Modeling the Water Balance Processes for Understanding the Components of River Discharge in a Non-conservative Watershed

Abstract

The study was conducted in the Shibetsu watershed, eastern Hokkaido, Japan, to examine the possibility of using the Soil and Water Assessment Tool (SWAT) model in a non-conservative watershed (the surface watersheds are lying on a discontinuous impervious horizon) with external contribution (EXT). After confirming the capability of model simulation, the EXT was estimated to understand the components of river discharge. The EXT is difficult to measure directly and simulate by SWAT due to its subsurface circulation. In this study, the EXT was roughly estimated from the water balance equation using measured data. The average daily flux of EXT (1.38 mm d-1) was assumed as a point-source discharge in SWAT. The simulation of daily streamflow during the calibration and validation periods produced satisfactory results, with R2 values of 0.65 and 0.66, respectively. In addition, the simulated daily baseflow, monthly streamflow, surface runoff, and evapotranspiration (ET) all showed good agreement with the corresponding observations. Our simulation suggested that the EXT assigned as the assumed discharge in SWAT can help us to reasonably simulate the streamflow in the Shibetsu watershed. The EXT was then investigated indirectly by considering the difference between the observed streamflow and simulated streamflow using calibrated SWAT without adding the assumed EXT. The result indicated that the EXT was an important water source in the Shibetsu watershed, accounting for 47% of streamflow during the study period.