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

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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 (ft 3 /s, m 3 /d) by multiplying rate times area –Assumes spatial homogeneity of rate

Infiltration Movement 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: f o = Initial infiltration capacity f p = Infiltration capacity f c = 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.

Heavy Machinery Affects Soil Infiltration Capacity Number of Vehicle Passes Infiltration rate (cm/ 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.

10x

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 (θ V ) –Volume of water per volume of soil –Maximum is porosity Field capacity –θ V soil moisture after free drainage –What soil can hold against gravity Wilting point –θ V at which plants cant obtain soil water –Not zero θ V, but zero AVAILABLE

Available Water Capacity

For unsaturated soil ESD = θ v 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?

θ v = V w / V s Calculating volumetric soil moisture volume water/volume soil (1 g water = 1 cm 3 ) 1.Sample a known volume 2.weigh-dry-weigh Cylinder Volume= 20cm 3 Wet weight = 30g Dry weight = 25g Θ v = (30-25) / 20cm 3 = 0.25g/cm 3

Equivalent Surface Depth of Soil Moisture (ESD) for unsaturated conditions ESD= Volumetric soil moisture * depth of soil θ= 0.25g/cm 3 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 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 = Cylinder Volume = 20cm 3 Wet weight = 30g Dry weight = 25g 25g 20cm 3 = 1.25 g/cm 3 Soil Volume (cm 3 )

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

Soil textureTotal porosityDrained porosity Bulk Density g/cm 3 Sand35-50%~35%1.5 Silts &Clay40-60%15-25%1.0 Organic>60%variable0.1

Wet BMP

Skidding Cycles

Less infiltration More runoff More erosion Less tree growth Compacted Soils: Less Storage

Next Time… Mid Term Exam

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