Lecture 7: Soils and Infiltration

1 Lecture 7: Soils and InfiltrationKey is a soil? are soil types classified? is Infiltration ? quantities control Infiltration rate? is soil is soil is capillarity?rain drop infiltrating into soilSoil type and thickness affect runoff and streamflowSoilis an unconsolidated aggregate of mineral and rock fragments ranging in size from clay, silt, sand, to gravel. Some Soils (near surface) also contain organic matter. Soil type (or classification) is determined by fragment (clast) size and distribution of clast SIZESSoil classification can get more descriptive, but for hydrology purposes we use clast Soil Texture Classificationrelative sizeFigure 1. Location of Whatcom County In Survey ofWhatcom County Area, WashingtonBy Alan Goldin, , Soil Conservation Service Washington State Department of Natural Resources United States Department of Agriculture, Soil Conservation ServiceDigital soil data (for ArcGIS) is available from the following data Coverage (USDA)Infiltrationis the movement of water INTO the soil surfaceinfiltrationpercolationPercolatio nis the movement of water WITHIN the soil matrix.

2 PercolationPercolation rate controls the Infiltration rate infiltrationponding and runoffWhen the rainfall rate exceeds the Infiltration rate, surface pondingand runoffoccursOpen the link below and read about infiltrationwet siltmoist sanddry sandwater is flowing in fastnothing happeningwater is coming outwater flowing in slowlynothing came out nothing came outnothing came outsoil storagesurface runoffgroundwater rechargeInfiltration(and runoff) is controlled by soil type, thickness, original water content, and precipitation characteristicsinfiltrationrunoffMoist Soils produce more runoff than dry soilssandy Soils allow water to infiltratesilt and clay Soils have slow Infiltration rates, hence more surface runoffUSDA Soil Texture Classificationclays have extremely low Infiltration rates (high runoff)siltshave low Infiltration ratessandshave high Infiltration ratesThe degree of Infiltration and runoff depends on the soil typeRainfall characteristics determine the amount of Infiltration versus runofflight rain more infiltrationheavy rain more runoffinfiltrationrunoffSoil storage is the amount of water the soil can hold.

3 It is controlled by the soil type, thickness, and a measure of void space in a soiltotal volume of dry soilwater volumesaturate soilvolume of voidssaturated soilwater lossvolume of voidssaturated soiltotal volumeporosity =total volumevolume of voidsporosity 40%porosity 25%uniform grain sizesmixture of grain sizesGrain-size distribution controls the magnitude of porosity nothing came outsoil water storagesurface runoffgroundwater rechargeAfter water has drained by gravity, some water is held between the pore spaces by tension (capillary forces)water is polar__++The polar nature of water causes it to adhereto solid surfaces and bond to other water molecules (cohesion)read about adhesion, cohesion, and surface tensionThe tension or capillary forces are created by molecular forces of attraction between the sand water air and are the same forces that bind sand together in the sculpture below Open the link below and read about water properties and capillaritySoils with smaller clasts have greater water holding capacity (stronger capillary forces) sandclaysiltwatershedQ= stream dischargeQTimeHydrographQsandy soilssilt and clay soilsQTimeHydrographQTimeHydrographatten uated responserapid high peak responseImpervious surfaces in urban settings don t allow any Infiltration , hence very rapid surface runoffQTimeHydrographvery rapid high peak responseWater flows to both sides and across Squalicum Parkway near the intersection with Roeder Avenue on Wednesday, January 7, 2009 Frozen Soils don t allow any Infiltration , hence very rapid surface runoffQTimeHydrographvery rapid high peak responseThe frost line is 4 to 6 feet in North Dakota.

4 March 13, 2010 thick, dry Soils can store a lot of rain, hence produce less runoff (if they are dry to begin with)QTimeHydrographthin Soils saturate quicker, hence produce more runoffQTimeHydrographsoils near saturation produce more runoffQTimeHydrographwater can flow through the Soils toward a streamQTimeHydrograph