Calibration and Validation of Watershed Models and Advances in Uncertainty Analysis in TMDL Studies

Watershed models are widely used in total maximum daily load (TMDL) studies to predict the impacts of pollutant discharges on the biochemical functioning and assimilative capacity of water bodies. The reliability of a TMDL is therefore tightly linked with the predictive capability of these models. While there has been an increasing availability and application of watershed models for TMDL studies, guidelines for model evaluation, including recommendations for an appropriate selection and implementation of calibration, validation, and uncertainty analysis strategies, remain at present limited. The ASCE Environmental and Water Resources Institute (EWRI) TMDL Analysis and Modeling Task Committee was established in part to identify existing and emerging challenges encountered by water resources professionals during any phase of a TMDL development and to produce documentation to address these challenges. This paper reviews existing approaches for model calibration, validation, and uncertainty analysis, including recommendations to establish baseline modeling practices to obtain a satisfactory watershed model.