Samuel J. Lamont

Watershed analysis with GIS: The watershed characterization and modeling system software application

The watershed characterization and modeling system (WCMS) was developed to support decision making and the management of water resources at a statewide level in West Virginia. Specific hydrological analysis functions were combined within a customized GIS interface to provide decision support capabilities to both technical and non-technical users. Components of the current system include: an overland flow path model that indicates optimum water quality sampling locations, flow estimation for all streams in an identified area, an instream water quality and loading model for pollutant levels, and a ranking model to prioritize treatment alternatives based on user defined criteria and preferences. The primary goals of this system are to provide consistent technical information related to natural watershed processes and to predict the impacts of alternative management scenarios for decision makers. WCMS is currently used by the West Virginia Department of Environmental Protection (WVDEP) to guide policy development and management decisions that address watershed and water quality issues throughout the state.

Curve Numbers and Urban Runoff Modeling—Application Limitations

The U.S.D.A. SCS (now the NRCS) Curve Number method has been in continuous use for about 50 years. As originally developed, the method yields a direct runoff depth from the accumulated 24 hour rainfall depth as function of the curve number CN. The method has since been extended to hydrograph generation and is found in commonly used hydrologic models applied to urban drainage design (e.g., WinTR55, SWMM and HEC-HMS). A number of recently published studies, including Curve Number Hydrology - State of the Practice, by the ASCE/EWRI Curve Number Hydrology Task Committee, have warned that it is inappropriate to use the method to generate runoff hydrographs, yet the practice continues with little awareness of this limitation by most users. A common misconception is that the CN method is an infiltration model, which can lead to significant errors in peak discharge predictions. CN values can be converted into equivalent physically based infiltration model parameters used in the Green-Ampt method in SWMM (or HEC-HMS), or in the infiltration component used in the PERLND module of EPAs HSPF model, each of which can produce a more acceptable hydrograph that matches CN method direct runoff depth.