Agricultural Watersheds Using Dwsm

DWSM, the dynamic watershed simulation model, was expanded with a subsurface and a reservoir flow routing schemes. The hydrology and sediment components of the model were applied to three agricultural watersheds in Illinois, Big Ditch (100 km2), Court Creek (250 km2), and Upper Sangamon River (2,400 km2), to simulate spatially and temporally varying surface and subsurface storm water runoff, propagation of flood waves, upland soil and streambed erosion, and sediment transport; to evaluate these simulation capabilities through calibration and validation; and to conduct various watershed investigative analyses. The new schemes were selected from the literature. DWSM was able to simulate the major hydrologic, soil erosion, and sediment transport processes, and generate reasonable water and sediment discharges in the Big Ditch and Court Creek watersheds, considering complexities of the physical processes simulated and sizes of the drainage areas evaluated. Comparisons of predicted and observed sediment discharges during recession portions of the hydrographs were much better in Court Creek watershed than in Big Ditch because of depth−integrated observation samples in the former, which is necessary during recession and low flow periods when pronounced concentration gradients are expected. Some discrepancies in model predictions were found, which may be due to limitations of the model, especially its single−event nature and lack of backwater simulation, limitations and uncertainties of input data, and temporally constant values of input parameters. Addition of the subsurface flow (tile drain and base flow combined) routing scheme improved predictions of the recession and base flow portions of the subwatershed (100 to 290 km2) hydrographs in the Upper Sangamon River watershed. Significant improvements were noticed in larger subwatersheds. Scaling effect investigations on the Big Ditch watershed showed different overland Manning’s roughness coefficients, effective lateral saturated hydraulic conductivities, and flow detachment coefficients for a coarser and a finer representations (subdivisions) of the watershed. These input parameters required recalibration when watershed subdivision sizes were altered. After recalibration, simulated water and sediment discharges were approximately the same for both representations. DWSM provided a robust tool in ranking overland planes and channel segments in the Court Creek watershed based on comprehensive criteria for flooding and sediment production potentials. The rankings were useful to stakeholders in prioritizing critical parts of the watershed and planning restoration and education programs. The model also provided a robust tool for evaluating detention basins in controlling downstream water and sediment discharges, although evaluations on sediment discharges were limited to large basins.