MODIFIED CHANNEL ROUTING METHOD FOR SWAT MODEL

Abstract

Soil Water Assessment Tool model is a watershed-scale model, widely used for various watershed management, land use-land cover scenario, and climate change studies. Due to its deterministic nature, stream flow is a key component in comparison to others. In any modeling study, a watershed is calibrated for stream flow first before modeling other processes, such as nutrient and sediment transport. There has been a significant development in the area of stream flow routing. SWAT model uses “Muskingum Routing Method” (MRM) and “Variable Storage Routing Method” (VSRM) for routing water from upstream of a reach to downstream. Being hydrologic model in nature, SWAT can yield appreciable result only when good amount of data are available. So, sensitivity analysis of parameters along with calibration is required for good model efficiency. On the other hand, there are models like “Variable Parameter Muskingum Method” (VPMM, Perumal and Price 2013), which are physically-based and perform quite well when applied in its applicability range. The major advantage of this method is that it requires only inflow discharge data, channel parameter and geometry data in longitudinal as well as transverse direction. Due to less input data, this model can be handled more effectively. To achieve our objective, SWAT and VPMM models is set-up to simulate streamflow on a watershed in the Vansadhara basin with upstream and downstream sites as Gunupur (Odisha) and Kashinagar (Odisha), respectively. Calibration and validation of both models are done for 2004-2006 and 2008-2012 respectively. Contribution of lateral flow into the routing reach is taken into account in the VPMM model and for this purpose, calibrated and validated SWAT model outflow is used. Test statistics like Nash-Sutcliffe efficiency (NSE), coefficient of determination (𝑅2) are evaluated for performance evaluation of both models. NSE for VPMM, SWAT (VSRM) for calibration and validation period are found out to be 0.904, 0.926 and 0.7, 0.632 respectively, and NSE for the VPMM, SWAT (MRM) for calibration and validation period are found out to be 0.902, 0.933 and 0.702, 0.684 respectively. 𝑅2 for VPMM, SWAT (VSRM) for calibration and validation period are found out to be 0.905, 0.928 and 0.694, 0.635 respectively, and 𝑅2 for the VPMM, SWAT (MRM) for calibration and validation period are found out to be 0.904, 0.934 and 0.695, 0.678 respectively. It can be seen that both Modified Channel Routing Method for SWAT Model VPMM and SWAT (MRM) model are in good agreement with each other in terms of test statistics, whereas SWAT (VSRM) performance is lagging by 7% and 6% for NSE and 𝑅2 respectively in validation period. If actual flow data corresponding to the intermediate catchment were available, then result of VPMM model could have been improved. This is the reason that VPMM performance is slightly lagging to SWAT model performance in calibration period. As the performance of VPMM model is at par with the SWAT model and has a sound physical basis than the SWAT model, it can be concluded that VPMM model can be incorporated into SWAT model for better model performance.