The influence of impoundments on riverine nutrient transport: An evaluation using the Soil and Water Assessment Tool

SummaryStudies of nutrient dynamics within stream reaches and in lakes and impoundments have resulted in important advances in our understanding of each system. However, less effort has been directed at understanding how linkages between stream and impoundment systems interact to determine nutrient dynamics at the watershed scale, and the possible influence of position of lakes and impoundments within river systems. The Soil and Water Assessment Tool (SWAT) was applied to the Huron and Raisin watersheds in southeastern Michigan to better understand the effect of impoundments on riverine nitrogen (N) and phosphorus (P) exports. The two watersheds were calibrated and validated for stream discharge and water quality parameters using data from 1995 to 2005 by using time series plots and statistical measures to verify model predictions. Simulated hydrology and water quality parameters closely resembled observed data except for daily streamflow in the Huron watershed and monthly nitrate loads in the Raisin watershed. The presence of impoundments had a marked effect on nutrient export from both watersheds. Modeled total phosphorus (TP) and total nitrogen (TN) export loads approximately doubled when all impoundments were removed from the Huron watershed model. The Raisin watershed showed a greater absolute increase in TP loads and a similar absolute increase in TN loads compared to the Huron, but because nutrient loads were several times larger in the Raisin, the proportional change was less. Impoundments placed near river mouths or in N and P source areas were most effective at reducing export, and many smaller reservoirs caused a greater reduction in nutrient loads than did a single large reservoir. In addition, impoundments increased the interannual variability in nutrient loads. Based on simulations using SWAT, impoundments have a substantial effect on riverine nutrient export, and that effect varies with their size and location.