Modeling the Big Ditch Watershed in Illinois and Studying Scaling Effects on Water and Sediment Discharges

Abstract

A Dynamic Watershed Simulation Model (DWSM) was developed at the Illinois StateWater Survey to simulate surface and subsurface storm water runoff, propagation of flood waves,soil erosion, and transport of sediment and agricultural chemicals in agricultural and ruralwatersheds. The hydrology and soil erosion-sediment transport components were tested (calibratedand validated) on the Big Ditch watershed in Illinois, a 100-square-kilometer tributary subwatershedof the Upper Sangamon River basin draining into Lake Decatur. Two different divisions of thewatershed, one with coarse subdivisions and the other with fine subdivisions were used in thesimulations to investigate scaling effects on parameter values and model results. The fine divisionsof the Big Ditch watershed did not add noticeable accuracy to the model results; however, thespatially averaged model parameters were different from the coarse divisions. The overlandroughness for the coarse division was one third of the roughness for the fine division whereas theeffective lateral saturated hydraulic conductivity was five times of the same, which are all due to thepresence of more channel features in the coarse overland segments. The flow detachmentcoefficient for the coarse divisions was two thirds of its value for the fine divisions due to longer slopelengths in the coarse segments with higher flows and higher erosion potentials.