A generalized reservoir module for SWAT applications in watersheds regulated by reservoirs

Accurate estimates of reservoir outflow are important for simulating runoff of watersheds regulated by large reservoirs using hydrological models. The target release reservoir module widely used in the Soil and Water Assessment Tool (SWAT) shows poor performance in simulating the outflow and water level of large multipurpose reservoirs. In this study, a generalized reservoir operation chart-based reservoir module is developed as an alternative to the original reservoir module in SWAT (SWAT-Or). The proposed reservoir module includes a dynamic decision-based outflow framework integrated into the operation chart. The modified version of SWAT with the new reservoir module (SWAT-M) is not only suite for small reservoir with flood control function but also more applicable to large multipurpose reservoir. The predictions obtained from the original and new reservoir modules are compared with measured data from the three largest reservoirs in the Dongjiang River basin, South China. The performance of SWAT-M and SWAT-OR for daily and monthly runoff simulations of four hydrological stations in the basin are compared. The results shows that the proposed module produces results closer to the field measurements and has fewer errors than the original module, which predicts inappropriate low reservoir outflow during dry periods. In addition, the runoff simulation by SWAT-M shows a significant improvement, in particular for stations directly influenced by large reservoirs. The monthly (daily) scale R2 of SWAT-M is 0.02 to 0.21 (0.14 to 0.44) higher, and the NSE is 0.11 to 0.30 (0.12 to 7.16) higher than that of SWAT-Or. The proposed reservoir operation chart-based module significantly improves the applicability of SWAT to simulate runoff in watersheds regulated by large reservoirs.