Base flow simulation using a physically based subsurface model – The case of a tropical basin in the Western Ghats, India

Abstract

A novel framework that integrally uses groundwater and streamflow observations to calibrate a physically based sub surface model to generate base flow on a daily time step is presented in this study. Trend analysis on mean annual base flow time series showed that a declining trend was present in the basin though the upstream humid region was devoid of any trend. GLUE analysis was adopted to the simulation of base flow in the humid region and the most sensitive parameters were identified as the recharge factor (rf) and discharge factor (pd). The model was able to satisfactorily simulate base flow as observed values fell within the 95CI bands for the calibration period. The mean annual BFI values were computed at Muthankera to be 0.236 ± 0.02. The mean seasonal BFI varied from 48 % during post monsoon periods to 21 % during monsoon periods. The base flow simulated by the model was comparable to the output from the Eckhardt filter which has been widely cited to be a suitable baseflow separation method for many river basins. Due to the absence of groundwater level data in the transition region, a combination of model and digital filter was used to calibrate that region. The draft from the groundwater storage computed from the model was found to be highly nonlinear. The rf was seen to be lower and pd to be higher in the transition region than in the humid region. This would imply that in the transition region, the base flow is generated more gradually in comparison to the humid region. This was characteristic of forested basins where the uptake of water by deep rooted trees during evapotranspiration from the unsaturated zone causes delay in the water table response which in turn could result in delay in the base flow generation process. The consequence of the calibration exercise on Kabini basin was establishing that the heterogeneity in terms of precipitation and land use has resulted in the distinct hydrologic responses from the two regions in the basin.