9.7: Slope and Runoff
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The steepness of slope is determined in percent (%) calculated as [(rise/run) x 100] using an Abney level or clinometer. The Abney level or clinometer is held at eye level and parallel to the slope. The level is adjusted allowing the % slope to be read directly. The absolute percent slope is the same whether sighting up slope or down slope.
The amount of soil erosion is directly related to the amount of surface water runoff, which depends on the water infiltration rate and the % slope. The steeper the slope and the less rapid the water infiltration rate, the more rapid the water runoff rate for a given soil. Soils having granular structure and high porosity have slower water runoff rates than do soils with massive structure and low porosity. This occurs because more water infiltrates into granular soils with less total water being available for over the surface water runoff when compared to massive soils. Other factors including vegetative cover, the type of vegetation, and the soil moisture content influence the surface water runoff rate. The greater the extent of plant vegetation covering the soil, the less soil erosion because the plant tissue intercepts the falling rain drops and greatly dissipates the energy of the falling water mass from directly hitting the soil. If a soil is moister, it requires less rainfall to reach the point where the soil becomes saturated (all pores filled) and water runoff occurs. Generally, as surface water runoff increases, soil erosion increases. The following table can be used to estimate the surface runoff rate using soil texture and the percentage of slope.
|Textural class||Water runoff rate|
|0-1||All textural classes||Very slow|
|1-2||Sands and loamy sands||Very slow|
|1-2||All textures except sands and loamy sands||Slow|
|2-6||Sands and loamy sands||Slow|
|2-6||All textures except sands and loamy sands||Medium|
|6-12||Sands and loamy sands||Medium|
|6-12||Sandy loams, sandy clay loams, loams, clay loams, silt loams, silty clay loams||Rapid|
|6-12||Silty clay, clay, sandy clay||Very rapid|
|12-18||Sands and loamy sands||Rapid|
|12-18||All textures except sands and loamy sands||Very rapid|
|>18||All textures||Very rapid|