Water Budget IV: Soil Water Processes P = Q + ET + G + ΔS.

Water Budget IV: Soil Water Processes P = Q + ET + G + ΔS

Transcript of Water Budget IV: Soil Water Processes P = Q + ET + G + ΔS.

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

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

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

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

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.






0 2 3 4 10 20

Heavy Machinery Affects Soil Infiltration Capacity

Number of Vehicle Passes



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• 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.



• 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) ?


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?

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

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


Dry Soil (g) Bulk Density


Cylinder Volume = 20cm3

Wet weight = 30g

Dry weight = 25g


= 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

Skidding Cycles

Less infiltration

More runoff

More erosion

Less tree growth

Compacted Soils:

Less Storage

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• Mid Term Exam