Water Budget IV: Soil Water Processes

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Water Budget IV: Soil Water Processes. P = Q + ET + G + Δ S. Infiltration. Infiltration capacity: The maximum rate at which water can enter soil. - PowerPoint PPT Presentation

Transcript of Water Budget IV: Soil Water Processes

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Water Budget IV: Soil Water ProcessesP = Q + ET + G + S1InfiltrationInfiltration 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 rateThe rate at which infiltration takes place expressed in depth per unit time.Converted to volume (ft3/s, m3/d) by multiplying rate times areaAssumes spatial homogeneity of rate


InfiltrationMovement of water into the soilWater 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 3Wetting Profiles

4Matrix PotentialCapillary forcesWater has high surface tensionLeads to zone above the water table that where pores are saturatedCapillary RiseVaries from a few cm to m (!)Texture dependentAlso accelerates infiltration into unsaturated soils


Matrix + GravityWhen soil is saturated matrix force = 0 HORTON EQUATION:fo = Initial infiltration capacityfp = Infiltration capacity fc = Equilibrium infiltration capacity

If precipitation rate (L/T) < fc (L/T), then all rain infiltrates 6Generation of Overland Flow

What is contour tillage? What does it do?7


Soil Texture


What is the implicit assumption here? How might a shallow water table violate this assumption?10


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


13The 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 CapacityNumber of Vehicle PassesInfiltration rate (cm/ hour)

17 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.



20Calculating S from soil moisture dataS = storage end storage beginIn this example the watershed soil is 1 meter deep and is unsaturated at end and saturated at beginning. How do we determine S as Equivalent Surface Depth (ESD) ? P=Q+ET+G+ S21Soil Moisture TermsPorosityTotal volume of pores per volume soilSoil is saturated when pores are filledVolumetric soil moisture (V)Volume of water per volume of soilMaximum is porosityField capacityV soil moisture after free drainage What soil can hold against gravityWilting pointV at which plants cant obtain soil waterNot zero V , but zero AVAILABLE

22Available Water Capacity

23For unsaturated soil ESD = v x soil depthFor saturated soil ESD = Porosity x soil depthS= ESD end ESD beginIf soil saturated at beginning and unsaturated at end, what will be the sign of S?24v= Vw / VsCalculating volumetric soil moisture volume water/volume soil (1 g water = 1 cm3)Sample a known volumeweigh-dry-weighCylinder Volume= 20cm3Wet weight = 30g Dry weight = 25gv= (30-25) / 20cm3= 0.25g/cm325Equivalent Surface Depth of Soil Moisture (ESD) for unsaturated conditions ESD= Volumetric soil moisture * depth of soil = 0.25g/cm3 or just 0.25Soil depth = 1.00mESD= 0.25mThis concept (yield of water per unit area) is also called the specific yield 26Calculating ESD of saturated soilPorosity= volume of voids / total volumeMethod ASaturate known soil volume, weigh, dry, weigh.Method BDetermine Bulk density and use: Porosity = 1-Bulk Density2.6527

Dry Soil (g) Bulk Density =Cylinder Volume = 20cm3Wet weight = 30g Dry weight = 25g25g20cm3= 1.25 g/cm3Soil Volume (cm3)

28Porosity= 1-(1.25 / 2.65)= 0.530.53 * 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. 53mS= 0.25m 0.53m = -0.28m29Soil textureTotal porosityDrained porosityBulk Density g/cm3Sand35-50%~35%1.5Silts &Clay40-60%15-25%1.0Organic>60%variable0.1


Wet BMP31Skidding Cycles

32Less infiltrationMore runoffMore erosionLess tree growthCompacted Soils:Less Storage33

34Next TimeMid Term Exam