Evaluation of modified CN methods for watershed runoff estimation using a GIS-based interface

Accurate surface runoff estimation techniques suitable for ungauged watersheds are relevant to areas such as India where hydrologic gauging stations are not widely available. The natural resources conservation services curve number (NRCS-CN) method is one of the most widely used methods for quick and accurate estimation of surface runoff from ungauged watersheds. In this study, an interface in geographic information system (GIS) was developed using the in-built macro-programming language Visual Basic for Applications (VBA) of the ArcGIS® tool to estimate the surface runoff by adopting the NRCS-CN technique and its three modifications. The developed interface, or interface for surface runoff estimation using curve number techniques (ISRE-CN), was validated using the recorded data for the periods from 1993 to 2001 of a gauged watershed, Banha, in the Upper Damodar Valley, Jharkhand, India. The observed runoff depths for different rainfall events in this study watershed were compared with the predicted values of NRCS-CN methods and its three modifications using statistical significance tests. It was found that using all the rainfall data for different antecedent moisture conditions (AMCs), the modified CN I performed the best (R2=0.92; E=0.89), followed by the modified CN III method (R2=0.88; E=0.87), while the modified CN II (R2=0.42; E=0.36) failed to predict accurately the surface runoff from the Banha watershed. Moreover, under AMC-based conditions, the modified CN I method also performed the best (R2=0.95; E=0.95) for the AMC II condition, while the modified CN II performed the worst under all the AMC conditions. Overall, it was observed that the application of the modified CN I method in the ungauged watersheds that are hydrologically similar to the Banha watershed would result in an accurate surface runoff estimation.