Computation of probability of exceeding environmental load allocations from point and diffuse sources using the integrated modeling system GIBSI

In an integrated watershed management context, the implementation of any clean water program requires the evaluation of the contribution of pollutant loads associated with wet (nonpoint or diffuse) and dry weather (point) sources to the studied water quality problem. It is also necessary to set environmental load allocations (ELA) or total maximum daily loads (TMDL) for various anthropogenic activities and to link these loads to water pollutant concentrations. The ELA or TMDL is a numerical quantity determining the maximum load of pollutants from point and nonpoint sources as well as background sources, to receiving water bodies that will meet designated water uses (e.g., swimming or fishing) in terms of water quality standards (WQS). The American Congress included a TMDL program in the Clean Water Act of 1972 (US EPA, 1997; 1999). The outcome of the TMDL program corresponds to the drafting of a watershed management plan (NOVOTNY, 1999).