Investigating practical alternatives to the NRCS-CN method for direct runoff estimation using slope-adjusted curve numbers

Abstract

The Natural Resources Conservation Service (NRCS) Curve Number (CN) method is generally applied to estimate direct runoff from watersheds. However, its utility in estimating direct runoff in South Korean watersheds is limited due to steep slopes, shallow soil layers, less infiltration losses and sudden peak flow generation. The CN is the primary parameter used in the method and varies with watershed characteristics and measured rainfall runoff data. In this study, we evaluated the performance of NRCS-CN models employing weighted average CNs and watershed slope-adjusted CNs. Furthermore, various combinations of alternatives were investigated to determine the proper watershed average CN. Direct runoff was estimated using different CN approaches and these estimates were compared to the observed values. The results obtained in this study indicated that the slope-adjusted CNs were higher than the NRCS tabulated CNs. Similarly, the Weighted Runoff (WQ) method provided higher runoff estimates than the weighted CN (WCN) method. In conclusion, the runoff estimated using the combination of the WQ method and slope-adjusted CNs showed better agreement with the observed direct runoff compared to other methods.