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

The Natural Resources Conservation Service-Curve Number (NRCS-CN) method has been used widely for estimation of direct runoff. However, the determination methods of CN values for land covers from the National Engineering Handbook Chapter 4 (NEH-4) are inadequate due to uncertainty raised in determining the of the hydrologic soil group, which could change from reported soil survey data due to soil compaction and other human activities. To overcome these drawbacks, an asymptotic CN (ACN) regression equation for the relationship between measured streamflow and rainfall for gauged river basins (ACN-RB) was developed and used in various studies. The ACN-RB determines various CN values depending on the amount of rainfall in an event. The ACN-RB has limitations in application to ungauged river basin because the ACN-based CN values do not consider various land covers. Accordingly, 13 land cover-based asymptotic CN regression equations (LC-ACN-REs) were developed and evaluated in this study by comparing estimated direct runoff with measured data. The estimated direct runoff using the LC-ACN-REs, which were classified as “standard types”, matched observed direct runoff well, as evidenced by a Nash-Sutcliff efficiency (NSE) value of 0.78 for Jungrang A basin and a NSE value of 0.73 for Tancheon A basin.