Hydrologic Modeling Consistency

Computer applications to storm runoff prediction have revolutionized the watershed modeling techniques. In general, a watershed is modeled as a system with inputs and outputs (Chow 1964). A system can be lumped or distributed, depending upon the spatial and temporal characteristics of the design variables used in the model. A lumped parameter ignores the variability in space. On the other hand, taking the spatial variations into consideration, the system becomes a distributed model (Singh 1982). As a rule of thumb, a lump model is applicable to a small watershed characterized by its quick response to rainfall while a distributed model is recommended for large watersheds. For example, in Las Vegas valley, Nevada, the HMS or HEC-1 model is accepted for regional master drainage planning while the Rational method is recommended for local designs (HCDDM, 1999). Although the watershed size dictates the selection of hydrologic model, both methods must be calibrated to warrant a consistency in flood predictions.