Separation of surface flow from subsurface flow in catchments using runoff coefficient

Separating surface flow (SF) from subsurface flow (SSF) based on direct runoff measurements in river gauges is an important issue in hydrology. In this study, we developed a simple and practical method, based on runoff coefficient (RC), for separating SF from SSF. RC depends mainly on soil texture, land use and land cover, but we also considered the effect of slope and rainfall intensity. We assessed our RC-based method for three different soil types by comparing the value obtained with laboratory rainfall simulator data. The correlation coefficient between observed and calculated data exceeded 0.93 and 0.63 when estimating SF and SSF, respectively. The method was then used to separate SF and SSF in two catchments (Heng-Chi and San-Hsia) in Northern Taiwan, and the results were compared with those produced by the geomorphological instantaneous unit hydrograph (GIUH) model. Test revealed that, if RC is calculated accurately, the proposed method can satisfactorily separate SF from SSF at catchment scale.