Spatial Distributions and Stochastic Parameter Influences on SWAT Flow and Sediment Predictions

The Soil and Water Assessment Tool SWAT was implemented in Northwest Arkansas to investigate flow and sediment predictive ability at multiple subbasin and hydrologic response units HRU distributions. The objectives of this study were to use SWAT and identify differences in annual predicted flow and sediment response of a watershed considering two subbasin delineations with six different HRU distributions each; quantify the uncertainty in SWAT output when sensitive model parameters are considered to have stochastic distribution; and evaluate the ability of the model to describe flow and sediment predictions of an ungauged watershed by stochastically validating the model. Flow results from the Single-Factor Between-Subjects Analysis of Variance test =0.05 indicated that predicted flow means were not significantly different from each other for all SWAT subbasin/HRU combinations; however, predicted flow means and measured flow mean were significantly different. Sediment simulation results suggested significant differences were present amongst the different model subbasin/HRU delineations and measured values. The Monte Carlo simulation of the model, using curve number CN , soil evaporation compensation factor ESCO , groundwater revap coefficient GW_REVAP , and peak rate adjustment factor for sediment routing in the subbasin AMP as uncertain parameters, indicated that generally ESCO induced most uncertainty in predicted flow. However, sediment prediction uncertainty was affected most by uncertainty in AMP. Results indicated that SWAT applications on ungauged watersheds should include small subbasin sizing 2% of watershed area , HRUs that reflect actual land cover composition, a check to evaluate surface runoff and ground water contributions and modification of parameters as needed to reflect site conditions, sensitivity analysis, and uncertainty analysis that includes several sensitive parameters. DOI: 10.1061/ ASCE 1084-0699 2008 13:4 258 CE Database subject headings: Spatial distribution; Stochastic process; Parameters; Sediment; Arkansas.