Hydrograph separation for a small agricultural watershed: The role of irrigation return flow

Abstract

Abstract The streamflow components in a small agricultural watershed were determined using a three-component hydrograph separation model. Rainfall, throughfall, soil water, groundwater, irrigation water, and stream water samples were collected from a headwater catchment consisting of a natural forested area, and from the downstream agricultural catchment outlet, where rice paddy fields are developed extensively along the stream. The purpose of this study was to compare streamflow generation patterns between the forested and agricultural watersheds. We differentiated pre-event and event water contributions to streamflow for seven rainfall events using chemical and isotopic tracers. All of the stream water samples collected from the upper headwater station were plotted within the three-component end-members (groundwater, soil water, and precipitation); whereas, at the lower catchment outlet station, stream water samples collected during the agricultural season deviated clearly from the three-component mixing analysis, indicating the addition of a new water source (irrigation return flow) to the streamflow. Our findings indicate that pre-event groundwater was the major contribution to streamflow in the headwater catchment, whereas event rainwater and irrigation return flow were the major contributors to streamflow in the lower part catchment during the agricultural season.