SOIL – PLANT – WATER
RELATIONSHIPS,
Eto &
Crop water requirements
M. Sachin Dutt
Asst. Director of Agriculture
Faculty, WALAMTARI
WHY Soil – Plant – Water Relations ?
For designing, operating, managing efficiently any
irrigation system, so as to suit different crops
SOIL:
Soil is a natural body in which plants grow.
Soil is a three phase system. The interfaces between phases
exhibit phenomena such as adsorption, surface tension,
adhesion, swelling & shrinking, capillarity etc.
Soil is a storehouse for
plant nutrients, an
anchorage for plants
and a reservoir that
holds water needed by
plants.
HOW SOIL HOLDS WATER ?
Soil holds water in two ways, as
a thin film on individual soil
particles, and as water stored in
the pores of the soil.
Water stored as a thin film on
individual soil particles is said to
be in adsorption. Adsorption in
simple terms is a thin film of
water adhering to the outside
layers of soil particles.
Water stored in the pores of the
soil is said to be in capillary
storage.
SOIL MOISTURE TENSION (SMT):
•The force or tenacity with which water is
retained or held in the soil is SMT / soil
moisture potential / soil moisture suction. It
indicates the force per unit area that must be
exerted to remove water from the soil.
•SMT is a result of surface tension
(capillarity), adhesion & cohesion.
SOIL MOISTURE CONSTANTS
FIELD CAPACITY (FC):
FC is upper limit of available moisture range in soil to plants.
The Soil moisture tension at FC varies from -0.1 to -0.3
atmospheres (atms).
PERMANENT WILTING POINT (PWP):
It is the soil moisture content at which plants can no longer obtain
enough moisture to meet the transpiration requirements and
remain wilted under water is supplied to the soil. At PWP the
plant growth stops. PWP is lower limit of available moisture
range in soil to plants. The moisture tension at PWP is -15 atms.
HYGROSCOPIC COEFFICIENT (HC):
It is the amount of water held very tightly around soil particles.
Much of the water is in non-liquid phase and can move only in
vapour form. Water potential at HC is about -28 atms.
60
Gravitational
water
40
30
Available
water
10
20
Permanent Wilting Point
Unavailable
Water
0
Soil Moisture (%)
50
Field Capacity
-0.01
-0.1
-1
-15
-100
Soil Moisture Potential (Atms)
-1000
SOIL MOISTURE CURVE OF A LOAMY SOIL
30
20
5
10
Permanent
Wilting Point
0
Moisture %
25
Field
Capacity
Sand
Sandy Loam
Silt
Clay Loam Loam
Increase in fineness of Soil Texture
RELATION OF SOIL TEXTURE AND MOISTURE CONSTANTS
PLANT WATER RELATIONS:
1. Water Constitutes 80-90% of plant cells and tissues where
there is active metabolism.
2. It is in the form of a continuous liquid phase from root hairs
to leaf epidermis (Outer layer).
3. Plant roots take up water and transfer through conducting
vessels (Xylem) and then to mesophyll cells of leaves and
reach Stomata (Transpiration site) in the liquid state and
finally to the atmosphere through Stomata as vapour.
4. Water acts as a solvent for carrying minerals, gases etc in
the plant body and hence essential for overall growth of
plants.
5. Plant Roots develop when there is enough moisture in the
soil, Mineral availability, Soil air, presence of impervious soil
layer etc.
LEAF STRUCTURE & STOMA
EFECTIVE ROOT ZONE DEPTH:
When soil and other growing conditions are favorable, crops develop well
branched root system that penetrates the soil to the depth unique to that
crop. This depth is referred to as Effective Root Zone Depth. Crops absorb
Moisture and Nutrients from this depth.
Depending on the Root Zone , Crops are classified as follows:
1. Shallow Rooted (60 cm Depth), Eg: Rice, Onion, Cabbage,
Cauliflower, Potato etc.
2. Moderately deep rooted(90 cm), Eg: Groundnut, Castor,
Tobacco, Wheat, Chillies etc.
3. Deep rooted (120 cm), Eg: Cotton, Maize, Jowar, Bajra.
4. Very Deep rooted (180 cm), Eg: Sugarcane, Citrus, Safflower,
Coffee, Grapevine etc.
The Quantity of water available in the root zone determines the irrigation
interval and the quantity of water to be given at each irrigation.
MOISTURE EXTRACTION PATTERN:
Most of the feeder roots of the crops are in the upper part of the
root zone (top 45 cm) and near the base of plant. Plants do not
draw moisture equally from the entire root zone.
Most active absorption of water occurs in plants through root
hairs and zones of root elongation behind growing root tips.
MOISTURE SENSITIVE PERIODS OF CROP GROWTH:
The Visible stages of crop growth are:
1. Germination & Emergence
2. Vegetative growth
3. Flowering
4. Grain formation
During certain stages plants are most sensitive or critical to
shortage of water which are known as Moisture Sensitive
periods. Moisture stress during critical stages will reduce yield
and adequate provision of water and fertilizer at other stages
will not help in recovering yield lost.
• Water shortage during Germination and early seedling stage
has harmful effects because of the tenderness of leaves and
roots.
• In the Vegetative stage, Water shortage reduces leaf area and
hence subsequent growth.
• Moisture stress during Flowering reduces grain formation and
results in shriveled grain.
MOISTURE SENSITIVE STAGES OF CERTAIN CROPS (CRITICAL PERIODS)
Rice
: Primordial development, heading and flowering
Sorghum : Booting, earing, milking
Maize
: Knee height, Tasseling-silking, Early grain formation
Wheat
: Crown root initiation, shooting, earing
Groundnut : Flowering, peg penetration, seed development
Sunflower : Budding, Head formation, Flowering, seed filling
Safflower : Rosette, flowering
Cotton
: Flowering to boll formation
CROP WATER REQUIREMENT:
Estimation of Crop water requirement is one of the basic needs
for crop planning and for planning any irrigation project.
The Quantity of water required by a crop in a given period of
time for its normal growth under field conditions.
CWR = E.T/C.U + Application losses + special needs
(land preparation, transplanting, leaching etc)
IR = CWR – (ER + S)
Irrigation requirement = Water requirement – (Effective rainfall
+ Groundwater contribution)
EVAPOTRANSPIRATION (E.T.):
Transformation of water from Liquid to Vapour phase from Soil
surface, Water surface, Plant surface.
Evaporation from plant surface is referred as Transpiration.
Consumptive Use (CU): The quantity of water used by plants for
Evapotranspiration and for its metabolic activities. Since the water
required for metabolic activities is negligible, the term CU is taken
equivalent to E.T.
Potential Evapotranspiration (P.E.T) or Reference E.T (ET0):
The Concept of ET0 is to characterize the microclimate of the field in
terms of the Evaporative Demand i.e., the maximum evaporation
rate which the atmosphere is capable of extracting from a field.
CROP EVAPOTRANSPIRATION (ETC):
The amount of Water required for the crop for the purpose of
meeting Evaporation & Transpiration demands. It is related to ETO as
follows:
ETC = ETO X KC
Wherein Kc is referred as Crop-Coefficient. It is dependent on the
stage of growth of crop.
The growth period of any crop is divided into 4 stages:
1. Initial stage: Germination period and early growth period of the
crop (<10% crop coverage)
2. Crop development stage: Soil cover by the crop is about 70-80%.
3. Midseason stage: Flowering, Start of maturity indicated by
discoloring of leaves.
4. Late season stage: Full maturity or harvesting.
Kc Values of some important crops:
Crop development stages
Crop
Cotton
Groundnut
Maize
Beans
Cabbage
Potato
Rice
Safflower
Sorghum
Soybean
Sugarcane
Sunflower
Tobacco
Tomato
Initial
Crop
development
0.4-0.5
0.4-0.5
0.3-0.5
0.3-0.4
0.4-0.5
0.4-0.5
1.1-1.15
0.3-0.4
0.3-0.4
0.3-0.4
0.4-0.5
0.3-0.4
0.3-0.4
0.4-0.5
0.7-0.8
0.7-0.8
0.7-0.85
0.65-0.75
0.7-0.8
0.7-0.8
1.1-1.5
0.7-0.8
0.7-0.75
0.7-0.8
0.7-1
0.7-0.8
0.7-0.8
0.7-0.8
Midseason Late season At harvest
1.05-1.25
0.95-1
1.05-1.2
0.95-1.05
0.95-1.1
1.05-1.2
1.1-1.3
1.05-1.2
1.0-1.15
1.0-1.15
1.0-1.3
1.05-1.2
1.0-1.2
1.05-1.25
0.8-0.9
0.75-0.85
0.8-0.95
0.9-0.95
0.9-1
0.85-0.95
0.95-1.05
0.65-0.7
0.75-0.8
0.7-0.8
0.75-0.8
0.7-0.8
0.9-1.0
0.8-0.95
0.65-0.7
0.55-0.6
0.55-0.6
0.85-0.95
0.8-0.95
0.7-0.75
0.95-1.05
0.2-0.25
0.5-0.55
0.4-0.5
0.5-0.6
0.35-0.45
0.75-0.85
0.6-0.65
Total
growing
period
0.8-0.9
0.75-0.8
0.75-0.9
0.85-0.95
0.7-0.8
0.75-0.9
1.05-1.2
0.65-0.7
0.75-0.85
0.75-0.9
0.85-1.05
0.75-0.85
0.85-0.95
0.75-0.9
ESTIMATION OF POTENTIAL EVAPOTRANSPIRATION OR ETO
Different methodologies have been developed to predict the
amount of water for optimal crop production based on
Climatological data. The FAO scientists have recommended the
following 4 empirical formulae for estimation of ETo.
1.
2.
3.
4.
Modified Penman method.
Radiation method.
Blaney – Cridle method.
Pan Evaporation method.
The modified Penman method is probably the most accurate
and it requires data on Temperature, Humidity, Wind and
Solar Radiation. Other components are derived from these
parameters.
The Equation is as follows:
ETO = C [ W . Rn + (1-W) . f(U) . (ea – ed)]
Radiation term
Aerodynamic term
Wherein
C = Adjustment factor to compensate the effect of day
and night weather condition (Wind speed in day and
night, Relative Humidity (Max), Incoming shortwave
radiation (Rs).
W = Temperature and altitude related weightage factor
for effect of radiation.
Rn = Net Radiation in equivalent evaporation (mm/day)
Rn = 0.75 Rs – Rnl
Rs = Incoming Short wave radiation
Rs = (0.25 + 0.50 n/N) Ra
Ra = Extra terrestrial radiation in equivalent evaporation (mm/day)
n = Actual measured bright Sunshine hours.
N = Maximum possible Sunshine hours.
Rnl = Net Long wave radiation (mm/day) which is a function of
Temperature, Vapour pressure, Sunshine duration.
Rnl = f(T) . f(ed) . f(n/N)
f(U) = Wind related function (Wind velocity in Km/day at 2 mt
height).
f(U) = 0.27 (1+U/100), U = Mean Wind velocity in Km/day
ea = Saturated vapour pressure at mean temperature (mbars)
ed = Mean actual Vapour pressure of air (mbars)
ed = ea x RH min/100
Calculate the ETO based on the following data:
Latitude = 15 degree North in June.
Altitude = 200 m
Temp mean = 30 degree C
Day Wind velocity = 15 km/hr
Night Wind velocity = 12 km/hr
Mean Sunshine hours (n) = 8 hr/day
RH max = 60%
RH min = 40%
Item from
S. No
ETo
formula
1
2
3
4
W
ea
ed
Rn
Calculation
ed = ea x RH min/100
ed = 42.4 x 40/100 = 16.96 mb
Rn = 0.75 Rs - Rnl
Rs = [0.25+0.5(n/N)] Ra
Rnl = f(T) . f(ed) . f(n/N)
Rn = 0.75x8.81 - 1.76 = 4.85 mm/day
5
f(U)
6
7
ea-ed
C
8
ETo
Value from
Table
0.78
42.4 mb
n = 8 (given), N =
13, Ra = 15.8
Final Value
W = 0.78
ea = 42.4 mb
ed = 16.96 mb
Rs = 8.81 mm/day
f(T) = 16.7, f(ed) = Rnl = 16.7x0.16x0.66 =
0.16, f(n/N) =
1.76 mm/day
0.66
Rn = 4.85 mm/day
f(U) = 0.27(1+U/100)
U mean = (15+12/2)x24 = 324 km/day
-
-
f(U) = 0.27(1+324/100) = 1.15
42.4-16.96 = 25.44
for RH max 60%, Uday/Unight = 15/12=1, Rs
= 8.81, U day in m/s = 4.16 m/s (15 km/hr)
-
f(U) = 1.15
ea-ed = 25.44 mb
C = 0.94
ETo = 0.94[0.78x4.85 + (1-0.78) x 1.15 x
25.44] = 0.94 [3.783 + 6.436] = 0.94 x 10.22
= 9.606
-
ETo = 9.6 mm/ day or
9.6 x 30 = 288
mm/month of June
Calculate the ETO based on the following data:
Latitude = 13 degree North in July.
Altitude = 410 m
Temp mean = 29 degree C
Day Wind velocity = 20 km/hr
Night Wind velocity = 10 km/hr
Mean Sunshine hours (n) = 9 hr/day
RH max = 80%
RH min = 55%
EFFECTIVE RAINFALL (ER):
It is the utilizable rainfall. It is that part of the total amount of
rainfall which is directly or indirectly useful for crop production.
Ineffective rainfall is that part of the rainfall which is lost by
surface runoff, deep percolation losses etc.
EMPERICAL ESTIMATION OF EFFECTIVE RAINFALL (Pe):
Pe = 0.8P – 25 (When P i.e.,Rainfall is more than 75mm/month)
Pe = 0.6P – 10 (when P is less than 75mm/month)
In general 40-50% rainfall is considered Effective.
IRRIGATION REQUIREMENT:
When to Irrigate?
Only after knowing the quantity of water available for the
plants in the root zone and the Evapotranspiration demand,
one can judge the time to irrigate.
Water availability can be quantified by many methods viz.,
Gravimetric method, Tensiometers, Electrical resistance, Pan
method etc.
For the purpose of understanding and demonstration the use
of Tensiometers is dealt hereunder.
TENSIOMETERS:
Tensiometers are simple, reliable instruments which provide a
measure of the moisture status of the soil. It reads from 0 to -85
centibars (cb).
Tensiometers help answer the irrigators' questions of when and
how much irrigation water to apply. Maintaining proper
moisture conditions is necessary for achieving optimal plant
growth and quality. Monitoring the moisture status of the soil
allows for timely and efficient irrigations, and for avoiding
unnecessary irrigations.
•At a typical tensiometer station, two tensiometers are installed. One, located
in the upper root zone, monitors the active root area and is used to determine
when an irrigation is needed.
•A second tensiometer, installed near the bottom of the root zone, is used to
adjust the irrigation amount or system run-time in order to ensure that
sufficient water is being applied, and to avoid over-irrigation and loss of water
and chemical amendments due to drainage beyond the root zone.
•The moisture status of the soil is monitored by reading the tensiometers
periodically.
•When the tension readings in the upper tensiometer reach a certain level, an
irrigation is needed. This tension level is determined by the irrigator, and
depends upon such factors as crop type, soil condition, and root depth.
•The lower tensiometer is monitored to ensure that enough water has been
applied to refill the root zone.
•When the upper unit indicates high soil suction values, irrigation is started.
•Irrigation is continued until the reading on the lower unit drops, indicating
that the irrigation water has penetrated to that depth and the whole active
root zone has been re-wetted.
CALCULATE THE CROP WATER REQUIREMENT OF GROUNDNUT
CROP WITH THE FOLLOWING DATA:
Area = 2000 Ha
Duration = 105 days
Date of sowing = 15th October
ETO values for October 5.23 mm/day with RF of 69mm
ETO values for November 4.7 mm/day with RF of 24mm
ETO values for December 4.05 mm/day with RF of 2mm
ETO values for January 4.58 mm/day with RF of 0mm
Kc values for Groundnut crop:
1. Establishment stage – 10 days – Kc = 0.4
2. Vegetative phase – 25 days – Kc = 0.7
3. Flowering phase – 30 days – Kc = 1.0
4. Yield formation – 30 days – Kc = 0.75
5. Ripening – 10 days – Kc = 0.6
S. No
Item
October
I
November
II
(1day)
I
(16 days)
December
II
(15 days)
I
(15 days)
January
II
(15 days)
I
(16 days)
TOTAL
II
(15 days)
(12 days)
105 days
1
ETo
5.23
5.23
4.7
4.7
4.05
4.05
4.58
4.58
2
Kc
0.4
0.4
0.7
0.7
1
1
0.75
0.75
(for 1 day) (for 9 days)
(for 15 days) (for 3 days)
0.7
1
(for 7 days)
ETc
3 (mm/day)
2.09
2.09
3.29
2.09
18.83
6
ETc Total
Special
needs
2.09
49.35
9.87
66.27
4.05
3.44
3.04
60.75
56.40
49.35
(for 10 days) 42 days
12.15
51.53
39.49
60.75
51.64
102.09
3.44
2.75
6.87
27.48
51.53
100
7 Effective RF
NIR
8 (mm/fortnt)
44.46
4.05
63 days
0.6
(for 13 days)
4.70
25.63
5
3.29
(for 15 days) (for 2 days)
0.75
(for 12 days)
3.66
ETc
4 (mm/fortnt)
(for 15 days) (for 3 days)
34.35
50 510.43
31.4
4.4
13.06
44.95
66.27
60.75
51.64
51.53
84.35
474.6
CALCULATE THE CROP WATER REQUIREMENT OF
TOMATO CROP WITH THE FOLLOWING DATA:
Month
Jan
Feb
Mar
Apr
May
June
July
ETo
(mm/day)
4
5.0
5.8
6.3
6.8
7.1
6.5
Humidity
medium
(60%)
Wind
speed
medium
3
m/sec
Duration of growing period (from sowing): 150 days
Planting date: 1 February (direct sowing)
Initial stage
Dev. Stage
Mid-season stage
Late season stage
Harvest
35 days (Kc = 0.45)
40 days (Kc = 0.70)
50 days (Kc = 0.95)
25 days (Kc = 1.15)
0 days (Kc = 0.85)
WATER REQUIREMENT OF VARIOUS CROPS:
CROP
Rice:
Groundnut:
Sugarcane:
Ragi:
Bajra:
Maize:
Tobacco:
Sesame:
Cotton:
Castor:
Sunflower:
Soybean:
Chillies:
Banana:
Water Required
1240 mm
550 mm
2800 mm
400-450 mm
400-500 mm
400-550 mm
300-400 mm
350-400 mm
650-850 mm
500 mm
350-500 mm
450 mm
500 mm
1600-2250 mm