Lecture 7: Soils and Infiltration Key Questions 1. What is a soil? 2. How are soil types classified? 3. What is infiltration? 4. What quantities control infiltration rate? 5. What is soil storage 6. What is soil porosity 7. What is capillarity? rain drop infiltrating into soil
Soil type and thickness affect runoff and streamflow
Soil is 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 sizes. RELATIVE SIZES
Soil classification can get more descriptive, but for hydrology purposes we use clast size.
USDA Soil Texture Classification relative size
Soil Survey of Whatcom County Area, Washington By Alan Goldin, Ph.D., Soil Conservation Service Washington State Department of Natural Resources United States Department of Agriculture, Soil Conservation Service Figure 1. Location of Whatcom County In Washington.
Digital soil data (for ArcGIS) is available from the following data base.
Soil Coverage (USDA)
Infiltration is the movement of water INTO the soil surface infiltration
Percolation is the movement of water WITHIN the soil matrix. percolation
Percolation rate controls the infiltration rate infiltration percolation
When the rainfall rate exceeds the infiltration rate, surface ponding and runoff occurs ponding and runoff
Open the link below and read about infiltration
wet silt dry sand moist sand
nothing happening water is flowing in fast
water is coming out water flowing in slowly
nothing came out nothing came out
surface runoff soil storage nothing came out groundwater recharge
Infiltration (and runoff) is controlled by soil type, thickness, original water content, and precipitation characteristics infiltration runoff
Moist soils produce more runoff than dry soils
sandy soils allow water to infiltrate silt and clay soils have slow infiltration rates, hence more surface runoff
The degree of infiltration and runoff depends on the soil type clays have extremely low infiltration rates (high runoff) sands have high infiltration rates USDA Soil Texture Classification silts have low infiltration rates
Rainfall characteristics determine the amount of infiltration versus runoff light rain more infiltration heavy rain more runoff infiltration runoff
Soil storage is the amount of water the soil can hold. It is controlled by the soil type, thickness, and porosity.
Porosity is a measure of void space in a soil
total volume of dry soil
water volume
saturate soil
water loss volume of voids saturated soil
volume of voids saturated soil total volume porosity = volume of voids total volume
Grain-size distribution controls the magnitude of porosity uniform grain sizes mixture of grain sizes porosity 40% porosity 25%
surface runoff soil water storage nothing came out groundwater recharge
After 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 adhere to solid surfaces and bond to other water molecules (cohesion) read about adhesion, cohesion, and surface tension
The 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 capillarity
Soils with smaller clasts have greater water holding capacity (stronger capillary forces) clay sand silt
watershed Q Hydrograph Q Q = stream discharge Time
sandy soils silt and clay soils Hydrograph Hydrograph Q attenuated response Q rapid high peak response Time Time
Hydrograph Q very rapid high peak response Impervious surfaces in urban settings don t allow any infiltration, hence very rapid surface runoff Time
Water flows to both sides and across Squalicum Parkway near the intersection with Roeder Avenue on Wednesday, January 7, 2009
The frost line is 4 to 6 feet in Minnesota Hydrograph Q very rapid high peak response Frozen soils don t allow any infiltration, hence very rapid surface runoff Time
Fargo North Dakota: March 13, 2010 http://www.ndsu.edu/fargo_geology/fldphotos2010.htm
thick, dry soils can store a lot of rain, hence produce less runoff (if they are dry to begin with) Hydrograph Q Time
thin soils saturate quicker, hence produce more runoff Hydrograph Q Time
soils near saturation produce more runoff Hydrograph Q Time
water can flow through the soils toward a stream Hydrograph Q Time