Multicriterial Calibration of SWAT to Assess Small Water Reservoirs' Impact on Extreme Flow Mitigation

ABSTRACT The study is focused on the evaluation of multi‐site and multi‐variable calibration of the Soil and Water Assessment Tool model. The variables used for the multi‐calibration approach comprised snow water equivalent, soil water content, and discharge. The calibrated model was further used to investigate the influence of small water reservoirs on the runoff regime in the Upper Vydra catchment, Czech Republic (89.9 km 2 ). The multi‐calibration approach led to significantly improved estimation of snow accumulation/melt, soil water storage, as well as discharge at incorporated subbasins. The values of the Nash‐Sutcliffe coefficient in the validation period improved from 0.65 to 0.82 for snow water equivalent, from −25.6 to 0.13 for soil water storage, and from 0.39 to 0.41 for discharge on average for all measuring stations. At the expense of the multi‐calibration approach, the Nash‐Sutcliffe coefficient decreased from 0.61 to 0.53 for discharge simulation in the closing profile. Due to their small storage volumes, the small water reservoirs had a modest effect on mitigating low‐flow conditions. In two pronounced drought periods in 2015 and 2018, the number of days with the discharge below minimum sustainable flow was reduced by 7 and 8 days, respectively. The volume of four inspected peak flows was reduced by less than 1%, indicating minimal impact of the reservoirs when used as the only flood mitigation measure.