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

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Transcript of 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 + S Slide 2 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 Slide 3 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 Slide 4 Wetting Profiles Slide 5 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 Slide 6 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 Slide 7 Generation of Overland Flow What is contour tillage? What does it do? Slide 8 Slide 9 Soil Texture Slide 10 What is the implicit assumption here? How might a shallow water table violate this assumption? Slide 11 Slide 12 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 Slide 13 Slide 14 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. Slide 15 Slide 16 Slide 17 Heavy Machinery Affects Soil Infiltration Capacity Number of Vehicle Passes Infiltration rate (cm/ hour) Slide 18 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. Slide 19 10x Slide 20 Slide 21 Lysimeters Measure flows below the surface Useful for quantity AND quality Works variably in very sandy soils Slide 22 Calculating S from soil moisture data S = 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 S as Equivalent Surface Depth (ESD) ? P=Q+ET+G+ S Slide 23 Soil Moisture Terms Porosity Total volume of pores per volume soil Soil is saturated when pores are filled Volumetric soil moisture ( V ) Volume of water per volume of soil Maximum is porosity Field capacity V soil moisture after free drainage What soil can hold against gravity Wilting point V at which plants cant obtain soil water Not zero V, but zero AVAILABLE Slide 24 Available Water Capacity Slide 25 For unsaturated soil ESD = v x soil depth For saturated soil ESD = Porosity x soil depth S= ESD end ESD begin If soil saturated at beginning and unsaturated at end, what will be the sign of S? Slide 26 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 v = (30-25) / 20cm 3 = 0.25g/cm 3 Slide 27 Equivalent Surface Depth of Soil Moisture (ESD) for unsaturated conditions ESD= Volumetric soil moisture * depth of soil = 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 Slide 28 Specific Yield Slide 29 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 Slide 30 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 ) Slide 31 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 S= 0.25m 0.53m = -0.28m Slide 32 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 Slide 33 Wet BMP Slide 34 Skidding Cycles Slide 35 Less infiltration More runoff More erosion Less tree growth Compacted Soils: Less Storage Slide 36 Slide 37 Next Time Mid Term Exam