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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
InfiltrationMovement of water into the soil
Water moves through spaces between soil particles (SLOW)
Water moves through old root channels, animal burrows, and between soil blocks (FAST)
Percolation is the movement of water through soil
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
When soil is saturated matrix force = 0
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
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.
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Heavy Machinery Affects Soil Infiltration Capacity
Number of Vehicle Passes
Infil
trati
on ra
te (c
m/
hour
)
• 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.).
• Compaction reduces infiltration and increases runoff.
• Soils may recover in 3-10 years if undisturbed.
Lysimeters
• Measure flows below the surface– Useful for quantity AND
quality– Works variably in very
sandy soils
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) ?
P=Q+ET+G+ ΔS
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
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?
θv= Vw / Vs
Calculating volumetric soil moisture 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 volumeMethod A
Saturate known soil volume, weigh, dry, weigh.
Method BDetermine Bulk density and use:
Porosity = 1-Bulk Density
2.65
Dry Soil (g) Bulk Density
=
Cylinder Volume = 20cm3
Wet weight = 30g
Dry weight = 25g
25g20cm3
= 1.25 g/cm3
Soil Volume (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) = 0.25mBegin S (saturated ) = 0. 53m
ΔS= 0.25m – 0.53m = -0.28m
Soil texture Total porosity Drained porosity
Bulk Density g/cm3
Sand 35-50% ~35% 1.5
Silts &Clay 40-60% 15-25% 1.0
Organic >60% variable 0.1
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