Unit hydrograph method for curve number validation in hydrological modeling: case study of Welaran Watershed, Karangsambung, Kebumen

Abstract

Unit hydrographs, which have long been used in hydrological analyses, assume or require evenly distributed rainfall as their input. In practice, such a requirement is met when the methods are applied to small watersheds. This study aim to validate the curve number in hydrological modelling used unit hydrograph. The Welaran Watershed in Karangsambung has an area of 11.6 km2. More than 50% of it is covered by rice fields, while the rest is by dry agricultural lands, multiple-species plantations, and settlements. This watershed has been installed with a stream gauge equipped with AWLR and ARR. Based on the automatically recorded flood and rainfall data, a hydrological analysis was performed to generate unit hydrographs. Baseflow (losses), separated from the hydrograph, was compared with a loss of water using the Curve Number (CN) method. CN was derived from a reference table containing land use (LU), soil hydrological group, andantecedent moisture condition (AMC).The CN of the study area was 67, which is the weighted value of the mapping unit. Insome rainfall-runoff events, the unit hydrograph approach yielded curve numbers varying between 23-82 for eight selected rainfall events. It indicates that many factors shape the diversification of rainfall-runoff interaction in a watershed, including the actual distribution of rain. Also, AMC seemed to be distributed according to not only the amount of previous rain but also other factors that need further study.