Assessing reservoir effect on water quality in the Missouri River basin using the soil and water assessment tool (SWAT) model

Abstract

AbstractThe flow regimes of the Missouri River Basin (MORB) have been significantly altered by hundreds of large and small reservoirs distributed throughout the basin. However, the impacts of these reservoirs on water quality dynamics have not been systemically studied. Therefore, we applied the Soil and Water Assessment Tool (SWAT) to evaluate how these reservoirs would affect sediment and nutrient loads in the MORB. Calibration and validation of the model indicated that the simulated streamflow and water quality at each gauge site across the MORB was in agreement with observation. Results showed that sediment and nutrient export in the MORB would be significantly reduced by the reservoirs. The mean annual total suspended sediment (TSS), total nitrogen (TN), and total phosphorous (TP) loads at the outlet of MORB were reduced by 11.9%, 7.6%, and 14.6%, respectively between 1997 and 2016. In addition, many small reservoirs could have an equivalent reduction effect on sediment and nutrient loads at the outlet as a large one due to similar trapping effects of these reservoirs and the weakening effect along the channel. Moreover, reservoirs near the mouth of the river reduced more TSS than those located in the upper watershed. But they made slightly less TN and TP reductions than those upstream ones because of not at the source area of the nutrients. These results suggested that reservoirs had substantial effects on water quality in the MORB, and the effects varied with the reservoirs' size and location.