Slope length and sediment yield from hilly cropland

In the Universal Soi Loss Equation (USLE), the geomorphological elements affecting erosion are slope length and gradient of the plot. In humid climates under natural conditions, a hilly landscape evidences slope length and gradient in dynamic equilibrium with environment, so that erosion is low enough to allow vegetation growth. Erosion and drainage density strongly increase, and average slope length (L°) consequently decreases, as the landscape reacts to new conditions (bare soils and fixed time tillage) tending to a new equilibrium, which is never reached because further practices prevent drainage network increase. This trend results in greater erosion and soil loss. In order to reduce erosion, slopes are subdivided into strips of short length (L) by drainage ditches, banks, etc. Erosion on cropped slopes depends on L. The authors suggest the most suitable value of L as L* L°/n, where n is empirically determined for that landscape and valid for any value of L°. This paper describes a methodology for automatic computation and mapping of L°. Some examples are reported concerning small instrumented basins where sediment yield has been measured.