Comparing Calculation Techniques for Effective Rainfalls Using NRCS-CN Method: Focused on Introducing Weighted Average and Slope-based CN

Abstract

The NRCS-CN method is generally used to estimate effective rainfalls in a basin. However, since the curve number which plays a critical role in the NRCS-CN method was originally developed for US watersheds, it is limited to be directly applied to other basins outside the United States. Therefore various modifications have been suggested to revise the NRCS-CN for specific watershed condition. This study introduced the weighted average method and the slope-based CN to estimate effective rainfalls available for Korean watersheds and compared with the observed direct runoff. The overall results achieved from this study indicated that the adjusted slope-based CN considerably increases effective rainfalls in general and makes the duration of effective storm longer. Based on the statistical error analysis performed for various modifications of NRCS-CN, the weighted average method with the adjusted slope-based CN has highest precision with the observed direct runoff. In addition, after analyzing the relation between the initial loss estimated from rainfall-runoff observations and the potential maximum retention from GIS-based data, it turns out that the assumption of linear relationship between the initial loss and the potential maximum retention is not available for Korean watersheds.

Approach of Land Cover Based Asymptotic Curve Number Regression Equation to Estimate Runoff

“…factors. To date, various direct runoff estimation approaches (Moon et al , ) have been developed and used such as the Φ‐index (McCuen, ), the ω‐index (Subramanya, ), the Natural Resources Conservation Service‐Curve Number (NRCS‐CN, Andrews, 1954) and Hydrograph Analysis (Subramanya, ), among others. The NRCS‐CN method developed by Andrews (1954) calculates direct runoff using rainfall events.…”