Modified Channel-Routing Scheme for SWAT Model

Because of its relatively easy quantification in comparison with the variables of other hydrological components, streamflow is an important and primary component for modeling in a watershed study. This study attempts to enhance one of the channel routing tools, the Muskingum routing method (MRM) used in the soil and water assessment tool (SWAT) model. This study advocates its replacement by a well-tested alternative physically based model known as the variable parameter McCarthy-Muskingum (VPMM), capable of varying the routing parameters at every routing interval, and thereby accounting the nonlinear characteristics of flood wave method movement in steep, intermediate, and small slope channels and rivers. However, the routing capability of the VPMM model is subject to the limitation of the inflow hydrograph being characterized by the criterion $(1/So)\partial y/\partial x<0.5$, where $\partial y/\partial x$ denotes the slope of the longitudinal water surface gradient. A small watershed of approximately $986.9{\mathrm{km}}^2$ in the Vansadhara basin of the Odisha state in India with upstream and downstream sites at Gunupur and Kashinagar, respectively, is studied to demonstrate the routing capability of the VPMM scheme in comparison with the MRM, and variable storage routing method (VSRM) routing schemes used in the SWAT model. The routing simulations carried out using these three schemes are evaluated using measures like the Nash-Sutcliffe efficiency (NSE), coefficient of determination ($R^2$), and mean absolute percent error (MAPE). The NSE estimates for VPMM, MRM, and VSRM routing schemes for the calibration and validation period were estimated to be 0.89, 0.92 and 0.91, and 0.72, 0.71, and 0.63, respectively; similarly, the $R^2$ estimates for VPMM, MRM, and VSRM routing schemes for the calibration and validation period were estimated to be 0.89, 0.93 and 0.92, and 0.71, 0.70, and 0.63, respectively. As the performance of the VPMM model is at par with the the MRM routing module of the SWAT model and has a sound physical basis than the MRM routing scheme, it can be recommended that the VPMM routing scheme can be incorporated in the SWAT model for enhancing its routing capability. The added advantage of the VPMM scheme is that it also estimates the stage hydrograph corresponding to the routed discharge hydrograph and thereby increasing the utility of the SWAT model for sediment transport, in-stream nutrient, and operational purposes.