CURVE NUMBER HYDROLOGY IN WATER QUALITY MODELING: USES, ABUSES, AND FUTURE DIRECTIONS

Although the curve number method of the Natural Resources Conservation Service has been used as the foundation of the hydrology algorithms in many nonpoint source water quality models, there are significant problematic issues with the way it has been implemented and interpreted that are not generally recog- nized. This usage is based on misconceptions about the meaning of the runoff value that the method computes, which is a likely funda- mental cause of uncertainty in subsequent erosion and pollutant loading predictions dependent on this value. As a result, there are some major limitations on the conclusions and decisions about the effects of management practices on water quality that can be sup- ported with current nonpoint source water quality models. They also cannot supply the detailed quantitative and spatial informa- tion needed to address emerging issues. A key prerequisite for improving model predictions is to improve the hydrologic algo- rithms contained within them. The use of the curve number method is still appropriate for flood hydrograph engineering applications, but more physically based algorithms that simulate all streamflow generating processes are needed for nonpoint source water quality modeling. Spatially distributed hydrologic modeling has tremen- dous potential in achieving this goal. (KEY TERMS: nonpoint source pollution; curve number; hydrologic modeling; water quality; agricultural hydrology; geographic infor- mation systems.)