Detachment: Dislodgement of soil particles from the soil mass due to the kinetic energy of raindrops or shear stress of flowing water. Usually described as a function of the kinetic energy of water and the cohesiveness or shear strength of the soil. Sediment transport: Movement of dislodged soil particles from one point to another. Function of the kinetic energy of raindrops and overland flow, land surface characteristics (cover, roughness), and sediment characteristics (particle size, density, shape, fall velocity). Deposition: Process by which sediment is lost (settles out) from flowing water. Occurs when flow no longer has sufficient kinetic energy to transport the sediment load. The deposition rate will be heavily influenced by the fall velocity of sediment particles. Methods for Estimating Sediment Yield: 1. Monitoring - very time consuming, expensive, and difficult to collect good data. How do you take a representative sample? 2. Reservoir sedimentation surveys (time consuming, expensive) 3. Sediment delivery ratio methods - both gross erosion and sediment delivery are difficult to estimate, considerable uncertainty in estimates. 4. Bedload prediction equations/models - assume an infinite supply of noncohesive material is available for bedload transport. Transport a function of transport capacity and sediment characteristics. Some researchers have correlated bedload transport with washload in an effort (poor) to simulate gross sediment transport. Does not consider upland landuse. 5. Cesium-137 testing. Amount of Cesium-137 (from atmospheric testing of nuclear weapons in the 1950's and 1960's) in the soil or sediment is a function of the amount of soil loss or deposition in an area. Reduced concentrations indicate erosion while higher concentrations indicate deposition of increased amounts of Cesium. 6. Empirical erosion equation/models - USLE, equation, and other regression equations. Estimate sediment detachment, not
Buffer Strip Cropping: Alternate strips of grass or legumes between other crops (normally in rotation). Often plant buffer strips on steep unfarmable portions of fields. MAJOR WEAKNESSES OF THE USLE • Empirically based equation which uses variables that are to a degree indices of factors that affect soil erosion. • Erosion processes are not explicitly represented as physical processes in the USLE. Poor representation of erosion, infiltration, slope, slope length, and especially contouring. • USLE is a regression equation developed from a limited data base. When the equation is applied to new situations, new experimental data are usually required. • Most of the USLE data are from cropland. Extension of the USLE to range land, forest land, and construction sites without an extensive data base has been highly controversial. • Not designed for individual storm analysis. Does not predict particle size distributions required for water quality work. • Problems with contour tillage accurately representing field slopes. Problems with individual USLE factors: • EI - Does not accurately reflect rainfall characteristics in all regions. • K - The more we study it, the less we seem to know (Romkens). • S - Too large on steep slopes. The relationship is more linear on steep slopes. • C - Canopy and soil roughness effects are not represented well. • P - For contouring should vary with ridge height and EI of 10 year storm.