Numerical Evaluation of the Role of Runon on Sediment Transport over Heterogeneous Hillslopes

Abstract

During rainfall events, the runon process allows water from upstream saturated areas to infiltrate into downstream regions where moisture deficit has not been satisfied. To date, several studies have clarified the role of the runon process in describing field-scale infiltration and surface runoff behavior. This study focuses on the role of the runon process in estimating field-scale sediment discharge over hillslopes with spatially-varying infiltration properties. This was accomplished by performing Monte Carlo realizations of saturated conductivity fields with varying properties to reflect different soil conditions. Numerical solutions were sought for flow and sediment transport processes over each realization, and field-scale ensemble averages and variances of sediment discharge hydrographs were obtained both with and without representation of runon in the models. Examination of results indicates that neglecting runon can cause serious discrepancies in estimation of field-scale sediment discharges. Using model differences, the cases when runon is important were identified, and the role of rainfall intensity, duration, slope, and soil type are discussed in this paper. It is observed that only when the rainfall intensity is much larger than the mean conductivity and when variance of the conductivity is very low that runon can be ignored for sediment discharge computations.