Modeling at catchment scale and associated uncertainties

This study describes application of a catchment scale model, SWAT (Soil Water Assessment Tool), to a small scale agricultural watershed in northern Maryland. It covers the steps involved in model application and associated model uncertainty as affected by variability in input parameters using Latin Hyper Cube Sampling (LHS) with Constrained Monte Carlo Simulation (MCS). SWAT model predictions of the impact of environmentally friendly practices are discussed within the context of input variability. Results indicate that SWAT is a reasonable monthly predictor of hydrology, but does not provide strong association between measured and simulated nitrate loss at that time scale. SWAT was found to perform very well when used for annual nitrate loss predictions. Results also show that using average input parameter values without considering their variability due to media heterogeneity produces simulation outputs that can be misleading and should not be given 100% confidence. It was concluded that in developing TMDL (Total Maximum Daily Load) plans for a given watershed one has to assert associated uncertainty levels in model's inputs and simulation results for proper resource management.