Modeling Soil Erosion with Emphasis on Steep Slopes and the Rilling Process

A soil erosion model, known as KYERMO, was developed for use in soil erosion research. The model was written in Microsoft FORTRAN, which is a subset of the ANSI FORTRAN 77 standard, allowing the model to be run on an IBM-PC as well as a mainframe computer. The model structure consisted of many interrelated subroutines which facilitated model development, testing, and future expansion. A sensitivity analysis of the detachment component was performed, examining the number of rills, the number of space and time steps, and the detachment parameters. This analysis indicated that the model performed as expected for the detachment limiting case. Field plots for the collection of justification data were designed and constructed on steep slopes (28 and 30.5 percent). Their surfaces were shaped to provide controlled rill patterns. Rainfall was applied through use of an irrigation nozzle rainfall simulator. The field simulations included initial full plot (22.1 m) runs on unrilled surfaces with two and six rill watersheds, full plot runs on established surfaces, and half plot runs on established surfaces. Eight field simulations were conducted. Measured or sampled quantities included runoff rate, delivered sediment concentration, delivered sediment size distribution, rainfall intensity, soil moisture content, plot surface shape, and rill cross-sections. Each of the field erosion events was simulated using KYERMO. Hydrologic parameters were fitted to provide the proper runoff characteristics to allow justification of the erosion component of the model. The detachment parameters were initially set using parameters and relationships from the literature. Prediction was comparable to that reported by other researchers. The detachment parameters were then fitted individually to gain insights about their effects. The resultant values were then considered in light of plot characteristics. These analyses indicated that a more complete knowledge of the rill bed particle size distribution could be used to increase accuracy and that soil structure changes due to tillage should be considered. The detachment rate was found to be the limiting factor rather than the sediment transport rate