Determination of the initial abstraction ratio and curve number of the upper catchment area of the Sawaga river watershed, Bukidnon

Abstract

The Curve Number (CN) method has been widely used for estimating runoff from rainfall. However, some uncertainties in the method have been recognized by various researchers all over the world. One of which is the NRCS-assumed initial abstraction ratio (Ia/S) of 0.20. In this study, the Ia/S and the CN for the upper catchment area of the local watershed of the Sawaga river in Bukidnon were determined using rainfall-runoff event analysis of rainfall events with a total precipitation depth ranging from 21.50 mm to 57.90 mm. The event Ia/S values ranged from 0.0019 to 0.4603. The representative values of Ia/S and CN are 0.03 and 62.3, respectively. Evaluating the performance of both the original NRCS and the locally derived values through their direct runoff prediction, the standard error, coefficient of determination, and Nash-Sutcliffe efficiency indicated that both are good and have predicted the direct runoff satisfactorily. However, these indicators showed that the locally derived values gained higher accuracy in general. The percentage of mean bias displayed the most significant difference between the two, classifying the original NRCS-CN values as unsatisfactory while the locally adapted values as very good. These findings suggest that the CN method with certain adjustments on Ia/S and CN values is appropriate for the direct runoff estimation in the upper catchment area of the Sawaga river, Bukidnon.