall. The reduction
factor is defined as function of the rainfall amount (van Diepen et
al., 1988).
Note that the non infiltrating fraction refers to rainfall only. Irrigation
water is assumed to infiltrate freely.
Reduction
factor of the
non
infiltrating
fraction as a
function of
rainfall.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…PERKOLASI
If the root zone soil moisture content is above field capacity, water
percolates to the lower part of the potentially rootable zone and the
subsoil. A clear distinction is made between percolation from the
actual rootzone to the so-called lower zone, and percolation from
the lower zone to the subsoil. The former is called Perc and the
latter is called Loss. The percolation rate from the rooted zone can
be calculated as:
where
1. Perc : Percolation rate from the root zone to the lower zone [cm
d-1]
2. Wrz : Soil moisture amount in the root zone [cm]
3. Wrz,fc : Equilibrium soil moisture amount in the root zone [cm]
4. Dt : Time step [d]
5. Ta : Actual transpiration rate [cm d-1]
6. Es : Evaporation rate from a shaded soil surface [cm d-1]
The equilibrium soil moisture amount in the root zone can be
calculated as the soil moisture content at field capacity times the
depth of the rooting zone:
where
1. Wrz,fc : Equilibrium soil moisture amount in the root zone [cm]
qfc : Soil moisture content at field capacity [cm3 cm-3] RD :
Actual rooting depth [cm]
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…LAJU PERKOLASI
. The percolation rate and infiltration rate are limited by the conductivity of
the wet soil, which is soil specific and should be given by the user. Note
that the percolation from the root zone to the lower zone can be limited by
the uptake capacity of the lower zone. Therefore, the value calculated with
equation 6.21 is preliminary and the uptake capacity should first be
checked. The percolation from the lower zone to the subsoil, the so-called
Loss, should take the water amount in the lower zone into account. If the
water amount in the lower zone is less than the equilibrium soil moisture
amount, a part of the percolating water will be retained and the percolation
rate will be reduced. Water loss from the lower end of the maximum root
zone can be calculated as:
where Loss : Percolation rate from the lower zone to the subsoil [cm d-1]
Perc : Percolation rate from root zone to lower zone [cm d-1] Wlz : Soil
moisture amount in the lower zone [cm] Wlz,fc : Equilibrium soil moisture
amount in the lower zone [cm] Dt : Time step
Water loss from the potentially rootable zone, is also limited by the
maximum percolation rate of the subsoil, which is soil specific and should
be provided by the user. The equilibrium soil moisture amount in the lower
zone can be calculated as the soil moisture content at field capacity times
the root zone depth:
where Wrz,fc : Equilibrium soil moisture amount in the lower zone [cm] qfc :
Soil moisture content at field capacity [cm3 cm-3] RDmax : Maximum rooting
depth [cm] RD : Actual rooting depth [cm]
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…SIMPANAN LENGAS TANAH
. For rice an additional limit of five percent of the saturated soil conductivity
is set to account for puddling (a rather arbitrary value, which may be easily
changed in the program). The saturated soil conductivity and is calculated
using equating with pF= -1.0 (i.e. a hydraulic head of 0.1 cm). The
percolation rate from the lower zone to the sub soil is not to exceed this
value (van Diepen et al., 1988). As mentioned before, the value calculated
with equation, should be regarded as preliminary; the storage capacity of
the receiving layer may become limiting.
The storage capacity of the lower zone, also called the uptake capacity, is
the amount of air plus the loss (van Diepen et al., 1988). It can de defined
as:
where
1. UP : Uptake capacity of lower zone [cm d-1]
2. RDmax : Maximum rooting depth [cm]
3. RD : Actual rooting depth [cm]
4. Wlz : Soil moisture amount in lower zone [cm]
5. qmax : Soil porosity (maximum soil moisture) [cm3 cm-3]
6. Dt : Time step [d]
7. Loss : Percolation rate from the lower zone to the subsoil [cm d-1]
Percolation to the lower part of the potentially rootable zone can not
exceed the uptake capacity of the lower zone. Therefore the percolation
rate is set equal to the minimum of the calculated percolation rate and the
uptake.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…INFILTRASI AWAL
The infiltration rate depends on the available water and
the infiltration capacity of the soil. If the actual surface
storage is less then or equal to 0.1 cm, the preliminary
infiltration capacity is simply described as:
where
1. INp : Preliminary infiltration rate [cm d-1]
2. FI : Maximum fraction of rain not infiltrating during
time step t [-]
3. CI : Reduction factor applied to FI as a function of
the precipitation intensity [-]
4. P : Precipitation intensity [cm d-1]
5. Ie : Effective irrigation [cm d-1]
6. SSt : Surface storage at time step t [cm]
7. Dt : Time step [d]
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
LAJU INFILTRASI AWAL
. The maximum fraction of rain not infiltrating during time step t, FI
can be either set to a fixed value or assumed to be variable by
multiplying FI with a precipitation dependent reduction factor CI
which is maximum for high rainfall and will be reduced for low
rainfall. The user should provide FI. The CI table is included in the
model and is assumed to be fixed. The calculated infiltration rate is
preliminary, as the storage capacity of the soil is not yet taken into
account. If the actual surface storage is more than 0.1 cm, the
available water which can potentially infiltrate, is equal to the water
amount on the surface (i.e. supplied via rainfall/irrigation and
depleted via evaporation):
where
1. INp : Preliminary infiltration rate [cm d-1]
2. P : Precipitation intensity [cm d-1]
3. Ie : Effective irrigation [cm d-1]
4. Ew : Evaporation rate from a shaded water surface [cm d-1]
5. SSt : Surface storage at time step t [cm]
6. Dt : Time step [d]
However, the infiltration rate is hampered by the soil conductivity
and cannot exceed it. Soil conductivity is soil specific and
should be given by the user.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…INFILTRASI YANG TERKOREKSI
Total water loss from the root zone can now be
calculated as the sum of transpiration, evaporation and
percolation. The sum of total water loss and available
pore space in the root zone define the maximum
infiltration rate. The preliminary infiltration rate cannot
exceed this value. The maximum possible infiltration
rate is given by:
where
1. INmax : Maximum infiltration rate [cm d-1]
2. qmax : Soil porosity (maximum soil moisture) [cm3 cm-3]
3. qt : Actual soil moisture content [cm3 cm-3]
4. RD : Actual rooting depth [cm] Dt : Time step [d]
5. Ta : Actual transpiration rate [cm d-1]
6. Es : Evaporation rate from a shaded soil surface [cm d-1]
7. Perc : Percolation rate from root zone to lower zone [cm d-1]
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
INFILTRASI
Infiltrasi mencerminkan kecepatan meresapnya air
ke dalam tanah melalui permukaan tanah.
Semakin tinggi infiltrasi, semakin banyak air yang akan
tersedia bagi tanaman dan semakin sedikit air runoff di
permukaan tanah, semakin sedikit pula erosi dan
pencucian unsur hara.
Seresah tanaman, tumbuhan hidup, atau permuakan
yang kasar, akan menghambat aliran air di permukaan
tanah, sehingga air mempunyai kesempatan untuk
meresap ke dalam tanah.
Kerak tanah dapat mereduksi infiltrasi dan dapat
diminimumkan dengan jalan membiarkan seresah
tumbuhan tetap di permukaan tanah, memperbaiki
kandungan bahan organik tanah, dan memacu aktivitas
biologis.
Diunduh dari:
….. 13/11/2012
PENGELOLAAN KAPASITAS LAPANG
.. Beberapa praktek pengelolaan tanah yang dapat meningkatkan
kapasitas lapang dan memperbaiki infiltrasi:
1. Pengelolaan bahan organik. Bahan organik dapat
meningkatkan kemampuan tanah menyimpan air (waterholding capacity, WHC) melalui dua cara. Bahan organik
mampu menyimpan dan menahan banyak air, dan dapat
memperbaiki struktur tanah – meningkatkan total volume dan
ukuran pori yang dapat menyimpan air dan mencegah
pembentukan kerak tanah di permukaan.
2. Praktek pengolahan tanah. Membiarkan seresah sisa panen di
permukaan atanah dapat memperlambat runoff dan mencegah
pembentukan kerak tanah di permukaan. Seresah ini dapat
mendorong perkembangan populasi cacing tanah dan
organisme lain yang membuat liang dalam tanah, dan air hujan
dapat dengan cepat meresap ke dalam tanah melalui lubanglubang tersebut.
3. Pencegahan pemadatan. Pemadatan tanah dapat mereduksi
WHC, karena berkurangnya jumlah dan ukuran pori tanah.
4. Pengendalian erosi. Erosi tanah dapat mereduksi kedalaman
tanah (solum tanah menjadi tipis) dan menurunkan WHC.
Diunduh dari:
….. 13/11/2012
KONSERVASI TANAH - INFILTRASI
Praktek konservasi yang dapat memperburuk infiltrasi:
1. Pembakaran dan pengangkutan sisa-sisa panen, membiarkan
tanah bera dan peka terhadap erosi
2. Metode pengolahan tanah yang merusak koneksi pori dengan
permukaan tanah, dan mencegah akumulasi bahan organik
tanah
3. Lalu lintas peralatan dan ternak, terutama pada saat tabnah
dalam kondisi bawah, yang menyebabkan pemadatan dan
reduksi porositas tanah.
Beberapa praktek konservasi membantu mempertahankan atau
memperbaiki infiltrasi air ke dalam tanah :
1. Meningkatkan tutupan vegetatif di permukaan tanah,
2. Mengelola residu vegetatif, dan
3. Meningkatkan bahan organik tanah.
Biasanya, praktek-praktek ini meminimumkan gangguan tanah dan
pemadatan tanah, melindungi tanah dari erosi, dan mendorong
perkembangan struktur tanah yang baik dan ruang pori yang
kontinyus.
Sebagai solusi jangka pendek mengatasi buruknya infiltrasi adalah
membongkar kerak permukaan dengan membajak tanah, dan
lapisan tanah yang kompak dapat dibongkar dengan pengolahan
tanah secara dalam.
Diunduh dari:
….. 13/11/2012
MEMPERBAIKI INFILTRASI
Praktek konservasi yang memperbaiki laju infiltrasi:
1. Pergiliran tanaman
2. Tanaman penutup tanah
3. Grazing terkendali
4. Pengelolaan residu dan seresah tanaman, serta
Pengolahan tanah
5. Pemanfaatan sisa panen.
Nilai Maksimum Laju Infiltrasi berbagai tipe tanah
Laju Infiltrasi
(inch/hr)*
Tipe Tanah
Pasir = Sand
2
Pasir berlempung =Loamy sand
1.8
Lempung berpasir = loam
1.5
Lempung = Loam
1
Debu = Silt dan Lempung Liat
0.5
Liat = Clay
0.2
. (*) Asumsi tanaman penutup tanah penuh. Laju pada Tanah bera sebesar
½ dari laju pada tanah dengan tumbuhan penutup tanah penuh.
Diunduh dari:
….. 13/11/2012
…LIMPASAN PERMUKAAN
Surface runoff is also taken into account by defining a maximum
value for surface storage. If the surface storage exceeds this value
the exceeding water amount will run off. Surface storage at time
step t can be calculated as:
where
1. SSt : Surface storage at time step t [cm d-1]
2. P : Precipitation intensity [cm d-1]
3. Ie : Effective irrigation rate [cm d-1]
4. Ew : Evaporation rate from a shaded water surface [cm d-1]
5. IN : Infiltration rate (adjusted) [cm d-1]
Surface runoff can be calculated as:
where
1. SRt : Surface runoff at time step t [cm]
2. SSt : Surface storage at time step t [cm]
3. SSmax : Maximum surface storage [cm]
4. SSmax is an environmental specific variable and should be
provided by the user.
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
SURFACE RUNOFF
Surface runoff is the water flow that occurs when soil
is infiltrated to full capacity and excess water from rain,
meltwater, or other sources flows over the land. This is
a major component of the hydrologic cycle.
Runoff that occurs on surfaces before reaching a
channel is also called a nonpoint source. If a nonpoint
source contains man-made contaminants, the runoff is
called nonpoint source pollution.
A land area which produces runoff that drains to a
common point is called a watershed. When runoff flows
along the ground, it can pick up soil contaminants such
as petroleum, pesticides, or fertilizers that become
discharge or nonpoint source pollution.
Diunduh dari:
….. 13/11/2012
LIMPASAN PERMUKAAN
. Faktor Meteorologi yang mempengaruhi runoff:
1. Tipe presipitasi (rain, snow, sleet, etc.)
2. Intensitas hujan
3. Jumlah hujan
4. Lamanya hujan
5. Distribution of rainfall over the watershedS
6. Direction of storm movement
7. Antecedent precipitation and resulting soil moisture
8. Other meteorological and climatic conditions that affect
evapotranspiration, such as temperature, wind, relative
humidity, and season.
Karakteristik Fisik yang mempengaruhi runoff:
1. Land use
2. Vegetasi
3. Tipe Tanah
4. Drainage area
5. Bentuk daerah tangkapoan air
6. Elevation
7. Kemiringan
8. Topography
9. Direction of orientation
10. Pola aliran Drainage
11. Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent
or alter runoff from continuing downstream.
Diunduh dari:
….. 13/11/2012
RUNOFF & TIPE TANAH
Kapasitas infiltrasi suatu tanah dipengaruhi oleh porositas tanah,
yang menentukan kapasitas simpanan air dan mempengaruhi
resistensi air untuk mengalir ke lapisan tanah yang lebih dalam.
Porositas suatu tanah berbeda dengan tanah lainnya. Kapasitas
infiltrasdi tertinggi dijumpai pada tanah-tanah yang gembur, tekstur
berpasir; sedangkan tanah-tanah liat dan berliat biasanya
mempunyai kapasitas infiltrasi lebih rendah. Bagan-bagan berikut
menyajikan beragam kapasitas infiltrasi yang diukur pada berbagai
tipe tanah.
Kapasitas infiltrasi juga tergantung pada kadar lengas tanah pada
akhir periode hujan sebelumnya. Kapasitas infiltrasi aweal yang
tinggi dapat menurun dengan waktu (asalkan hujan tidak berhenti)
hingga mencapai suatu nilai konstan pada saat profil tanah telah
jenuh air.
Kurva kapasitas
infiltrasi untuk
berbagai tipe
tanah yang
berbeda.
Diunduh dari:
….. 13/11/2012
Koefisien Runoff
Selain faktor-faktor yang bersifat spesifik-lokasi, perlu diperhatikan
juga adalah homogenitas kondisi fisik daerah tangkapan air.
Meskipun pada sekala mikro, ternyata juga ada variasi kemiringan,
tipe tanah, vegetasi penutup dll. Oleh karena itu setiap daerahtangkapan air mempunyai respon-runoff yang spesifik, dan respon
ini juga akan tergantung pada ragam kejadian hujan. Disain
sarana pemanenan air memerlukan pengetahuan tentang jumlah
runoff yang akan dihasilkan oleh hujan dalam suatu daerah
tangkapan. Biasanya diasumsikan bahwa volume runoff sebanding
dengan kedalaman (jumlah) hujan.
Runoff [mm] = K x Rainfall depth [mm]
Dalam kondisi daerah-tangkapan di pedesaan yang tidak ada
bagian kedap air, koefficien K, yang mencerminkan persentase
runoff dari suatu kejadian hujan, bukanlah merupakan faktor yang
konstan. Nilai koefisien ini sangat beragam dan tergantung pada
faktor-faktor spesifik lokasi dan karakteristik hujannya.
Misalnya, dalam suatu daerah tangkapan tertentu, dengan kondisi
initial yang sama (misalnya kadar lengas tanah awal), kejadian
hujan selama 40 menit dengan intensitas rataan 30 mm/jam akan
menghasilkan persentase runoff lebih kecil dibandingkan dengan
kejadian hujan selama 20 menit tetapi dengan rataan intensitas 60
mm/jam, walaupun total hujan keduanya sama.
Diunduh dari:
….. 13/11/2012
…PERUBAHAN KANDUNGAN LENGAS TANAH
The rates of change in the water amount in the root and lower zone are
calculated straightforward from the flows found above:
where
1. DWrz : Change of the soil moisture amount in the root zone [cm]
2. DWlz : Change of the soil moisture amount in the lower zone [cm] Ta :
Actual transpiration rate [cm d-1]
3. Es : Evaporation rate from a shaded soil surface [cm d-1]
4. IN : Infiltration rate [cm d-1]
5. Perc : Percolation rate from root zone to lower zone [cm d-1]
6. Loss : Percolation rate from lower zone to sub soil [cm d-1]
7. Dt : Time step [d]
Due to extension of the roots into the lower zone, extra soil moisture
becomes available, which can be calculated as:
where
1. RDt : Rooting depth at time step t [cm]
2. RDt-1 : Rooting depth at time step t-1 [cm]
3. RDmax : Maximum rooting depth [cm]
4. Wlz : Soil moisture amount in the lower zone [cm]
5. DWrz : Change of the soil moisture amount in the root zone [cm]
6. DWlz : Change of the soil moisture amount in the lower zone [cm]
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
JUMLAH LENGAS TANAH DI ZONE AKAR
The actual water amount in the root zone and in the
lower zone can be calculated according to:
where
1. Wrz,t : Soil moisture amount in the root zone at time
step t [cm]
2. Wlz,t : Soil moisture amount in the lower zone at time
step t [cm]
3. Wrz,t-1 : Soil moisture amount in the root zone at time
step t-1 [cm]
4. Wlz,t-1 : Soil moisture amount in the lower zone at
time step t-1 [cm]
5. DWrz : Rate of change of the soil moisture amount in
the root zone [cm]
6. DWlz : Rate of change of the soil moisture amount in
the lower zone [cm].
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
…KANDUNGAN LENGAS TANAH AKTUAL
The actual soil moisture content can now be calculated
according to:
where
1. qt : Actual soil moisture content at time step t [cm3
cm-3]
2. Wrz,t : Soil moisture amount in the root zone at time
step t [cm]
3. RD : Actual rooting depth [cm]
Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012
KANDUNGAN LENGAS PROFIL TANAH
Modeled soil water content over the 10-m profile on 30 Mar. 1997
(maximum soil water storage), 22 Aug. 1997 (beginning dry season), and 8
Jan. 1998 (minimum soil water storage).
Diunduh dari: https://www.soils.org/publications/sssaj/articles/67/6/1672 ….. 13/11/2012
SOIL WATER CONTENT
The distribution of soil water content in the 0–200 cm soil profile for
two shrubs. (a) The driest profile for C. korshinkii, (b) The wettest
profile for C. korshinkii, (c) The driest profile for S. psammophila, (d)
The wettest profile for S. psammophila. The vertical dashed lines
represent soil water content at the permanent wilting point of
0.064 cm3 cm− 3.
Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0016706111003648 …..
13/11/2012
POROSITAS TANAH
Patterns of vertical distribution of bulk density (a), porosity (b), and
organic matter content (c) within the soil profile above the landslide.
The different position of the black layer zone in c reflects an
excavation slightly upslope of the pit where bulk density/porosity
cores were collected (a and b).
Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0016706109002274 …..
13/11/2012
POROSITAS TANAH
Air-filled porosity for the switch plow treatment of the bare fallow
and residue covered field as a function of the soil profile. Values
are means, n = 4. Values in each profile layer followed by the same
letter are not significantly different at the P < 0.05 level.
Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0167198706000493…..
13/11/2012
POROSITAS TANAH
Subsoil denitrification is a potential sink for leached nitrate
(NO3–) that may otherwise contaminate ground water.
Following rainfall and irrigation, subsoil N2O concentrations
increased rapidly. Within days of NO3– leaching below 1 m,
high concentrations of NO3–, Br–, and N2O were observed at 7m depth.
Diunduh dari:
https://www.soils.org/publications/sssaj/abstracts/76/4/1333?access=0&view=article…..
13/11/2012
Soil profile and physical and chemical
characteristics
Grain-size distribution and soil horizons
Diunduh dari: http://www.kgs.ku.edu/Publications/Bulletins/GW12/05_site7.html …..
12/11/2012
Soil profile and physical and chemical
characteristics
Organic carbon content and cation-exchange capacity
profiles
Diunduh dari: http://www.kgs.ku.edu/Publications/Bulletins/GW12/05_site7.html …..
12/11/2012
Soil profile and physical and chemical
characteristics
Soil bulk density distributions determined by the core and
clod methods.
The bulk density of the soil profile, determined by the clod and core
methods, shows relatively high compaction at the bottom of the
upper 30 cm (1 ft). The soil becomes less compact below this level
until the silty clay soil is reached, at which point compaction
increases again.
Diunduh dari: http://www.kgs.ku.edu/Publications/Bulletins/GW12/05_site7.html …..
12/11/2012
.
Soil profile and physical and chemical
characteristics
Available water-holding capacity of the soil profile.
The available water capacity of the soil profile WAS RELATED TO THE
SOIL TEXTURE, it is generally lower because of the sandier nature of
the soil profile, except where the silty clay soil is encountered at
depth [below 180 cm (6 ft)]. The nature of the silty clay soil causes
the available water capacity to increase significantly.
Diunduh dari: http://www.kgs.ku.edu/Publications/Bulletins/GW12/05_site7.html …..
12/11/2012
Simpanan Lengas Tanah = SOIL WATER
STORAGE
Dalam kaitannya dengan irigasi dan pengairan, kapasitas
simpanan air tanah (SWS) didefinisikan sebagai jumlah total
air yang disimpan dalam tanah pada zone perakaran tanaman.
Tekstur dan struktur tanah, serta kedalaman perakaran
tanaman akan menentukan besarnya SWS ini.
Semakin dalam perakaran tanaman, berarti semakin banyak air
yang dapat disimpan dalam tanah dan semakin besar pula
cadangan air tersedia bagi tanaman selama periode tidak ada
penambahan air.
Forms of Soil Water Storage (sumber: bettersoils.soilwater.com.au)
. Bagaimana menentukan SWS dan Defisit
maksimum lengas tanah (MSWD)
1. Tahap 1. Menentukan kedalaman perakaran tanaman, RD (m).
2. Tahap 2. Menentukan kapasitas simpanan air tersedia, AWSC
(mm/m), Table 2
3. Tahap 3. Menghitung total simpanan lengas tanah, SWS (mm)
SWS (mm) = RD (m) x AWSC (mm/m) …………….. (1)
4. Tahap 4. Menentukan koefisien eketersediaan air bagi tanaman,
AC (%), Table 3
5. Tahap 5. Menghitung Defisit maksimum lengas tanah, MSWD
(mm)
MSWD = SWS (mm) x AC (%) ………………. (2)
Kedalaman Efektif Perakaran tanaman Dewasa.
Shallow
0.45 m (1.5 feet)
Medium Shallow
0.60 m (2 feet)
Medium Deep
0.90 m (3 feet)
Deep
1.20 m (4 feet)
Kubis, Timun,
Sawi, Bawang
merah,Lobak, dll
Kentang, Wortel,
Tomat, Kacang
kapri, Buncis, dll
Jagung, Terong,
Cabe, dll
Asparagus,
Anggur, Beet, dll
Kapasitas simpanan air tersedia dari beberapa tipe
tanah
Tekstur Tanah
Available Water Storage Capacity (AWSC)
Clay = Liat
(in. water / in.
soil)
0.21
(in. water / ft.
soil)
2.5
(mm water / m
soil)
200
Clay Loam
0.21
2.5
200
Silt loam
0.21
2.5
208
Clay loam
0.20
2.4
200
Loam
0.18
2.1
175
Fine sandy
loam
loam
0.14
1.7
142
0.12
1.5
125
Loamy sand
0.10
1.2
100
Sand
0.08
1.0
83
Koefisien ketersediaan air tanah
Tanaman
Kacang Kapri
Kentang
Pohon buah-buahan
Anggur
Tomat
Other crops
Maximum Percent (%)
35
35
40
40
40
50
…
LENGAS
TANAH
SOIL MOISTURE
…AIR TANAH = LENGAS TANAH
Air terdapat dalam tanah karena ditahan (diserap) oleh massa
tanah, tertahan oleh lapisan kedap air, atau karena keadaan
drainase yang kurang baik.
Air dapat meresap atau ditahan oleh tanah karena adanya gayagaya adhesi, kohesi, dan gravitasi.
Karena adanya gaya-gaya tersebut maka air dalam tanah dapat
dibedakan menjadi:
1. Air hidroskopik, adalah air yang diserap tanah sangat kuat
sehingga tidak dapat digunakan tanaman, kondisi ini terjadi
karena adanya gaya adhesi antara tanah dengan air. Air
hidroskopik merupakan selimut air pada permukaan butir-butir
tanah.
2. Air kapiler, adalah air dalam tanah dimana daya kohesi (gaya
tarik menarik antara sesama butir-butir air) dan daya adhesi
(antara air dan tanah) lebih kuat dari gravitasi. Air ini dapat
bergerak secara horisontal (ke samping) atau vertikal (ke atas)
karena gaya-gaya kapiler. Sebagian besar dari air kapiler
merupakan air yang tersedia (dapat diserap) bagi tanaman.
Diunduh dari: ….. 12/11/2012
Kandungan Air Tanah (KAT)
KAT menyatakan banyaknya air yang ada dalam tanah.
KAT dapat dinyatakan sebanyak banyaknya air ( mm
kedalaman air) yang ada dalam satu meter kedalaman
tanah. Misalnya: kalau sejumlah air (mm kedalaman air)
150 mm ada dalam satu meter kedalaman tanah, maka
kandungan air tanah sebesar 150 mm/m.
KAT dapat juga dinyatakan sebagai persen volume.
Dalam hal contoh perhitungan di atas, 1 m3 volume
tanah (misalnya kedalaman 1 m, dan luas permukaan 1
m2) mengandung 0.150 m3 air (missal dengan
kedalaman tanah of 150 mm = 0.150 m dan luas
permukaannya 1 m2). Hal ini menghasilkan nilai KAT
dalam persen volume:
Dengan demikian, kandungan air 100 mm/m setara
dengan kadar air tanah 10 % v/v.
Diunduh dari: ….. 12/11/2012
AIR DALAM TANAH
• Kadar (Kandungan) air tanah
Mw
m 
Ms
– Mass water content (m)
– m = mass water content (fraction)
– Mw = mass of water evaporated, g
(24 hours @ 105oC)
– Ms = mass of dry soil, g
• Kadar air volumetrik (v)
Vw
v 
Vb
V = volumetric water content (fraction)
Vw = volume of water
Vb = volume of soil sample
At saturation, V = 
V = As m
As = apparent soil specific gravity = b/w
(w = density of water = 1 g/cm3)
– As = b numerically when units of g/cm3 are
used
–
–
–
–
–
–
• Ekuivalen kedalmana air (d)
– d = volume of water per unit land area = (v
A L) / A = v L
– d = equivalent depth of water in a soil layer
– L = depth (thickness) of the soil layer
Volumetric Water Content & Equivalent
Depth
(cm3)
Equivalent
Depth
(g)
(g)
(cm
3)
Volumetric Water Content & Equivalent
Depth
Typical Values for Agricultural Soils
Soil Solids (Particles): 50%
0.50
in.
Total Pore
Space:
50%
1
in.
Very Large Pores:
15% (Gravitational
Water)
0.15
in.
Medium-sized Pores:
20% (Plant Available
Water)
0.20
in.
Very Small Pores:
15% (Unavailable
Water)
0.15
in.
Water-Holding Capacity of Soil
Effect of Soil Texture
Coarse Sand
Clay Loam
Silty
Dry Soil
Gravitational Water
Water Holding
Capacity
Available Water
Unavailable Water
WHC = WATER HOLDING CAPACITY
The water holding capacity of a soil is a very important agronomic
characteristic. Soils that hold generous amounts of water are less
subject to leaching losses of nutrients or soil applied pesticides.
This is true because a soil with a limited water holding capacity (i.e.
a sandy loam) reaches the saturation point much sooner than a soil
with a higher water holding capacity (i.e. a clay loam). After a soil is
saturated with water, all of the excess water and some of the
nutrients and pesticides that are in the soil solution are leached
downward in the soil profile.
Soil water holding capacity is controlled primarily by the soil texture
and the soil organic matter content. Soil texture is a reflection of the
particle size distribution of a soil. An example is a silt loam soil that
has 30% sand, 60% silt and 10% clay sized particles.
In general, the higher the percentage of silt and clay sized
particles, the higher the water holding capacity. The small particles
(clay and silt) have a much larger surface area than the larger sand
particles. This large surface area allows the soil to hold a greater
quantity of water.
The amount of organic material in a soil also influences
the water holding capacity. As the level of organic
matter increases in a soil, the water holding capacity
also increases, due to the affinity of organic matter for
water.
PENENTUAN WHC
The water holding capacity of the soil is determined by the amount
of water held in the soil sample vs. the dry weight of the sample.
The amount of pressure applied in these different methods can be
as low as 1/3 atmosphere of pressure (about 5 psi) up to 15
atmospheres of pressure (about 225 psi).
. Soil Water Holding Capacity
One of the main functions of soil is to store moisture and supply it to plants
between rainfalls or irrigations. Evaporation from the soil surface,
transpiration by plants and deep percolation combine to reduce soil
moisture status between water applications. If the water content becomes
too low, plants become stressed. The plant available moisture storage
capacity of a soil provides a buffer which determines a plant’s capacity to
withstand dry spells.
Forms of Soil Water Storage
Water is held in soil in various ways and not all of it is available to plants.
Chemical water is an integral part of the molecular structure of soil
minerals. It can be held tightly by electrostatic forces to the surfaces of
clay crystals and other minerals and is unavailable to plants.
The rest of the water in the soil is held in pores, the spaces between the
soil particles. The amount of moisture that a soil can store and the amount
it can supply to plants are dependent on the number and size of its pore
spaces
Gravitational water is held in large soil pores and rapidly drains out under
the action of gravity within a day or so after rain. Plants can only make use
of gravitational water for a few days after rain.
Capillary water is held in pores that are small enough to hold water
against gravity, but not so tightly that roots cannot absorb it. This water
occurs as a film around soil particles and in the pores between them and is
the main source of plant moisture. As this water is withdrawn, the larger
pores drain first. The finer the pores, the more resistant they are to
removal of water. As water is withdrawn, the film becomes thinner and
harder to detach from the soil particles. This capillary water can move in all
directions in response to suction and can move upwards through soil for
up to two metres, the particles and pores of the soil acting like a wick.
WHC - TEKSTUR
Water holding capacity (mm/cm depth of soil) of main texture
groups. Figures are averages and vary with structure and organic
matter differences.
Texture
Coarse sand
Fine sand
Loamy sand
Sandy loam
Field Capacity
0.6
1.0
1.4
2.0
Wilting point
0.2
0.4
0.6
0.8
Available water
0.4
0.6
0.8
1.2
Light sandy clay
loam
2.3
1.0
1.3
Loam
Sandy clay loam
Clay loam
Clay
2.7
2.8
3.2
4.0
1.2
1.3
1.4
2.5
1.5
1.5
1.8
1.5
Self-mulching clay
4.5
2.5
2.0
Source: Department of Agriculture Bulletin 462, 1960
Potential Air Tanah
• Deskripsi:
– Measure of the energy status of the
soil water
– Important because it reflects how hard
plants must work to extract water
– Units of measure are normally bars or
atmospheres
– Soil water potentials are negative
pressures (tension or suction)
– Water flows from a higher (less
negative) potential to a lower (more
negative) potential
Potential Air Tanah
• Komponennya:
t  g  m  o
– t = total soil water potential
– g = gravitational potential (force of
gravity pulling on the water)
– m = matric potential (force placed on
the water by the soil matrix – soil
water “tension”)
– o = osmotic potential (due to the
difference in salt concentration across
a semi-permeable membrane, such
as a plant root)
– Matric potential, m, normally has the
greatest effect on release of water
from soil to plants
• Soil Water Release Curve
– Curve of matric potential (tension) vs.
water content
– Less water  more tension
– At a given tension, finer-textured soils
retain more water (larger number of
small pores)
Matric Potential and Soil Texture
The tension or suction created by small
capillary tubes (small soil pores) is greater
that that created by large tubes (large soil
pores). At any given matric potential coarse
soils hold less water than fine-textured soils.
Height of
capillary rise
inversely related
to tube diameter
KAPASITAS LAPANG (FC or fc)
–Soil water content where gravity
drainage becomes negligible
–Soil is not saturated but still a very wet
condition
–Traditionally defined as the water content
corresponding to a soil water potential of 1/10 to -1/3 bar
Permanent Wilting Point (WP or wp)
–Soil water content beyond which plants
cannot recover from water stress (dead)
–Still some water in the soil but not
enough to be of use to plants
–Traditionally defined as the water content
corresponding to -15 bars of SWP
KAPASITAS LAPANG
"The amount of water held in soil after excess water has
drained away and the rate of downward movement has
materially decreased, which usually takes place within 2 - 3
days after a rain or irrigation in pervious soils of uniform
structure and texture".
Diunduh dari:
lawr.ucdavis.edu/classes/.../chapter5-00.pdf -….. 15/11/2012
KAPASITAS LAPANG
Factors affecting field capacity
1. Texture. The finer the texture of the soil particles,
the higher is the apparent field capacity and the
slower it is attained.
2. Type of Clay. Soils high in montmorillonite have
higher field capacity values
3. Organic Matter. Increases field capacity (as high as
100% in organic soils)
4. Depth of initial wetting. In general (but not always),
The wetter the lower soil profile at the beginning of
redistribution, and the greater the depth of wetting,
the slower the rate of redistribution, and the greater
the value of field capacity
5. Impeding layers. Inhibit redistribution and increase
field capacity
6. Evapotranspiration. Modifies redistribution and
affects field capacity.
Diunduh dari:
lawr.ucdavis.edu/classes/.../chapter5-00.pdf -….. 15/11/2012
KAPASITAS LAPANG
Field capacity minus wilting point is the amount of water
available to plants.
Diunduh dari: http://www.extension.umn.edu/distribution/cropsystems/components/7399_02.html
….. 15/11/2012
KAPASITAS LAPANG
Field capacity is the volumetric water content at a soil water
suction of 0.33 bars or remaining after a prolonged period of gravity
drainage without additional water supply. Wilting point is the
volumetric water content at a suction of 15 bars or the lowest
volumetric water content that can be achieved by plant
transpiration. These moisture retention parameters are used to
define moisture storage and relative unsaturated hydraulic
conductivity.
The field capacity must be greater than the wilting point and less
than the porosity. Total porosity must be greater than the field
capacity but less than 1 (one). The general relation among
moisture parameters and soil texture class is shown below.
Diunduh dari: http://msw.cecs.ucf.edu/AndFiles/hlp2.html ….. 15/11/2012
KAPASITAS LAPANG
1.
2.
3.
4.
5.
Conservation practices resulting in available water
capacity favorable to soil function include:
Conservation Crop Rotation
Cover Crop
Prescribed Grazing
Crop Residue and Tillage Management
Salinity and Sodic Soil Management
Diunduh dari:
http://soils.usda.gov/sqi/assessment/files/available_water_capacity_sq_physical_indicator_sheet
.pdf….. 15/11/2012
KAPASITAS LAPANG
Hygroscopic water content (θH) and bulk density (ρb) as a function of
organic matter content (OM).
The effect of OM content on ρb and θH is significant. . Both properties
were linearly correlated to OM content (R2 = 0.91 for ρb and 0.97 for θH).
Even though the different treatments imposed some changes on the OM
content, for a given soil these changes resulted in a limited range of both
θH and ρb values. The increasing trend of θH as a function of OM content
can be explained by the significantly higher vapor adsorption capacity of
OM substances (Chen and Schnitzer, 1976).
Diunduh dari: https://www.crops.org/publications/sssaj/articles/71/3/692 ….. 15/11/2012
KAPASITAS LAPANG
The proportion of rainwater that infiltrates into the soil
depends on the amount of soil cover provided.
The figure shows that on bare soils (cover = 0 tonnes/ha)
runoff and thus soil erosion is greater than when the soil is
protected with mulch.
Crop residues left on the soil surface lead to improved soil
aggregation and porosity, and an increase in the number of
macropores, and thus to greater infiltration rates.
Effect of amount of soil cover on rainwater
runoff and infiltration.
Diunduh dari:
http://www.fao.org/docrep/009/a0100e/a0100e08.htm….. 15/11/2012
LENGAS TANAH – MIKRO PORI
.
Soil is a porous structure. Ideally about 50% of the volume is
pore space, with half of it consisting of “macropores” of
visible to invisible size (0.05 mm to several millimetres in
diameter) and half being smaller “micro-” or “capillary” pores.
During an extended, heavy rainfall, all pores may become
filled with water. The water then drains from the larger pores
by downward gravitational flow, leaving them air-filled; this is
important for supplying oxygen to roots and soil life.
The larger the pores, the more rapidly they drain, providing
that such drainage is not impeded (e.g. by a hardpan).
After those larger pores have drained—about 1 day after the
rain stops in lighter soils, and 3 days in heavier soils—all of
the micropores remain filled with water and the soil is said to
be at field capacity.
The micropores do not drain by gravitational flow;
rather they hold water by a type of electrostatic
attraction between the water and the soil surfaces.
The smaller the pores or the closer you get to the
surface of the particles, the more tightly the water is
held.
Diunduh dari: http://www.cog.ca/documents/Water.pdf ….. 15/11/2012
Available water capacity (AWC)
Water that is held in macropores drains too quickly to be
of much use to plants; water held in micropores below
the permanent wilting point cannot be used at all. Thus
the water available to plants is that held between the
field capacity and the permanent wilting point. This
quantity of water is called the available water capacity
(AWC) of the soil. Of this amount, one half is usually
readily available, meaning that until AWC has dropped to
one half, there are no serious water limitations.
THE AWC values for soils of different textures. In
general, loams hold the most available water, and also
have the best balance of macro- and micropores.
Consequently, loams have both good water storage as
well as good drainage and aeration.
Sandy soils, with a predominance of larger pores, are
well drained, but they do not hold much water and they
dry out quickly.
At the other end of the textural scale, clay with many
very small pores holds a lot of water but much of it is
not available. Clay also lacks larger pores so drainage
and aeration are poor.
Diunduh dari: http://www.cog.ca/documents/Water.pdf ….. 15/11/2012
KEDALAMAN PERAKARAN
Roots can pull water from a distance of only 1–2
cm. So, the depth of soil from which crops can
extract water is, in general, the depth to which roots
are growing. This means that shallow rooting crops
are, in general, more prone to water limitations than
are deep rooting crops.
On the other hand, deep rooting crops can take
advantage of water deeper in the soil only if their
growth downwards is unrestricted. Removing
barriers (such as plowpans) facilitates both water
movement and root growth into deeper horizons.
Double-digging, with deep incorporation of
compost, is a good way to do it in gardens. Planting
deep rooting cover crops with large tap roots, such
as oilradish, or using rotations with alfalfa or sweet
clover are effective biological ways to open up the
subsoil.
The direction of root growth is always towards the
more moist regions. For that reason, frequent
watering discourages deep growth of roots.
Watering deep and less frequently encourages deep
growth.
Diunduh dari: http://www.cog.ca/documents/Water.pdf ….. 15/11/2012
KAPASITAS AIR TERSEDIA
Increasing available water capacity
The total water holding capacity of a soil can
be improved by:
1. Adding more soil,
2. Opening up the subsoil as discussed
above,
3. Changing the texture, or
4. Increasing the organic matter content.
Texture should be a consideration when soil
is being imported to make a garden.
Manufactured topsoil used in new
developments is often much too sandy for
optimal water retention in a garden.
Diunduh dari: http://www.cog.ca/documents/Water.pdf ….. 15/11/2012
BOT & KAPASITAS AIR TERSEDIA
Organic matter is about 5 fold lighter than mineral
soil, and a small amount of organic matter by
weight has a big impact on pore space.
“Within all textural groups, as organic matter
increased from 1 to 3%, the available water capacity
approximately doubled.
When organic matter content increased to 4%, it
then accounted for more than 60% of total AWC“.
Organic matter increases the bulk of the soil so you
actually have more soil depth as well as greater
AWC per unit depth.
It also improves infiltration, drainage and aeration,
not to mention the benefits of improved soil
nutrition and soil life.
Diunduh dari: http://www.cog.ca/documents/Water.pdf ….. 15/11/2012
MEMANEN AIR HUJAN
We want to capture as much of the precipitation as
possible. The maximum rate at which water can be
absorbed is referred to as the infiltration rate. When
rainfall intensity exceeds the infiltration rate, water
runs off, or forms ponds or puddles on the soil
surface.
The infiltration rate can vary from a few millimetres
to more than ten centimetres per hour. The rate
increases with the soil macro-porosity, which is
naturally high in pure sands. In finer soils, the
infiltration rate is very dependent on the formation
of soil crumbs through binding of particles by
humus, microbial gums and fungal hyphae, and on
the channels formed by soil fauna and roots.
Hence, it can be increased by adding compost and
by regularly feeding the soil biota with plant
residues and manures.
Worm holes are especially important as they carry water
quickly into deeper layers. Pulverizing soil through intensive
cultivation or compacting soil with heavy machinery can
quickly and drastically reduce macroporosity and infiltration.
Diunduh dari: http://www.cog.ca/documents/Water.pdf ….. 15/11/2012
. Ketersediaan air (Water availability)
Ketersediaan air adalah berapa besar cadangan air
yang tersedia untuk keperluan irigasi. Ketersediaan air
ini biasanya terdapat pada air permukaan seperti
sungai, danau, dan rawa-rawa, serta sumber air di
bawah permukaan tanah.
Pada prinsipnya perhitungan ketersediaan air ini
bersumber dari banyaknya curah hujan, atau dengan
perkataan lain hujan yang jatuh pada daerah tangkapan
hujan (catchment area/ watershed) sebagian akan
hilang menjadi evapotranspirasi, sebagian lagi menjadi
limpasan langsung (direct run off), sebagian yang lain
akan masuk sebagai infiltrasi. Infiltrasi ini akan
menjenuhkan tanah atas (top soil), kemudian menjadi
perkolasi ke ground water yang akan keluar menjadi
base flow
Di samping data meteorologi, dibutuhkan pula data
cahaya permukaan (exposed surface), dan data
kelembaban tanah (soil moisture).
Untuk rumus run off adalah
Run off = base flow + direct run off.
Diunduh dari: ….. 12/11/2012
Air Tersedia
• Definisi
– Water held in the soil between field capacity and
permanent wilting point
– “Available” for plant use
• Available Water Capacity (AWC)
– AWC = fc - wp
– Units: depth of available water per unit depth of soil,
“unitless” (in/in, or mm/mm)
– Measured using field or laboratory methods (described
in text)
Kandungan air tersedia sangat tergantung pada tekstur dan struktur tanah.
Kisaran nilai-nilai pada beragam tipe tanah sbb:
Soil
Available water content in mm water depth per m soil depth
(mm/m)
Sand
25 to 100
Loam
100 to 175
Clay
175 to 250
Sifat hidraulik tanah & Tekstur tanah
• Fraction available water depleted (fd)
 fc  v 
fd  

 fc  wp 
– (fc - v) = soil water deficit (SWD)
– v = current soil volumetric water content
• Fraction available water remaining (fr)
 v  wp 
fr  

 fc  wp 
– (v - wp) = soil water balance (SWB)
• Total Available Water (TAW)
TAW = (AWC) (Rd)
– TAW = total available water capacity within
the plant
root zone, (inches)
– AWC = available water capacity of the soil,
(inches of H2O/inch of
soil)
– Rd = depth of the plant root zone, (inches)
– If different soil layers have different
AWC’s, need to sum up the layer-by-layer
TAW’s
TAW = (AWC1) (L1) + (AWC2) (L2) + . . .
(AWCN) (LN)
- L = thickness of soil layer, (inches)
- 1, 2, N: subscripts represent each
successive soil layer
[Error on page 26 of text: change SWD 
TAW ]
Ketersediaan Air Tanah
Ketersediaan air dalam tanah dipengaruhi:
1. Banyaknya curah hujan atau air irigasi,
2. Kemampuan tanah menahan air,
3. Besarnya evapotranspirasi (penguapan langsung
melalui tanah dan melalui vegetasi),
4. Tingginya muka air tanah,
5. Kadar bahan organik tanah,
6. Senyawa kimiawi atau kandungan garam-garam, dan
7. Kedalaman solum tanah atau lapisan tanah.
Jumlah air yang tersedia bagi tanaman adalah jumlah
air yang disimpan dalam tanah pada kondisi kapasitas
lapang dikurangi dengan jumlah air yang masih
tertinggal dalam kondisi titik layu permanen.
Kandungan air tersedia =
Kandungan air pada kapasitas lapang – kandungan air
pada titik layu permanen
Diunduh dari: ….. 12/11/2012
Gravity vs.
Capillarity
Vertical
movement due
largely to gravity
Horizontal
movement due to
capillarity
Infiltrasi Air
Masuknya air ke dalam tanah
Faktor-faktor yang berpengaruh :
1. Soil texture
2. Initial soil water content
3. Surface sealing (structure, etc.)
4. Soil cracking
5. Tillage practices
6. Method of application (e.g., Basin vs.
Furrow)
7. Water temperature
Infiltrasi Kumulatif vs. Waktu
Berbagai Tekstur tanah
Infiltration Rate vs. Time
For Different Soil Textures
Laju Infiltrasi dan Tekstur Tanah
Soil Infiltration Rate vs. Constant
Irrigation Application Rate
Soil Infiltration Rate vs. Variable Irrigation
Application Rate
INFILTRASI & SIFAT-SIFAT TANAH
As water infiltrates into the soil, the length of the transmission zone
increases, and the infiltrating water wets the soil’s wetting zone,
which subsequently moves down in the soil profile.
Diunduh dari:
lawr.ucdavis.edu/classes/.../chapter5-00.pdf -….. 15/11/2012
INFILTRASI & SIFAT-SIFAT TANAH
For small times, vertical infiltration behaves as if
horizontal infiltration because soil water pressure
potential gradients dominate over the gravitational
gradient.
Example: Infiltration from an irrigation furrow into an
initially dry soil.
Diunduh dari:
lawr.ucdavis.edu/classes/.../chapter5-00.pdf -….. 15/11/2012
INFILTRASI & SIFAT-SIFAT TANAH
Effect of Soil Moistture on Infiltration
Diunduh dari:
lawr.ucdavis.edu/classes/.../chapter5-00.pdf -….. 15/11/2012
INFILTRASI & SIFAT-SIFAT TANAH
Effect of Soil Texture on Infiltration
Diunduh dari:
lawr.ucdavis.edu/classes/.../chapter5-00.pdf -….. 15/11/2012
INFILTRASI & SIFAT-SIFAT TANAH
Effects of Surface crusts on infiltration:
Soil surface crusts Can develop by:
· Soil surface compaction
· Slaking of soil agggregates at soil surface by rainfall / irrigation
Diunduh dari:
lawr.ucdavis.edu/classes/.../chapter5-00.pdf -….. 15/11/2012
INFILTRASI & SIFAT-SIFAT TANAH
Soil layering:
· Effect of soil layering depends on soil texture variations between layers
· Generally, any soil layers that are present will decrease water infiltration
1. If clay layer: Clay impedes infiltration because of lower saturated
hydraulic conductivity
2. If sand layer: Sandy layer will reduce infiltration rate temporarily and
retards moving of the wetting front due to its lower unsaturated
hydraulic conductivity
Diunduh dari:
lawr.ucdavis.edu/classes/.../chapter5-00.pdf -….. 15/11/2012
Kedalaman Penetrasi
•
•
•
Can be viewed as sequentially filling the soil profile in
layers
Deep percolation: water penetrating deeper than the
bottom of the root zone
Leaching: transport of chemicals from the root zone due to
deep percolation
Deep Percolation
Deep percolation is
when water moves
down through the
soil profile below
the root zone and
cannot be utilized
by plants.
Diunduh dari:
http://www.ca.uky.edu/agripedia/glossary/deepperc.htm….. 14/11/2012
Simpanan Air dalam Profil Tanah
. SIMPANAN LENGAS TANAH =
Soil Moisture Storage (ST).
Simpanan lengas tanah adalah jumlah air yang ditahan dalam
tanah selama waktu tertentu.
Jumlah air dalam tanah tergantung pada sifat tanah seperti tekstur
tanah dan kandungan bahan organik tanah.
Jumlah maksimum air yang dapat ditahan dalam tanah disebut
KAPASITAS LAPANG.
Tanah-tanah berbutir halus mempunyai kapasitas lapang lebih
besar dibandingkan dengan tanah-tanah berbutir kasar (tanah
berpasir).
Dengan demikian lebih banyak air yang tersedia untuk
evapotranspirasi aktual dari tanah-tanah yang teksturnya halus
daripada tanah yang teksturnya kasar.
Batas maksimum simpanan lengas rtanah adalah KAPASITAS
LAPANG, batas minimumnya adalah nol (0) ketika tanah telah
mengering.
SIMPANAN LENGAS TANAH
Pemanfaatan lengas tanah merupakan faktor penting yang
membatasi produksi tanaman.
Pengetahuan mengenai simpanan air tersedia dalam tanah
sangat penitng dalam pengelolaan pertanian lahan kering.
Kajian tentang simpanan air tanah, komponen-komponen dari
siklus air, sangat diperlukan dalam perhitungan neraca air
lahan.
Neraca air lahan menyajikan informasi tentang masukan air
(hujan dan irigasi), kehilangan air (evapotranspirasi, run-off
dan drainage), serta perubahan simpanan lengas tanah yang
terjadi selama periode waktu tertentu.
Pengelolaan simpanan lengas tanah secara efisien dapat
dicapai dengan jalan memanipulasi neraca air lahan. Hal ini
melibatkan pemantauan dan pengendalian berbagai proses
aliran lengas tanah, termasuk infiltrasi, redistribusi, drainage,
evaporasi dan penyerapan air oleh tanaman.
Bahan organik tanah mempunyai peran penting dalam
mengendalikan semua pproses-proses fisika ini.
KAPASITAS SIMPANAN AIR TERSEDIA
Air hujan yang dipanen disimpan dalam tanah di daerah lahan budidaya
tanaman. Kapasitas tanah untuk menyimpan air dan membuat air tersebut
tersedia abagi tanaman disebut KAPASITAS SIMPANAN AIR TERSEDIA.
Kapasitas ini tergantung pada (i) jumlah dan ukuran pori tanah (tekstur)
dan (ii) kedalaman tanah. Kapasitas simpanan air tersedia dinyatakan
dalam mm kedalaman air (air simpanan) per meter kedalaman tanah,
mm/m.
Kapasitas Simpanan Air Tersedia:
Tipe Tanah
Pasir = sand
Lempung Berpasir = sandy loam
Lempung Liat = clay loam
Liat = clay
Air Tersedia (mm/m)
55
120
150
135
Suatu tanah lempung dengan WHC air tersedia yang cukup
baik 120 mm per meter kedalaman tanah, akan kehilangan
nilainya kalau SOLUM TANAH hilang dan tanah menjadi
dangkal.
Misalnya, 40 cm tanah pada suatu batuan induk hanya
menyediakan 48 mm air tersedia bagi tanaman.
KEDALAMAN PERAKARAN TANAMAN
Kapasitas simpanan air tersedia dan kedalaman tanah mempunyai
implikasi penting bagi disain sistem pemanenan air tersedia. Pada tanah
yang solumnya dalam, misalnya, 2 m dengan kapasitas simpanan air yang
besar (150 mm/m) , maka kapasitas simpanan airnya sebesar 300 mm air
dan tidak akan terjadi genangan air runoff pada lahan pengolahan hingga
kedalaman lebih dari 300 mm (30 cm).
Kalau jumlah air melebihi 30 cm kedalaman, maka akan hilang melalui
drainage-dalam dan juga akan dapat menimbulkan bahanya
penggenangan. Kapasitas air tersedia dan kedalaman solum tanah juga
mempengaruhi jenis tanaman yang akan ditanam. Suatu tanah yang
solumnya dalam dengan kapasitas simpanan air tersedia yang besar
hanya dapat digunakan secara efektif oleh tanaman yang mempunyai
perakaran yang dalam.
Misalnya, bawang-Onions, mempunyai perakaran sedalam 30 - 40 cm,
sehingga ia tidak dapat memanfaatkan sepenuhnya semua lengas tanah
yang tersimpan dalam solum. Tabel 7 menyajikan kedalaman perakaran
beberapa jenis tanaman.
Kedalaman perakaran efektif beberapa tanaman (Doorenbos et al., 1979).
Jenis Tanaman
Kedalaman akar efektif (m)
Kacang buncis = Bean
0.5 - 0.7
Jagung = Maize
1.0 - 1.7
Bawang merah = Onion
0.3 - 0.5
Padi = Rice
0.8 - 1.0
Sorghum
1.0 - 2.0
Bunga matahari = Sunflower
0.8 - 1.5
KEDALAMAN TANAH
This gives an indication of the soil volume which can be
utilised by the plant and which is conducive to moisture
retention.
Effective soil depth is the depth where adequate
moisture, nutrients and air occur.
Effective soil depth can be lowered by rocky layers, a
high clay content, waterlogged layers, limestone layers,
acid subsoil and compacted layers.
Various crops have different requirements concerning
effective soil depth.
A hypothetical soil profile.
Diunduh dari:
http://www.fao.org/Wairdocs/ILRI/x5546E/x5546e04.htm….. 14/11/2012
Generalized Soil Profile
Diunduh dari: http://www.princetonsoilinstitute.com/special-topics/arkoserocks/soils/generalized-soil-profile.html….. 14/11/2012
The top layer of soil is called the SURFACE LITTER LAYER. This is where
all the “litter” of any ecosystem lies. It includes leaves, branches, animal
scats and bodies, mushrooms and other rotting matter.
Diunduh dari:
http://www.exploringnature.org/db/detail.php?dbID=27&detID=1206…..
14/11/2012
PROFIL TANAH ALFISOL
Alfisols develop in
humid and
subhumid climates,
have average
annual precipitation
of 500-1300 mm.
They are frequently
under forest
vegetation.
Characteristic
features:
1. Clay
accumulation in
a Bt horizon,
2. Thick E horizon,
3. Available water
much of the
growing season,
4. Slightly to
moderately acid.
Diunduh dari:
http://www.soilandwetlandscientistscertification.net/home/soil-science-basics…..
14/11/2012
PROFIL TANAH INSEPTISOL
.
Inceptisols, especially
in humid regions, have
weak to moderated
horizon development.
Horizon development
have been retarded
because of cold
climated, waterloged
soils, or lack of time for
stronger development.
Characteristic feature:
Texture has to be finer
than loamy very fine
sand.
Diunduh dari:
http://www.soilandwetlandscientistscertification.net/home/soil-science-basics…..
14/11/2012
PROFIL TANAH ENTISOL
Entisols have no
profile development
except a shallow
marginal A horizon.
Many recent river
floodplains, volcanic
ash deposits,
unconsolidated
deposits with
horizons eroded
away, and sands are
Entisols.
Diunduh dari:
http://www.soilandwetlandscientistscertification.net/home/soil-science-basics…..
14/11/2012
PROFIL TANAH MOLISOL
Mollisols are frequently
under grassland, but
with some broadleaf
forest-covered soils.
Characteristic features:
Deep, dark A horizons,
they may have B
horizons and lime
accumulation.
Diunduh dari:
http://www.soilandwetlandscientistscertification.net/home/soil-science-basics…..
14/11/2012
PROFIL TANAH ULTISOL
Ultisols are
ectensively
weathered soils of
tropical and
subtropical climates.
Characteristic
features:
Thick A horizon, clay
accumulation in a Bt,
strongly acid.
Diunduh dari:
http://www.soilandwetlandscientistscertification.net/home/soil-science-basics…..
14/11/2012
PROFIL TANAH VERTISOL
Vertisols exist most in
temperate to tropical
climated with distinct
wet and dry seasons.
They have a high
content of clays that
swell when wetted and
show cracks when dry.
Characteristic features:
1. Deep self-mixed A
horizon ,
2. Top soil falls into
cracks seasonally,
3. Gradually mixing
the soil to the depth
of the cracking.
Diunduh dari:
http://www.soilandwetlandscientistscertification.net/home/soil-science-basics…..
14/11/2012
PROFIL TANAH OKSISOL
Oxisols are
excessively weathered,
whereas few original
minerals are left
unweathered.
They develop only in
tropical and subtropical
climates.
Characteristic features:
1. Often Oxisols are
over 3 m deep,
2. Have low fertility,
3. Have dominantely
iron and aluminium
clays,
4. Acid.
Diunduh dari:
http://www.soilandwetlandscientistscertification.net/home/soil-science-basics…..
14/11/2012
…
PENTINGNYA
NERACA AIR
TANAH
.. Water balance, transpiration and canopy
conductance in two beech stands
Agricultural and Forest Meteorology. 2000. 100 (4). 291-308
Granier, A., Biron, P., Lemoine, D.
Measurements of sap flow, vapour fluxes, throughfall
and soil water content were conducted for 19 months in
a young stand dominated by beech (Fagus sylvatica)
growing at low elevation, in the Hesse forest, Germany.
Study of the radial variation of sap flow within tree stems
showed a general pattern of sap flux density in relation to the
depth below cambium. Among-tree variation of sap flow was
also assessed, in order to determine the contribution of the
different crown classes to the total stand transpiration.
Stand sap flow and vapour flux, measured by the eddy
covariance technique, were well correlated, for half
hourly as well for daily values, the ratios of the fluxes for
both averaging periods being 0.77.
A strong canopy coupling to the atmosphere was found,
with the omega factor ranging between 0.05 and 0.20
relative to the wind speed. Canopy conductance
variation was related to a range of environmental
variables: global radiation, vapour pressure deficit, air
temperature and soil water deficit. In addition to the
effect of radiation and of vapour pressure deficit often
found in various other tree species, here beech
exhibited a strong reduction in canopy conductance
when air temperature decreased below 17oC.
Maize canopies under two soil water regimes. I. Diurnal
patterns of energy balance, carbon dioxide flux, and canopy
conductance
Agricultural and Forest Meteorology. 1998. 89 (3/4). 169-184
Steduto, P., Hsiao, T. C.
.
Diurnal patterns of fluxes of CO2, latent heat and other components of
energy were determined for adjacent fields of maize located at the
University of California, Davis, one being well watered and one growing
only on water stored in the soil, under clear and variable cloud
conditions.
In addition, it was demonstrated that the canopy responded to the
movement of clouds without measurable lag, with Acf, lambda E and
sensible heat (H) fluctuating in unison as the sun disappeared and
reappeared. Acf, reaching peak values of about 30-35 micro mol m-2 s-1,
was higher in the morning than in the afternoon for the same level of Qp
(photosynthetic photon flux density), and dropped to zero late in the
afternoon when Qp was still substantial as a result of increases in
respiration as the temperature rose from the morning to the afternoon.
Senescence of the canopy, whether normal as maturity was approached
or accelerated by water stress, was associated with a general reduction
in Acf for a given green leaf area index. Late in the life cycle, Acf was
light saturated at 1.3 to 1.6 mmol m-2 s-1. At that time, abnormally high
radiation due to focusing by clouds caused an increase in lambda E and
H, but not in Acf.
Canopy conductances were calculated using the Penman-Monteith big
leaf model, after aerodynamic conductances were derived from wind
data. Diurnal patterns indicated that canopy conductance was strongly
influenced by radiation and was independent from, and usually lower
than, aerodynamic conductance. Canopy conductance for water vapour
reached noon-time values /up to/ 40 mm s-1 under the most favourable
water conditions. Water stress and senescence reduced canopy
conductance. Aerodynamic conductance for water vapour ranged
between 15 and 90 mm s-1 for wind velocity between 1 and 5 m s-1
.. Water
and energy balance of a forested
Appalachian watershed
Agricultural and Forest Meteorology. 1997. 84 (1/2). 61-68
Tajchman, S. J., Fu, H., Kochenderfer, J. N.
This study focuses on the water and energy balance
of a central Appalachian watershed with an area of
39.2 ha, covered with upland oaks (Quercus spp.) and cove
hardwoods (Acer saccharum, Prunus serotina, Liriodendron
tulipifera, Fagus grandifolia), 80 years old, located near
Parsons, West Virginia, USA.
For 40 hydrological years (May-April, 1951-1990) the average
yearly sum of precipitation (P) was 145.5 cm, stream flow (R)
was 63.8 cm, and evapotranspiration (Et = P - R) was 81.7 cm.
Neglecting the energy used in photosynthesis, the
yearly sum of net radiation of the watershed (Rn)
equals latent heat of evapotranspiration (Et) plus
sensible heat exchange with the atmosphere (H). Rn
was computed for 432 terrain segments covering
the whole watershed area.
Its average value was 2.2 GJ m-2 year-1. From the
balance LEt = 2.02 GJ m-2 year-1, where L is the
latent heat of evaporation of water. The sensible
heat exchange (H = Rn - Et) was 0.18 GJ m-2 year-1
or 8% of Rn
.. Difficulties of estimating evapotranspiration from
the water balance equation
Agricultural and Forest Meteorology. 1995. 72 (3/4). 317-325
Villagra, M. M., Bacchi O. O. S., Tuon R. L., Reichardt, K.
The effect of soil spatial variability on the
estimation of evapotranspiration, using the water
balance equation, was evaluated using data from 25
experimental plots, distributed along a transect of
125 m, on a dark red Brazilian latosol.
The variability of soil water storage, total hydraulic
gradients, soil hydraulic conductivity and soil water
flux densities, and their influence on the calculation
of evapotranspiration, are discussed.
The variability of these parameters confers a
coefficient of variation of the order of 40% to
evapotranspiration estimates, indicating that
aerodynamic and empiric approaches are a better
choice for evapotranspiration estimation of
extensive field areas, in which spatial variability of
soil hydraulic characteristics is relevant.
What is Evapotranspiration?
Evapotranspiration is the loss of water to the atmosphere
through two processes: evaporation from soil and plant
surfaces, and transpiration from plant tissues.
Evapotranspiration is an indicator of how much water crops,
lawns, gardens and trees need for healthy growth and
productivity. By measuring evapotranspiration, only the
amount of water that is lost will be put back into the soil,
therefore reducing water waste.
Diunduh dari: http://www.discovercorona.com/City-Departments/Department-of-Water--Power/Tiered-Rate-and-Water-Budgets/Explaining-Evapotranspiration.aspx….. 15/11/2012
Grouping water storage properties of Indian soils
for soil water balance model applications
Agricultural Water Management. 1998. 36 (2). 99-109
Rao, N. H.
Soil water balance models are useful tools of
agricultural and water resources planning and
management. Data of water storage limits of soil
profiles, field capacity (FC) and permanent wilting point
(PWP), are essential for running soil water balance
models. A procedure for estimating these parameters
for Indian soils is presented. The values of FC and PWP
for each textural class are estimated independently for
alluvial, black and red soils.
The estimated parameters are tested by running a soil
water balance model for a case study area. The use of
the model in saving irrigation water is also
demonstrated. The estimated parameters of Indian soils
are also compared with corresponding values of US
soils. The comparisons are favourable for FC for all soil
texture groups, and for PWP for coarse to medium and
medium texture groups.
Differences in PWP between US and Indian soils
increase at finer soil textures as a result of differences in
clay fraction, mineralogy, and organic matter content.
Water balance and water use efficiency of different land uses
in western Himalayan valley region
Agricultural Water Management. 1998. 37 (3). 225-240
Narain, P., Singh, R. K., Sindhwal ,N. S., Joshie, P.
.
Various land uses, including sole plantations of Leucaena and
Eucalyptus, maize-wheat, Chrysopogon grass or turmeric and their tree
crop mixtures were compared for nine years in two sequences for runoff,
water use and water use efficiency on nine large erosion plots on a 4%
slope.
Availability of water during summers and climatic evaporative (EP)
demand during winters appeared to be the governing factors for
seasonal water use. Approximately 70% of annual water consumption
occurs during the four months (July to October) of the rainy season.
During this season water use was approx equal to 3-4 times EP for trees
and grass and 2.5 times for maize.
The water use equals EP regardless of the land use during the winter
season, while it was reduced to approx. one-third to half of EP in the
summer season. Annual water use was closely linked with runoff
reduction efficiency of the land use. Sole plantations of Leucaena and
Eucalyptus showed negligible runoff losses and their water use
approximated annual rainfall.
Agroforestry land uses also reduced runoff and increased water use
and water use efficiency. Seasonal crops exploited 1.5 m depth of
profile more exhaustively than trees, whereas trees used soil water
down to 3.0 m depth. Therefore, in tree-crop mixtures more efficient
soil water use was observed as compared to monocropping systems.
Results of this study indicate that water conserved under sole tree
plantations and due to tree intervention in agroforestry land uses
through runoff reduction, is utilised to meet increased
evapotranspiration demand, and hence groundwater recharge in
appreciable quantities is unlikely.
AGRONOMIC EFFICIENCY OF WATER USE,
FAG
FAG = P/U
where P is crop production (total dry matter or the marketable
product, as the case may be) and U is the volume of water
applied.
As only a fraction of the applied water is actually absorbed
and utilized by the crop, it is necessary to consider the
various components of the denominator U:
U = R + D + Ep + Es + Tw + Tc
where R is the volume of water lost by runoff from the field, D
the volume drained below the root zone (deep percolation), Ep
the volume lost by evaporation during the conveyance and
application to the field,2 Es the volume evaporated from the
soil surface (mainly between the rows of crop plants), Tw the
volume transpired by weeds, and Tc the volume transpired by
the crop.
All these volumes pertain to the same unit area.
Accordingly:
FAG = P / (R + D + EP + ES + TW + TC )
Diunduh dari: http://www.fao.org/docrep/W3094E/w3094e04.htm ….. 15/11/2012
The water balance of a field
Under flood irrigation as commonly practised in river
diversion schemes, excessive water application often results
in considerable runoff, evaporation from open water surfaces
and transpiration by weeds. In the experience of the author,
these losses commonly amount to 20 percent or even 30
percent of the water applied. In addition, the loss of water due
to percolation below the root zone may be of the order of 30
percent or even 40 percent of the water applied. Consequently,
the fraction actually taken up by the crop is often below 50
percent and may even be as low as 30 percent.
Diunduh dari: http://www.fao.org/docrep/W3094E/w3094e04.htm ….. 15/11/2012
Water balance estimates over Greece
Agricultural Water Management. 1996. 32 (1). 85-104
Kerkides, P., Michalopoulou, H., Papaioannou, G., Pollatou, R.
.
Water balance for 31 locations in Greece was calculated on the basis
of long-term average monthly precipitation, evapotranspiration and
combined soil and vegetation characteristics, using the method of
Thornthwaite and Mather. Monthly evapotranspiration estimates
were calculated from 27 years (1960-1987) of routine meteorological
data using the original Penman method.
Soil and vegetation characteristics specific for the locations under
study were combined in the water capacity of the root zone (WCRZ).
Similar water balance calculations were carried out using fixed
values of WCRZ for all stations, to evaluate the effects of soil and
vegetation through the WCRZ in the final estimates of soil moisture
deficits.
Water balance calculations were performed using average monthly
evapotranspiration estimates calculated according to the empirical
Thornthwaite method. Results were compared to show differences
that could be attributed to the method of estimating
evapotranspiration.
Results obtained with a value of WCRZ fixed at 300 mm and
potential evapotranspiration estimated by the Thornthwaite
method for the period 1969-1987 were compared with
existing similar results over a longer period in the past
(1931-1968), in order to detect diachronic changes in the
water balance components over the same regions in
Greece.
. A soil water balance model for no-tillage and
conventional till systems
Agricultural Water Management. 1994. 26 (3). 155-168
Shanholtz, V. O., Younos, T. M.
A soil water balance model to simulate hydrological aspects of
no-tillage and conventional tillage systems when selected
soils are subjected to a range of climatic conditions was
developed.
Model components include procedures for determining plant
water interception, plant growth, root development, surface
runoff and infiltration, plant available soil water, evaporation,
and evapotranspiration.
A database for two growing seasons of maize (Zea mays) production
was used to develop mathematical expressions for various model
components and to calibrate and verify the model. Procedures for
parameter determination and adjustment are presented.
Results include a comparison of simulated soil water status to
field measurements for two tillage systems.
Good agreement between simulation results
and field measurements of soil water was
obtained at 0-30 cm soil depth for two different
climatic conditions.
Components of the water balance for tree species under
evaluation for agroforestry to control salinity in the wheatbelt
of Western Australia
Agroforestry Systems. 1994. 26 (3). 157-169
Eastham, J., Scott, P. R., Steckis, R.
.
The soil water balance technique was used to study evaporation (total
water loss by the processes of transpiration, evaporation from the soil
surface and evaporation of water intercepted by plant canopies) in
agroforestry trials using 2 fodder tree species (Acacia blakeyi and
Chamaecytisus proliferus), 10 Eucalyptus species (4 of single stemmed
form, and 6 of mallee form), and annual pasture over a 3-yr period in the
Western Australian wheatbelt.
Evaporation from both fodder trees and from 7 of the Eucalyptus spp. (E.
calycogona, E. camaldulensis, E. horistes, E. kochii subsp. kochii and
plenissima [E. plenissima?], E. loxophleba and E. spathulata) was greater
than from pasture for one or more of the study years. The maximum
difference in evaporation between trees and pasture was 82, 84 and 70
mm in the first, second and third study years, respectively. Higher
evaporation from trees was associated with greater depletions in soil
water than occurred beneath pasture.
Upward movement of water from wet soil beneath the root zone was
found under trees, with a maximum flux of 30 mm observed over a
one year period beneath E. camaldulensis.
The water use efficiency of fodder trees was significantly higher than
for most Eucalyptus species, due to greater yields from fodder trees.
Biomass production was a good indicator of the water use of
eucalypts over the first 2 yr of growth, but the relation between
productivity and water use differed for species with tree and mallee
forms. In the third year of study, obvious differences in the relation
between water use and yield were observed for some species of
eucalypts with high evaporation.
Obtaining soil hydraulic properties for water balance and
leaching models from survey data. 1. Water retention
Australian Journal of Agricultural Research. 9. 50 (2). 283-289
Smettem, K. R. J., Oliver, Y. M., Heng. L. K., Bristow, K. L., Ford, E. J..
A physico-empirical 2-parameter power law model of the draining
water retention curve (WRC) based solely on clay content was
described and further developed using 6 datasets obtained from
Australian and New Zealand soils. The slope of the WRC, or pore-size
distribution index, was well described by the model but the bubbling
pressure, or inflection point was poorly described. Without a good
estimation of the bubbling pressure it was not possible to scale the
physico-empirical model to the WRC.
To achieve the scaling, a single measured point on the WRC in the
unsaturated range was required.
The resulting estimated water contents may be satisfactory for
application within broad-scale leaching risk models and for
generalized extrapolation of results from detailed experimental sites
but caution is still required for quantitative applications of nitrate
leaching models at a particular site.
It is concluded that soil surveys could usefully
include a single WRC measurement in the field
at each sampling location to improve their
utility for water and chemical transport
modelling.
Typical soil - water retention curve
Diunduh dari:
http://www3.imperial.ac.uk/geotechnics/people/researchers/aikaterinitsiampousi
….. 15/11/2012
Obtaining soil hydraulic properties for water balance and
leaching models from survey data. 2. Hydraulic conductivity
Australian Journal of Agricultural Research. 1999. 50 (7).
Smettem, K. R. J., Bristow, K. L.
.
A simplified physico-empirical model to estimate the
'matrix', or textural saturated hydraulic conductivity, Km,
using estimates of the bubbling pressure derived entirely
from clay content data that are readily available in soil
surveys.
Model estimates were compared with in situ measurements
on surface soils obtained using a disc permeameter with a
negative pressure head at the supply surface of 40 mm.
Results were satisfactory for broad-scale water
balance and leaching risk models that require
specification of a matching point for the
unsaturated hydraulic conductivity function and
for modelling applications requiring generalized
application of results from experimental sites.
KONDUKTIVITAS HIDRAULIK
Hydraulic conductivity, symbolically represented as
K, is a property of vascular plants, soil or rock, that
describes the ease with which water can move
through pore spaces or fractures.
It depends on the intrinsic permeability of the
material and on the degree of saturation. Saturated
hydraulic conductivity, Ksat, describes water
movement through saturated media.
Soil hydraulic conductivity is a function of the water
potential of the soil.
Conductivity measures the ease with which water
moves through the soil.
As water content (and hence the water potential)
decreases, the hydraulic conductivity decreases
drastically.
The decrease in conductivity as the soil dries is due
primarily to the movement of air into the soil to
replace the water. As air moves in, the pathways for
water flow between soil particles become smaller
and more tortuous, and flow becomes more
difficult.
Diunduh dari:
http://5e.plantphys.net/article.php?ch=t&id=39….. 15/11/2012
KONDUKTIVITAS HIDRAULIK
Soil hydraulic conductivity as a function of the water potential of the
soil. Conductivity measures the ease with which water moves
through the soil.
The overall shape of this curve is representative of many soils, but
the shape for a particular soil may be influenced by the size
distribution of its particles and by its organic matter content. The
field capacity is the amount of water the soil is able to retain against
gravitational forces. The permanent wilting point is the soil water
potential value at which plants cannot regain turgor pressure even at
night, in the absence of transpiration.
Diunduh dari:
http://5e.plantphys.net/article.php?ch=t&id=39….. 15/11/2012
. Water balance of Pinus halepensis Mill. afforestation in an
arid region
Forest Ecology and Management. 1998. 105 (1/3). 121-128
Schiller, G., Cohen, Y.
Transpiration (T), needle water stress (MPa) and the water balance of an
Aleppo pine (Pinus halepensis) plantation, growing in an arid region at
the edge of the Israeli Negev desert, were studied over 1 yr (1992-93).
The heat pulse technique for the measurement of the heat flow velocity
was used for the estimation of the sap flow velocity (i.e. transpiration), in
the stems of 16 trees (27% of the trees in a plot of 1000 m2). A pressure
chamber was used to determine the needle water potential (MPa).
Climatic parameters were measured in the forest for the computation of
the potential transpiration (Tp) by means of the Penman-Monteith
equation.
During the rainy period, the transpiration (T) rate was maintained at a
level between 1 and 2 mm day-1, which then dropped after the last rain;
the decrease of T in small trees (diameter at breast height (DBH) <12 cm)
was much faster than that in larger trees (DBH >12 cm).
In May, T dropped to a nondetectable rate (about 0.02 mm day-1) and
remained at this level until the next significant rain in December.
A maximum T/Tp ratio of 0.3, which is nearly half of the ratio found in a
previous study by the authors in an Aleppo pine plantation under
Mediterranean climatic conditions (Schiller and Cohen, 1995), was
recorded in February. This difference in T/Tp ratio between the 2 sites is
attributed to their difference in basal area. Needle water potential at
sunrise decreased from -0.8 MPa in the rainy period to more than -3.0
MPa during the dry period.
The integrated T throughout the measurement period was
used to estimate the total water uptake by the stand, which
was 210.1 mm (93% of the effective rainfall, or 80.2% of the
total annual rainfall).
.. Hydraulic
properties and water balance of
a clay soil cropped with cotton
Irrigation Science. 1994. 15 (1). 17-23
Aydin, M.
A field study was carried out in Cukurova Region, Turkey, to
investigate the soil water balance and water uptake by roots in
relation to hydraulic properties of a clay (Vertic Luvisol) soil.
Bare soils and cotton plots were equipped with tensiometers,
gypsum blocks and neutron probe access tubes. Hydraulic
conductivity, evaporation, drainage and water withdrawal by
roots were determined from water flow equations using field
data.
Evaporation from bare soil was generally high from May to
July, varying between 1.0 and 4.5 mm/day. However, when the
soil water potential at 10 cm depth decreased to between 0.065 and -0.070 MPa, soil evaporation decreased to 0.4
mm/day. Drainage below 150 cm was highest (3.7 mm/day) at
the start of wetting cycles, but quickly decreased. The highest
values of capillary flux toward the surface layer and drainage
rate from cropped soil were 2.0 and 1.8 mm/day resp.
Rates of root water uptake from the soil profile, excluding the
0-10 cm layer, were high when compared with drainage and
upward fluxes, with values of 1.4-7.7 mm/day. Good agreement
between root length densities and water uptake was found: up
to 80% of roots grew in the top 50 cm of soil, where 78% of
water was extracted. Evapotranspiration declined as a cubic
function of available water in the top 120 cm of soil
..
Soil water balance changes in
engineered soil surface
Journal of Environmental Quality. 1995. 24 (2). 352-359
Sackschewsky, M. R., Kemp, C. J., Link, S. O., Waugh, W. J.
Recharge can be prevented by storing precipitation near the surface
so that it will be returned to the atmosphere via evapotranspiration.
Erosion can be reduced with gravel mulch, but thick gravel layers
may increase recharge. Gravel mixed into the surface soil may
provide erosion protection without increasing recharge. To compare
the effects of erosion control on infiltration, two lysimeter
experiments were conducted to examine the effects of sand and
gravel mulches and gravel admixtures, using two precipitation
regimes and with or without vegetation.
Sand and gravel mulch increased soil-column water storage
and decreased evapotranspiration compared with a plain soil
surface. Gravel admixtures did not significantly affect the soil
water balance compared with plain soil surfaces. Vegetation
increased evapotranspiration and decreased soil moisture
storage compared with nonvegetated treatments. Irrigation
greatly increased evapotranspiration but had little effect on
soil water storage. Drainage was detected from sand and
gravel-mulch lysimeters, but not from lysimeters with a plainsoil or gravel-admixture surface.
Results are significant for isolation barrier designs in arid
sites: (i) a nonvegetated gravel-mulch surface eventually will
result in recharge, even under low precipitation (160 mm/yr);
and (ii) a soil column with a plain-soil or gravel-admixture
surface is capable of recycling all water back to the
atmosphere, even under high-precipitation (450 mm/yr)
.. A conceptual model of catchment water balance:
Application to runoff and baseflow modeling
Journal of Hydrology (Amsterdam) 1995. 173 (1/4). 41-50
Ponce, V. M.; Shetty, A. V.
A conceptual model of catchment water balance
was used to simulate changes in runoff and
baseflow with annual precipitation. The model is
based on the sequential separation of annual
precipitation into surface runoff and wetting, and
wetting into baseflow and vaporization.
Runoff is the sum of surface runoff and baseflow.
Runoff gain is defined as the derivative of runoff
coefficient with respect to precipitation. Baseflow
gain is defined as the derivative of baseflow
coefficient with respect to precipitation.
Catchment data show that runoff and baseflow
gains are always positive. Runoff gain reaches a
peak value at a threshold precipitation Prt; baseflow
gain reaches a peak value at a threshold
precipitation Put.
Analysis of the runoff and baseflow functions helps
show the nature of the competition between runoff
and vaporization, and baseflow and vaporization.
.. Modelling effects of soil structure on the water
balance of soil-crop systems: a review
Soil & Tillage Research. 1998. 48 (1/2). 1-19
Connolly, R. D.
Poor soil structure, i.e. aggregation and porosity, is widely
acknowledged as a major limitation to infiltration, redistribution and
storage of water in a soil profile, leading to more runoff and erosion,
reduced available water for plants and reduced crop production.
Models of soil-crop systems are useful tools for evaluating
interactions between soil physical condition, climate, management
and crop growth.
An outline of the principal components of soil structure which influence
the water balance, hence crop growth, is given. Models that represent
these components are reviewed and classified according to their
representation of soil structure and the water balance. Few of the models
reviewed explicitly represented soil structure and none attempted to
simulate long-term changes in soil physical properties. About half the
models used a USDA Curve Number approach to represent runoff and a
'tipping bucket' approach to represent soil water redistribution, and were
classed as 'functional'.
Functional models were mostly used for predicting long-term trends
in erosion, soil fertility and crop growth. The remainder were termed
'mechanistic' as they used more physically based representations of
the water balance. Mechanistic models were typically used for
detailed simulations of soil water dynamics and solute movement
and were more complex than functional models. No one model or
modelling approach suites all applications; the problem the model is
being used to resolve define the features and complexity needed.
Future development of models of the soil-crop system would benefit from
better methods of parameterising and applying models, extension of
point-source models to simulate spatial variability and to handle varying
catchment scales, and a more complete representation of soil physical
properties
STRUKTUR TANAH
The components of soil structure. Soil minerals with organic
matter form soil structure units, called "peds".
Micropores inside the peds and macropores between the peds
carry air and water and facilitate root penetration.
Soil aggregation or soil structure refers to how the sand, silt
and clay come together to form larger granules. Good
aggregation is apparent in a crumbly soil with water-stable
granules that do not disintegrate easily. Well-aggregated soil
has greater water entry at the surface, better aeration, and
more waterholding capacity than poorly aggregated soil. A
stable system of soil pores allows easy exchange of air and
water.
Diunduh dari:
http://www.fao.org/docrep/009/a0072e/a0072e07.htm ….. 15/11/2012
Tillage and furrow diking effects on water
balance and yields of sorghum and cotton
..
Soil Science Society of America Journal. 1993. 57 (4). 1077-1083
Baumhardt, R. L., Wendt, C. W., Keeling, J. W.
The amount of rain conserved and the yields of forage sorghum and
cotton as affected by furrow dikes and tillage were compared for a 3-yr
period at a site in Texas, USA.
A clay loam soil was alternately cropped to cotton and sorghum. Forage
sorghum was grown in (i) disc or (ii) chisel-disc tilled 16 by 23.8 m field
plots with and without diking. Cotton was grown in rotation following
sorghum after (i) conventional mouldboard-disc or (ii) no-tillage, with
furrow dikes in one-half of the tillage treatment plots. Crop yield, rainfall
amount, soil water content, and runoff of natural rainfall and of simulated
rainfall, applied at 80 mm/h for 1 h, were measured.
Compared with conventionally tilled undiked plots, cumulative
nonponded infiltration of simulated rainfall was significantly greater with
no-tillage treatments and greater (not significant) in furrow-diked
treatments.
Runoff of natural rainfall from plots with furrow dikes
averaged approx equal to 22 mm less than from undiked plots,
it was as much as 57 mm less; however, runoff from diked
fields was observed. Under the conditions of this 3-yr study,
diking did not significantly increase crop water use and yield.
It was concluded that furrow dikes installed during the
growing season did not increase water conservation and
crop yields under the conditions of this 3-yr study due to
seasonal dike consolidation that reduced the detention
capacity and to the limited runoff from level fields. No-tillage
was more effective than chisel tillage for increasing water
conservation and crop yields for the conditions of this study
..
Transpiration from coppiced poplar and willow
measured using sap-flow methods
Agricultural and Forest Meteorology. 1998. 90 (4). 275-290
Hall, R. L., Allen S. J., Rosier P. T. W., Hopkins, R.
Transpiration rates from clones of poplar (Populus trichocarpa x
deltoides [P. interamericana] 'Beaupre') and willow (Salix burjatica [S.
aquatica]) clones, grown as short-rotation coppice (3-year-old stems
on 4-year-old stools) at a site in south-west England, were measured
through the summer of 1995. Area-averaged transpiration was
estimated by scaling sap-flow rates measured in individual stems to
a stand area basis using measurements of leaf area and stem
diameter distribution. Sap flow in poplar was measured using the
stem heat balance, heat pulse velocity and deuterium tracing
techniques, while in willow only the stem heat balance method was
used.
In June and early July the mean daily transpiration from the poplar
was 6 plus or minus 0.5 mm/day, stomatal conductances averaged
0.33 mol m-2/s for leaves in the upper layer of the canopy, and daily
latent heat flux often exceeded the daily net radiation flux. Similarly
high transpiration was estimated for the willow.
Transpiration rates were higher than any reported rates from
agricultural or tree crops grown in the UK and arose because
of high aerodynamic and stomatal conductances. The high
stomatal conductances were maintained even when
atmospheric humidity deficits and soil water deficits were
large. Much lower rates from both clones were recorded in
August at the end of a drought period.
It is suggested that extensive plantation of poplar or willow
short-rotation coppice may result in reduced drainage to
stream flow and aquifer recharge
TRANSPIRASI
Oak trees will lose water through a process called
transpiration as illustrated at the left. Transpiration
is part of the water cycle, and it is the loss of water
vapor from parts of the plant, mainly the leaves,
which also triggers a flow of mineral nutrients and
water from the roots back to the shoots of tree.
Diunduh dari:
http://tips.woodlandtree.com/terrantip2012_4.html ….. 15/11/2012
. Evaporation of intercepted precipitation based on an energy
balance in unlogged and logged forest areas of central
Kalimantan, Indonesia
Agricultural and Forest Meteorology. 1998. 92 (3). 173-180
Asdak C.; Jarvis P. G.; Gardingen P. V.
The effect of logging practices on rainfall interception loss was
investigated in a dipterocarp humid tropical rain forest in central
Kalimantan. The traditional volume balance method was used to
measure throughfall, stemflow and interception loss. The evaporation
rate during and after rainfall had ceased in canopy-saturated
conditions was calculated by an energy balance method, which relied
on the modified Penman equation using directly determined
microclimatic and canopy structure variables as inputs.
The results obtained showed that the evaporation from wet canopies
in this research area is driven more by advected energy than by
radiative energy. In the unlogged plot (581 trees/ha with diameter at
breast height (dbh) >10 cm), advective energy accounted for 0.38 mm
h-1 of the 0.51 mm h-1 of evaporation, whereas radiative energy
accounted for only 0.13 mm h-1.
A similar relationship between the major driving variables and the
rate of evaporation was also found in the logged over plot (278
trees/ha with dbh >10 cm) and this implies that logging activities did
not change the proportion of energy used for interception loss.
The Priestley-Taylor equation was found to be a poor model for
evaporation of intercepted water in tropical forests because advected
energy is very important at the canopy scale
Influence of vegetation on infiltration
capacity
Diunduh dari: http://hercules.gcsu.edu/~sdatta/home/teaching/hydro/slides/index.html …..
15/11/2012
. Hydrological impacts of converting native forests and
grasslands to pine plantations, South Island, New Zealand
Agricultural and Forest Meteorology. 1997. 84 (1/2). 69-82
Fahey B.; Jackson R.
Changes in water yield, flood hydrology, and low flows caused by
replacing indigenous forests and grasslands with commercial softwoods
have been investigated in New Zealand since the mid-1970s.
Planting the harvested areas caused the water yield from both
catchments to return to pre-harvesting levels within 8 yr, and an
estimated reduction in runoff of 340 mm within 5 yr at DC4. Mean flood
peaks increased after harvesting, especially for small and medium
storms on the skidder logged catchment (75-100%). The response of the
storm quickflows to harvesting was similar but much more subdued. Low
flows also increased after harvesting.
Tree growth brought storm peak flows, quickflows, and low flows back to
the levels of those in the original beech forest within 10 yr.
The second study examined the impact of converting tussock grasslands
to pine (Pinus radiata) plantations using data collected from two
catchments in the eastern uplands of southern New Zealand. After a 3year calibration period (1980-82) one catchment was planted with pines
over 67% of its area and the other was left as tussock. By 1989 the
difference in annual water yield from the planted catchment was 130 mm,
and between 1991 and 1994 it averaged 260 mm (27% of total runoff from
the control). Differences in low flows (represented by the minimum
annual 7-day mean) showed a similar trend, and suggest that in dry
periods, afforestation of tussock grasslands can reduce water yields by
0.18 mm/day.
Higher interception losses from increased canopy
evaporation was the main reason for the reduction in water
yield. After 10-12 yr of tree growth mean flood peaks had
fallen by between 55 and 65%, and quickflows had
decreased by approx equal to 50%
INTERSEPSI TAJUK POHON
The structure of a tree’s canopy causes it to intercept
raindrops as they fall, with the broad surfaces of the many
layers of leaves and branches either catching rainwater and
holding it until it evaporates, or letting it drip slowly to the
ground. This provides two primary benefits.
First, it minimizes the erosive effect of the droplets on barren
surfaces by decreasing their velocity as they reach the
ground. Second, it lessens the total amount of water that
reaches the ground, which reduces the volume of runoff on
the ground; this also makes rainwater less erosive.
Diunduh dari:
http://www.midwestutc.org/stormwater.aspx ….. 15/11/2012
. Evaporation from an eastern Siberian larch forest
Agricultural and Forest Meteorology. 1997. 85 (3/4). 135-147
Kelliher F M., Hollinger D Y., Schulze E D., Vygodskaya N N., Byers J N Hunt J E
McSeveny T M., Milukova I., Sogatchev A., Varlargin A., Ziegler W., Arneth A., Bauer G.
Total forest evaporation ( lambda E), understorey evaporation, and
environmental variables were measured on 9 summer days under
different weather conditions in a 130-yr-old stand of Larix gmelinii
located 160 km south of Yakutsk in eastern Siberia, Russia. Tree and
broadleaved understorey vegetation one-sided leaf area indices were 1.5
and 1.0, respectively. Agreement of lambda E and sensible heat flux (H),
both measured by eddy covariance, and the available energy (Ra) was
generally good: (H + lambda E ) = 0.83Ra + 9W m-2 with r2=0.92 for 364
half-hour periods and the mean plus or minus 95% confidence limit was
129 plus or minus 17 for (H + lambda E) and 144 plus or minus 19 for Ra.
Daily E was 1.6-2.2 mm, less than half of the potential evaporation
rate and accounting for 31-50% of Ra, with the lowest percentage on
clear days. A perusal of the sparse literature revealed that average
daily E of boreal coniferous forest during the tree growing season
(1.9 mm day-1 for this study) is relatively conservative, suggesting
that low evaporation rates are a feature of this biome's energy
balance. Using the Penman-Monteith equation, the maximum bulksurface conductance (Gsmax) was 10 mm s-1. E and Gs were
regulated by irradiance, air saturation deficit, and surface soil water
content during a week-long dry period following 20 mm rainfall. From
lysimeter measurements, 50% of E emanated from the understorey at
a rate proportional to Ra.
Based on the measurements and published climatological data,
including average annual precipitation of 213 mm, water balance
calculations indicated a growing season forest E of 169 mm, the
occurrence of a late summer-autumn soil water deficit, and annual
runoff of 44 mm by snowmelt.
Estimation of transpiration by single trees: comparison of sap
flow measurements with a combination equation
Agricultural and Forest Meteorology. 1997. 87 (2/3). 155-169
Zhang, H., Simmonds L. P., Morison J.I. L., Payne D,
.
Sap flow estimates for whole trees (scaled from measurements on
selected branches using the heat balance method) were compared
with estimates of transpiration based on porometry in a study of
poplar (Populus trichocarpa x Populus tacamahaca [P. balsamifera])
trees in an agroforestry system in the south of the UK.
Sap flow showed good agreement with the transpiration rate
estimated using the Penman-Monteith equation with measured
stomatal conductance (R2 = 0.886) on six selected days during the
season.
The dominant environmental variable influencing transpiration
was the vapour pressure deficit, as the aerodynamic term in
the Penman-Monteith equation accounted for more than 70%
of daily total transpiration, with the rest due to the radiation
component.
Stomatal conductance, estimated by inverting the PenmanMonteith equation from continuous measurements of sap flow
over 55 days, was used to determine the parameters for a
multiplicative stomatal conductance model.
For an independent data set there was better agreement between
measured sap flow and transpiration predicted from the stomatal
conductance than for calculated and predicted stomatal
conductance.
. Water regime of a pine forest under a
Mediterranean climate
Agricultural and Forest Meteorology. 1995. 74 (3/4). 181-193
Schiller G., Cohen Y.
The transpiration (T) of a 14-yr-old Aleppo pine (Pinus halepensis)
plantation on Mt. Carmel, at Ramat ha'Nadiv, Israel, was studied
during 1 year (autumn 1990 to the end of summer 1991) to evaluate
soil water storage and tree response to water stress. The heat pulse
method was used for continuous measurement (excluding rainy
days) of sap flow in the stems of 8 trees at hourly intervals. Climatic
parameters were measured in the forest for computation of potential
transpiration (Tp) using the Penman formula. Extrapolating T of
sampled trees to stand T was difficult because of a poor correlation
between T and stem diameter.
During the rainy season the average daily rate of T was linearly
related to Tp and the ratio T/Tp was 0.62, independent of Tp rate.
After the last rainfall, T/Tp decreased steadily, reaching a value of
0.05 approximately 30 days later; the leaf water potential ( psi l) at
sunrise also decreased, from -0.8 to -2.9 MPa. The drop in T and psi l
indicates that most of the available water in the main root zone had
been extracted during less than 30 days after the last rain.
Decreasing values of wood thermal diffusivity with the progress of
the dry season show that trees may use internally stored water when
soil water stress is intense.
The integrated T throughout the measurement period was used for
estimation of total water uptake by the stand and for evaluation of the
amount of water which was available to the trees following the last
rain.
SOIL MOISTURE REGIME
Soil-moisture regime :
The changing state of soil moisture through
the year, which reflects the changing
balance of monthly precipitation and
potential evapotranspiration above the
ground surface.
When the latter exceeds the former the
period is one of soil-moisture deficit in the
annual regime.
The water regime of the soil is determined
by the physical properties and arrangement
of the soil particles. The pores in a soil
determine its water-retention
characteristics. When all the pores are full of
water, the soil is said to be saturated.
(Source: DUNSTE)
Diunduh dari:
….. 15/11/2012
Soil Moisture Regimes - Descriptions
Soil moisture regimes are defined based on the watertable level and the
presence or absence of available water (water that can be used by
plants). All moisture regimes, except aquic, are based on regional
climate. Aquic moisture regimes are based on the length of the period that
the soil was saturated.
Soil moisture regimes are used as a soil classification criterion because
they affect soil genesis (formation), affect the use and management of
soils, and can be used to group soils with similar properties and
morphology.
The soil moisture regime classes include:
1. Aquic (or Perudic): Saturated with water long enough to cause oxygen
depletion.
2. Udic: Humid or subhumid climate.
3. Ustic: Semiarid climate.
4. Aridic (or Torric): Arid climate.
5. Xeric: Mediterranean climate (moist, cool winters and dry, warm
summers).
Management considerations vary based on different moisture
regimes. Soils with an aridic (torris) moisture regime require irrigation to
be used for crops. Soils with a ustic moisture regime can grow rain-fed
crops, but moisture will be limited during some of the growing season.
Soils with a udic moisture regime have sufficient moisture for crops. Crops
may be grown in the udic moisture regime without irrigation, but irrigation
is needed for crops in most years in an ustic moisture regime. Soils with
an aquic (perudic) moisture regime need artificial drainage for most
cropping practices.
Diunduh dari:
http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447033&topicord
er=11&maxto=13&minto=1 ….. 15/11/2012
Evaporation from bare soil in a temperate humid climatemeasurement using micro-lysimeters and time domain
reflectometry
Agricultural and Forest Meteorology. 1995. 76 (1). 1-17
Plauborg, F..
Direct measurements of evaporation from a loamy sand in Denmark were
made using micro-lysimeters. The accuracy of daily measurement was
approx equal to plus or minus 0.5 mm H2O/d. The micro-lysimeter
method was not valid in periods with high precipitation. The use of time
domain reflectometry (TDR) for measuring soil water content was
investigated using a manual interpretation of the trace. The precision of
changes in soil water content calculated from daily measurements with
TDR was approx equal to 1.3 mm H2O, when using probes of 50 cm
length. However, improved precision may be obtained by the use of an
automatic interpretation of the trace. Estimates of daily evaporation from
bare soil calculated from the water balance equation and measurements
of soil water content with TDR were compared with measurements with
micro-lysimeters.
The TDR technique was suitable for estimating bare soil evaporation
when the soil water content was integrated over a 0-50 cm soil profile
and drainage had ceased at the lower depths of the profile.
Evaporation during a 13 d drying period in spring, just after the soil
had been fully rewetted, was approx equal to 26 mm. During a 23 d
drying period later in the season the evaporation from the bare soil
was approx equal to 30 mm.
In both periods the accumulated evaporation was rather high
and equivalent to approx equal to 65 and 50% of the
accumulated potential evapotranspiration in the first and
second drying period, resp., even though the soil water
content in the 0-50 cm profile was well below field capacity at
the beginning of the second drying period.
Simulated and measured water uptake by Picea abies under
non-limiting soil water conditions
Agricultural and Forest Meteorology. 1994. 71 (1/2). 147-164 CD Volume:15
Cienciala E., Eckersten H., Lindroth A., Hallgren J.E..
A one-dimensional non-steady-state soil-plant-atmosphere
continuum (SPAC) model was applied to a stand of Picea abies trees
in southern Sweden. The simulated root water uptake was compared
with measured sap flow under non-limiting soil water conditions. Sap
flow was measured during the growing season using the tree-trunk
heat balance method.
The model included four resistances against water (soil-root, plant,
stomatal and aerodynamic), one pool of easily available plant water
and one compartment of intercepted water on the needle surface.
The bulk stomatal resistance was estimated as the product
of the combined effect of two independent variables vapour pressure deficit and solar radiation.
Good agreement between simulated and 'measured' water
uptake was obtained both on short- and long-term scales,
and the model explained 92-93% of the variation of
measured uptake for both hourly and daily values.
The pool of easily available water was found to be small
(0.5 mm), i.e. of the same order as for agricultural crops.
Radiation balance, transpiration and photosynthesis of an
isolated tree
Agricultural and Forest Meteorology. 1993. 64 (3/4). 210-221
Green S. R.
.
Radiation balance of an isolated walnut [Juglans nigra] tree was
measured using an experimental Whirligig device. The total amount
of all-wave radiation absorbed by the tree canopy was used to
estimate transpiration rates using a Penman-Monteith model.
Results compared favourably with tree water use, measured by the
heat-pulse technique. Total amount of photosynthetically active
radiation (PAR) absorbed by the tree canopy was combined with a
photosynthetic light response curve to estimate net photosynthesis
rates.
Results compare favourably with published data from other tree
canopies. Daily energy balance calculations showed that on average,
about two-thirds of total radiant energy absorbed by the tree canopy
was dissipated as latent heat in the form of transpiration. Dominant
environmental variable influencing transpiration was vapour
pressure deficit of air. Almost two-thirds of net latent heat flux was
attributable to vapour pressure deficit component, with the remainder
owing to the radiation component.
Daily transpiration-assimilation ratios varied from day to
day in response to changing environmental conditions, but
generally decreased with increasing net photosynthesis
and with increasing transpiration. This appears to be the
first time that such a direct measurement of energy balance
and photosynthesis of a single tree has been made.
. Effect of planting methods and soil moisture on cassava
performance in the semi-arid Sudan savanna belt of Nigeria
African Crop Science Journal. 9. 7 (1). 21-33
Okogbenin E., Ekanayake I J., Porto M. C. M.
The effects of planting methods and soil moisture on cassava
(Manihot esculenta) performance in the Sudan savanna region of
Nigeria were assessed under field conditions on Eutric Regosols at
the International Institute of Tropical Agriculture (IITA) in Minjibir,
Kano State. Six planting methods in monoculture were evaluated in
two crop seasons.
These were horizontal planting on furrows or ridges, inclined
planting on flat land or ridges, and vertical planting on flat land or
ridges. Two genotypes were compared: TMS 91934, an improved IITA
clone; and Dakata Uwariya, a land race. Dakata Uwariya was
significantly better than TMS 91934 in plant height and root dry
matter content; TMS 91934 was better in leaf formation and leaf
retention.
Ridge-based methods positively influenced root
yield production and leaf formation, while flat or
furrow methods were advantageous in terms of
the number of plants at harvesting. Horizontal
and inclined planting were the best methods in
general.
Water use and soil moisture depletion by olive trees under
different irrigation conditions
Agricultural Water Management. 1996. 29 (3). 315-325
Michelakis N., Vouyoukalou E., Clapaki G,
.
12-year-old olive cv. Kalamon trees irrigated at soil water potential ranges
of -0.02 to -0.06 MPa ( PSI 0.2) and -1 to -1.5 MPa ( PSI 15) were compared
with non-irrigated trees in Crete, Greece. Irrigation water was applied by
two drip laterals per row (DR2) or a microtube-fed basin at each tree
(BAS) for PSI 0.2 and one drip lateral per row (DR1) or a microtube-fed
basin at each tree for PSI 15.
Soil moisture depletion in the non-wetted areas of the irrigated
treatments was greater than that of the wetted ones, but about the same
as the non-irrigated treatment. Soil moisture depletion in the non-wetted
areas of the irrigated and non-irrigated treatments occurred during MayJune. Soil moisture reached wilting percentage at the end of September
in the non-wetted areas of the PSI 0.2, at the end of August in the nonwetted areas of PSI 15 and at the end of July in the non-irrigated
treatment. Total soil moisture reserve depletion was greater in the upper
soil layers of the wetted and non-wetted areas. The amount of irrigation
water used was higher at the PSI 0.2 than at the PSI 15 treatment, but it
was not noticeably different between the drip and basin irrigation
methods within the same soil water potential levels.
Evapotranspiration of irrigated trees during May-September period
was met mainly from irrigation water (75-90%), 10-20% from the
reserves of soil moisture and 2.5-5% from rainfall. Crop coefficient,
expressed as an evapotranspiration to class 'A' pan evaporation
ratio, increased from 0.4 to 0.65 for the PSI 0.2, maintained at approx
equal to 0.3 to 0.4 for the PSI 15 and decreased from 0.2 to 0.05 for
the non-irrigated treatment.
Crop coefficients were not markedly different between drip
and basin methods within the same soil water potential levels.
Improving water-use efficiency
Conservation of water
1. Reduce conveyance losses by lining channels or, preferably, by using
closed conduits.
2. Reduce direct evaporation during irrigation by avoiding midday sprinkling.
Minimize foliar interception by under-canopy, rather than by overhead
sprinkling.
3. Reduce runoff and percolation losses due to overirrigation.
4. Reduce evaporation from bare soil by mulch-ing and by keeping the interrow strips dry.
5. Reduce transpiration by weeds, keeping the inter-row strips dry and
applying weed control measures where needed.
Enhancement of crop growth
1.
2.
3.
4.
5.
Select most suitable and marketable crops for the region.
Use optimal timing for planting and harvesting.
Use optimal tillage (avoid excessive cultivation).
Use appropriate insect, parasite and disease control.
Apply manures and green manures where possible and fertilize effectively
(preferably by injecting the necessary nutrients into the irrigation water).
1. Practise soil conservation
for long-term sustainability.
2. Avoid progressive
salinization by mon-itoring
water-table elevation and
early signs of salt
accumulation, and by
appropriate drainage.
3. Irrigate at high frequency
and in the exact amounts
needed to prevent water
deficits, taking account of
weather conditions and
crop growth stage.
Diunduh dari:
http://www.fao.org/docrep/W3094E/w3094e04.htm….. 15/11/2012
Soil moisture relations at the tree/crop interface in black
locust alleys
Agroforestry Systems. 1994. 25 (2). 135-140
Ssekabembe C. K., Henderlong P. R., Larson M.
.
A study was undertaken at the Ohio State Agricultural Farm to
determine whether the presence of black locust (Robinia
pseudoacacia) hedgerows would increase water shortage on crop
land. Water was applied to bare soil which had carried a pure stand of
maize in the previous growing season, and to previously established
(2 yr old) alley cropping plots, some of which had 100 cm deep
below-ground fibreglass partitions to prevent root competition for
soil moisture in the alleys. Spacing within hedgerows was 1.5 m and
alleys were 5.5 m wide. No crops were grown during the course of the
experiment and all plants (weeds) other than the trees were removed.
In alleys without below-ground partitions, the hedgerows reduced
soil moisture content of the alleys by about 8% at site 1, and 32% at
site 2, 8 days after water application. In the top 45 cm depth of the
soil at site 1, the influence of the hedgerows in the same treatment
was large within 76 cm of the hedgerows but declined farther inside
the alleys.
For the soil at site 2 (where there was more gravel
in the lower soil layers preventing deep growth of
black locust roots), the influence of the hedgerows
was pronounced throughout the alleys but was also
most marked within 76 cm of the hedges.
Pattern of soil moisture depletion in alley cropping under
semiarid conditions in Zambia
Agroforestry Systems. 1994. 26 (2). 89-99
Chirwa P W., Nair P K R., Nkedi Kizza P..
The pattern of soil moisture changes was studied during a cropping
season in an alley cropping experiment with maize (var. MM603) and 2
hedgerow species (Leucaena leucocephala and Flemingia macrophylla)
at the SADC/ICRAF Agroforestry Research Station (Chalimbana
Agricultural Research Station) in a semiarid region near Lusaka, Zambia
(28 deg 29'56"E. and 15 deg 21'32"S.). Factorial combinations of the 2
hedgerow species and 2 fertilizer rates (none, and 150 kg/ha urea applied
as a split dose + a basal dose of diammonium phosphate at 200 kg/ha)
formed the main plots of a split-plot experiment, and the maize rows
formed the subplots. Each treatment unit consisted of 2 double
hedgerows with 6 rows of maize between them and 3 on each side. Maize
was planted at within- and between-row spacings of 25 and 75 cm. Soil
moisture potential was monitored at regular intervals (daily after every
major rainfall event for 4-5 days, and then less frequently) over the maize
growing season (0-90 days after planting) using tensiometers installed at
15, 30 and 45 cm depths in fertilized and unfertilized alleys within the
double hedgerow, and in the first, second and third rows of maize in the
alleys.
Soil moisture moved mostly towards the top horizon during very dry
conditions. Alleys that had received a combination of fertilizer and
hedgerow prunings depleted more moisture than those that had
received only prunings. There were no differences in moisture
utilization patterns between leucaena and flemingia hedgerows.
The hedgerows depleted the same amount of moisture as the maize
plants. However, during dry conditions, there was a higher soil
moisture content under the hedgerows than in maize rows, indicating
that there was no apparent competition for moisture between the
hedgerows and the maize plants.
Root distribution and soil moisture depletion
pattern of a drought-resistant soybean plant
introduction
Agronomy Journal. 1996. 88 (3). 478-485
Hudak C. M., Patterson R. P.
.
The drought-resistant soyabean genotype PI 416937 was compared
with cv. Forrest for soil moisture utilization and root distribution on a
Dothan loamy sand (fine-loamy, siliceous, thermic Plinthic Paleudult)
in Johnston County, North Carolina. Water treatments (well-watered
and water-deficient) were established during R4 to late R6 with rain
exclusion shelters and differential irrigation.
Soil moisture utilization in row and interrow areas was monitored via
neutron attenuation and tensiometry.
A direct measurement of root distribution was obtained on a second
site by a trench profile technique. It is suggested that PI 416937's
advantage may reside in its ability to exploit upper soil horizons
(above 68 cm) with a network of fibrous roots.
Localized measurements of soil moisture tension indicated that PI
416937's rate of soil desiccation was slower than Forrest's, but it
appeared to exploit a larger total soil volume.
Trench profile results indicated that the lateral spread of PI 416937's root
system was greater than that of Forrest. The usefulness of PI 416937 may lie
in the opportunity it provides soyabean breeders for adding diversity to the
root morphology of present soyabean cultivars.
. A weather-soil variable for estimating soil moisture stress
and corn yield probabilities
Agronomy Journal. 1995. 87 (6). 1115-1121
Dale R. F., Daniels J. A.
The SIMBAL (simulation of soil water balance) program was used to
calculate the soil moisture under maize and the ratio of actual to
potential evapotranspiration (ET/PET) for each day of the 1961-92
growing seasons for poorly drained and well-drained soils in
Tippecanoe County, Indiana.
The interaction regression model of maize yield on the soil moisture
stress variable sc (the sum of modelled daily ET/PET ratios) and
technology trend (T = year, average annual yield increase due to
improving agricultural technology) to Tippecanoe County was
associated with 70% of the variance in the 1961-92 average county
maize yields when Sc was a 90-d period (S90) from 39 d before
silking to 50 d after.
With no moisture stress (S90 = 90), the technology trend over the last
32 years was 0.17 t ha-1 year-1 (2.7 bu acre-1 year-1). With 1992
technology, each deficit unit of S90 reduced the yield 0.19 t ha-1 (3.1
bu acre-1). The distributions of S90 and predicted maize yield were
highly negatively skewed. The probability of having an S90 less than
85 (at least some moisture stress), and a county maize yield less than
9.5 t ha-1 (152 bu acre-1) is 69%, but the probability of severe stress
(S90 <75) and maize yield less than 7.5 plus or minus 0.8 t ha-1 (139
plus or minus 13 bu acre-1) is 22%.
For the same weather regime, the probability of moisture
stress and resulting maize yields differs greatly for individual
soils. For a poorly drained soil (Typic Argiaquoll) the
probability of have an S90 less than 85 is 41%, but for a welldrained soil (Typic Argiudoll) the probability is 90%.
. Fine-root dynamics, soil moisture and soil carbon content in
a Eucalyptus globulus plantation under different irrigation and
fertilisation regimes
Forest Ecology and Management. 1995. 74 (1/3). 1-12
Katterer T., Fabiao A., Madeira M Ribeiro C., Steen E.
The minirhizotron technique was used to study the temporal dynamics of
fine roots over a 10 month period in 1991 in a Eucalyptus globulus
plantation (established with 3-yr-old plants in 1986) in central Portugal.
Four treatments were applied: a control without irrigation or fertilizer
application (C), NPK fertilizer twice per year (F), irrigated without fertilizer
(I), and irrigated with fertilizer once each week, with fertilizer in the
irrigation water (IL). In I and IL a drip-tube system was used, and fertilizer
rates were adjusted based on the estimated plant nutrient demand.
Soil moisture content was measured during the same period at 5 cm
depth intervals down to 90 cm depth. Soil carbon content was measured
at planting, 30 months and 54 months after planting.
Fine-root counts peaked in late autumn in all treatments and declined
thereafter until March. Fine-root growth in spring and summer seemed to
be dependent on water supply; i.e. with an ample water supply (within
rows, close to the drip-tubes in I and IL), root counts increased almost
linearly between April and November. In the non-irrigated treatments (C
and F, as well as between rows in I and IL), no marked increase in root
counts occurred until late August, when it increased immediately after a
heavy rain.
Root growth in I was shallowest during spring and summer, while in F it
was shallowest during autumn and winter. In general, treatment means of
root counts were highest in IL, somewhat lower in I, and considerably
lower in C and F.
In addition to irrigation effects, treatment differences in soil water
content were enhanced by differences in soil carbon content, which in
turn could be attributed to root turnover, as reflected by the temporal
dynamics of root counts. The carbon flow from the trees to the soil,
which was probably associated mainly with root death, was highest in IL.
Thus this treatment should have enhanced soil fertility.
. Water absorption by roots
Intimate contact between the surface of root and the soil is essential
for effective water absorption. Root hairs are filamentous outgrowths
of root epidermal cells that greatly increase the surface area of the
root, thus providing greater capacity for absorption of ions and water
from the soil. Water enters the root most readily near the root tip. The
intimate contact between the soil and the root surface is easily
ruptured when the soil is disturbed. It is for this reason that newly
transplanted seedlings and plants need to be protected from water
loss for the first few days after transplantation.
Root hairs intimate contact with soil particles and greatly amplify the surface
area used for water absorption by the plant (source: Taiz L., Zeiger E., 2010)
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. Comparing different methods for estimating the soil moisture
supply capacity of a soil series subjected to different types of
management
Geoderma. 9. 92 (3/4). 185-197
Bouma J., Droogers P
The capacity of soils to supply water to growing plants was expressed in
a static and dynamic manner by calculating 'available water' (AW) and the
'soil moisture supply capacity' (MSC), respectively. Four methods of
increasing complexity were compared for the AW and two for the MSC,
using measured moisture retention and hydraulic conductivity curves
and data derived from class- and continuous pedotransfer functions.
Calculations were made for a Dutch Typic Fluvaquent (genoform). Three
phenoforms, defined by long-duration management, were distinguished:
BIO (biodynamic); CONV (conventional, high-tech) and PERM (permanent
grassland). Values for AW for a given treatment were significantly
different when using measured data or pedotransfer functions. AW can
be used to rank different soils but does not reflect the amount of water
the crop can take up.
Simulation models are needed to estimate MSC which cannot directly be
measured. Simple empirical models, using AW for the root zone in a
'tipping-bucket' approach, did not produce realistic values because
upward flow from the relatively shallow water table could not be
distinguished. The more complex mechanistic WAVE model, also
including hydraulic conductivity data, but still operating under the
implicit and incorrect assumption of soil homogeneity and isotropy,
produced soil water contents that were too high.
Realistic values were only obtained when considering bypass flow,
internal catchment and accessibility of peds for rooting, using a modified
WAVE model incorporating hydraulic data derived from continuous
pedotransfer functions. Values for AW and MSC were significantly
different for the three phenoforms, illustrating the need to distinguish
phenoforms, rather than only genoforms, when reporting basic physical
data for soil series
Development of a rainwater harvesting system for increasing
soil moisture in arid rangelands of Pakistan
Journal of Arid Environments. 1995. 31 (4). 471-481
Suleman S., Wood M K., Shah B H., Murray L.
.
Micro catchments 4-5 m long with 7-15% slopes
increased soil moisture by 59, 63 and 80% at depths
of 0-15, 15-30, and 30-45 cm, resp.
Soil moisture increased in late summer and in late
winter when precipitation is greatest.
Rill erosion increased with microcatchment length
and gradient, with erosion volumes of 14.9-26.3
litres from areas of 120 and 150 m2.
PANEN AIR BHUJAN DAN SIMPAN DALAM
TANAH
To increase RAINWATER capture and
storage in soil:
1. Build up soil organic matter and feed soil
regularly
2. Break up hardpans, open up subsoil
3. Deep dig gardens and incorporate compost
4. Mulch or cover crop over winter
5. Mulch between rows
6. On slopes, plant along contours
7. Practice water harvesting (e.g. by making water
basins around plants, hilling between rows)
8. Apply water below the canopy and close to
plants
9. Apply more water at fewer times rather than a
little water frequently,
10. Allow more drying down as deep rooting crops
grow deeper
11. Plant shelterbelts in windy landscapes .
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WATER BALANCE IN SOIL
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TEKSTUR TANAH & LENGAS
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