Water Budget IV:
Soil Water Processes
P = Q + ET + G + ΔS
Infiltration
• Infiltration capacity: The maximum
rate at which water can enter soil.
• Infiltration capacity curve: A graph
showing the time-variation of
infiltration capacity if the supply were
continually in excess of infiltration
capacity.
• Infiltration rate
– The rate at which infiltration takes place
expressed in depth per unit time.
– Converted to volume (ft3/s, m3/d) by
multiplying rate times area
– Assumes spatial homogeneity of rate
Infiltration
Movement of water into the soil
Water moves through old root
Water moves through spaces
channels, animal burrows, and
between soil particles (SLOW)
between soil blocks (FAST)
Percolation is the movement of water through soil
Wetting Profiles
Matrix Potential
• Capillary forces
– Water has high surface tension
• Leads to zone above the “water
table” that where pores are
saturated
– Capillary Rise
– Varies from a few cm to m (!)
– Texture dependent
• Also accelerates infiltration into
unsaturated soils
Matrix + Gravity
HORTON EQUATION:
fo = Initial infiltration capacity
fp = Infiltration capacity
fc = Equilibrium infiltration
capacity
If precipitation rate (L/T) < fc
(L/T), then all rain infiltrates
When soil is saturated
matrix force = 0
Generation of
Overland Flow
What is contour
tillage?
What does it do?
Soil Texture
What is the implicit assumption here? How might a
shallow water table violate this assumption?
During a rainfall, millions
of drops fall at velocities
reaching 30 feet per
second. They explode
against the ground,
splashing exposed soil as
high as 3 feet in the air
and as far as 5 feet from
where they hit.
Impact energy breaks up soil particles
into smaller units that can clog soil
pores
The forest floor plays a key role in the infiltration process
by adsorbing the energy of the rainfall (throughfall)
preventing dispersed colloidal material from clogging soil
pores and detaining water to give it time to infiltrate.
Infiltration rate (cm/ hour)
Heavy Machinery Affects Soil Infiltration Capacity
80
60
40
20
0
0
2
3
4
10
Number of Vehicle Passes
20
• Compaction reduces infiltration and increases runoff.
• Wet & fine textured soils compact the most.
• Most of the compaction occurs in the first 3 trips.
• Compaction reduces root growth, nutrient and gas exchange, and
site productivity (46% less volume for loblolly in N.C.).
• Soils may recover in 3-10 years if undisturbed.
10x
P=Q+ET+G+ ΔS
Calculating ΔS from soil moisture data
ΔS = storage end – storage begin
In this example the watershed soil is 1
meter deep and is unsaturated at end and
saturated at beginning.
How do we determine ΔS as Equivalent
Surface Depth (ESD) ?
Soil Moisture Terms
• Porosity
– Total volume of pores per volume
soil
– Soil is saturated when pores are
filled
• Volumetric soil moisture (θV)
– Volume of water per volume of soil
– Maximum is porosity
• Field capacity
– θV soil moisture after free drainage
– What soil can hold against gravity
• Wilting point
– θV at which plants can’t obtain soil
water
– Not zero θV , but zero AVAILABLE
Available Water Capacity
For unsaturated soil ESD = θv x soil depth
For saturated soil ESD = Porosity x soil depth
ΔS= ESD end – ESD begin
If soil saturated at beginning and unsaturated at end,
what will be the sign of ΔS?
Calculating volumetric soil moisture
θv= Vw / Vs
volume water/volume soil (1 g water = 1 cm3)
1. Sample a known volume
2. weigh-dry-weigh
Cylinder Volume= 20cm3
Wet weight = 30g
Dry weight = 25g
Θv= (30-25) / 20cm3= 0.25g/cm3
Equivalent Surface Depth of Soil Moisture
(ESD) for unsaturated conditions
ESD= Volumetric soil moisture * depth of soil
θ= 0.25g/cm3 or just 0.25
Soil depth = 1.00m
ESD= 0.25m
This concept (yield of water per unit area) is
also called the specific yield
Calculating ESD of saturated soil
Porosity= volume of voids / total volume
Method A
Saturate known soil volume, weigh, dry, weigh.
Method B
Determine Bulk density and use:
Porosity = 1-
Bulk Density
2.65
Dry Soil (g)
Bulk
=
Density
Soil Volume (cm3)
Cylinder Volume = 20cm3
Wet weight = 30g
Dry weight = 25g
25g
20cm3
= 1.25 g/cm3
Porosity= 1-(1.25 / 2.65)= 0.53
0.53 * 1m soil = 0.53m ESD for
saturated conditions.
For unsaturated conditions the ESD
was 0.25 m.
End S (unsaturated) = 0.25m
Begin S (saturated ) = 0. 53m
ΔS= 0.25m – 0.53m = -0.28m
Bulk Density g/cm3
Sand
Total porosity Drained
porosity
35-50%
~35%
Silts &Clay
40-60%
15-25%
1.0
Organic
>60%
variable
0.1
Soil texture
1.5
Wet
BMP
Skidding Cycles
Compacted Soils:
Less infiltration
More runoff
Less Storage
More erosion
Less tree growth
Next Time…
• Mid Term Exam