Subwatershed Monitoring in the McKenzie Brook Watershed in Vermont.

Hastings, B., Allen, A.R., and Regaldo, S. Vermont Department of Environmental Conservation. (2019). Subwatershed Monitoring in the McKenzie Brook Watershed in Vermont (Technical Report No. 95). Grand Isle, VT: Lake Champlain Basin Program

A phosphorus loading study was conducted for the Vermont portion of the agriculturally dominated Mckenzie Brook watershed, which drains several small subwatersheds directly to the southern portion of Lake Champlain. Water quality (7 sites), streamflow (2 gages), and precipitation (12 gages) monitoring was conducted from April – November in both calendar years 2017 and 2018. 342 total and dissolved phosphorus samples were collected in 2017, which was characterized by above normal spring-summer rainfall and normal to below normal monthly totals that fall, with a significant 2.59 inch event in early July. 281 total and dissolved phosphorus samples were collected in 2018, which was much drier with above normal rainfall in April and November but well below normal for most of the intervening months and many periods of zero flow. A watershed model based on the curve number approach was developed that includes soil moisture accounting routines, which was calibrated and validated to estimate daily mean streamflow at ungaged sampling sites. Poor and inconsistent concentration-discharge correlations necessitated the use of the Beale Ratio Estimator to estimate flow-weighted daily mean concentrations and total April – November loads for year at each site. In general, the sampled phosphorus concentrations were high, with some extremely high concentrations even at low flows. This study provided useful insight into the hydrologic and geochemical dynamics of this region, and results serve as a point of reference for reducing phosphorus concentrations and loads through implementation of agricultural best management practices moving forward.