Curve Numbers in Stormwater Runoff Simulation
Urban runoff models require the determination of excess rainfall for pervious areas in catchments. This is generally achieved by means of infiltration equations such as, for example, the Horton and Green-Ampt equations in the SWMM and MIDUSS models. Some models (MIDUSS, OTTHYMO) also use the SCS runoff curve number method. Computation of the excess rainfall by any of these methods requires the estimation of two or three parameters which are generally functions of the land use and soil properties, including the moisture content of the pervious areas. Values of the parameters can be obtained by infiltrometer measurements, but field measurements are time-consuming and may not be practical.
Typical parameter values are available in the literature but, in many cases, modelers use optimization techniques to estimate the “best” parameter values during the calibration of the model. Ideally, the parameter optimization should be guided by parameter values derived for the specific catchment from observed rainfall-runoff data, but independently of the calibration process. In this chapter an asymptotic method of estimating the curve number CN and initial abstraction Ia for a watershed, from observed rainfall and runoff events, is presented.
The proposed method is asymptotic in the sense that the estimated values approach the “true” values as the number of observations increases. The method provides estimates of spatially averaged CN and Ia for each event for the catchment area upstream of the streamflow gauging station.
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