Adequacy of HRU and Grid-based Hydrological Models for Streamflow Simulation in a Paddy-Dominated Agricultural Catchment

Abstract:

Catchment-scale streamflow assessment in the paddy-dominated catchments using the available hydrological models is highly intricate due to the presence of significant ponding water during the rainy season and irrigation events. While the Soil and Water Assessment Tool (SWAT) and Variable Infiltration Capacity (VIC) model could be some suitable options for this, the model conceptualization, discretization of spatial domain, involved computational time, and these integrated effects on streamflow simulation are yet to be evaluated. Hence, this study evaluates the performances of the semi-distributed hydrologic response unit (HRU) based SWAT-pothole and grid-based VIC model for catchment-scale streamflow simulation; and subsequently, assesses the effect of the spatial domain representation of the catchment on the model computational requirement. The selected study is the 12,014 km² Kangsabati River basin (KRB) of eastern India that consists of about 46% paddy land use with the tropical monsoon type climate having complex paddy field dynamics.

SWAT is set up for the whole KRB using the add-in reservoir module; whereas the VIC model, due to the absence of reservoir module, is set up individually for the upstream river catchment and the downstream reservoir-catchment command. The SWAT setup consisted of 44 and 191 discrete sub-basins and HRUs, respectively. Conversely, the VIC setup resulted in 31 grids with 0.25°0.25° spatial discretization. The model simulation results reveal that the SWAT-pothole approach performed better with the Nash-Sutcliffe Efficiency (NSE) estimate of 0.69 than the standalone VIC model (NSE=0.66). However, the other water balance components, viz. evapotranspiration, baseflow, and percolation are far away from reality in both the models which could be attributed to the non-accountability of irrigation return flow by the SWAT-pothole model. The inferior water balance simulation in VIC could be aggravated due to the absence of crop management module and ignorance of explicit paddy land use class in a VIC grid. These findings highlight the future scope of including more dynamic spatial representation of paddy land use in the SWAT and VIC model domain for their application in other similar world river basins.