Verification and Correction of the Hydrologic Routing in the Soil and Water Assessment Tool

The Soil andWater Assessment Tool (SWAT) is one of the most widely used eco-hydrological models. SWAT has been undergoing constant changes since its development. However, compartment review and testing of SWAT, especially the hydrologic routing functions, are comparably limited. In this study, the daily hydrologic routing subroutines of different SWAT versions were reviewed and tested using a well observed segment of the Weser River located in Germany. Results show several problems with the routing subroutines of SWAT. The variable storage subroutine of SWAT (Revision 664) does not transform the stream flow. Unphysical results could be obtained with the variable storage routing of SWAT (Revision 528). The Muskingum subroutine of SWAT (Revisions 664 and 528) overestimates daily channel evaporation (resulting in a bias of up to 6.3% in streamflow in our case studies) and underestimates daily transmission losses. Simulated results show that the timing and shape of flood waves, as well as the volume of low flows, could be improved with a corrected Muskingum subroutine. Based on the results of this study, we suggest that the SWAT user community review their existing SWAT models to see how the aforementioned issues will affect their methods, findings, and conclusions.