This report describes the hydrology and water quality of Powers Lake, a recreational lake in a densely populated area of southeastern Wisconsin, from October 16, 1986 - October 15, 1987.

The hydrologic budget for the study period showed that direct precipitation on the lake and ground water were dominant sources of water entering the lake (37 and 36 percent, respectively) and that streamflow dominated the outflow. Surface runoff contributed 27 percent of the inflow-23 percent from Powers Lake inlet and 4 percent from shoreline drainage. Streamflow through Powers Lake outlet accounted for 62 percent of the outflow and evaporation accounted for 38 percent. Based on the streamflow from Powers Lake outlet, the lake's hydraulic residence time was 3.8 years.

 During the study period, precipitation was 27.16 inches or 4.08 inches below long-term (1951-80) average. The data were adjusted or normalized to represent an average year of precipitation and runoff to help evaluate the water quality of the lake for an average year. For an average year, precipitation dominated inflow (42 percent), followed by ground water (32 percent), Powers Lake inlet (21 percent), and shoreline drainage (5 percent). Streamflow through Powers Lake outlet accounted for 61 percent of an average year's outflow budget and the remaining 39 percent was evaporation. Based on an average year's streamflow from Powers Lake outlet, the lake's hydraulic residence time was 4.2 years.

Phosphorus budgets were prepared for the study period and for an estimated normal year. The phosphorus budget for the study period showed that, of the total inputs (516 pounds), surface runoff contributed the largest amount; shoreline drainage contributed 44 percent, and Powers Lake inlet contributed 36 percent. Direct precipitation contributed 11 percent; ground water, 2 percent; and septic systems, 7 percent. Of the total outputs, 83 pounds (16 percent) was lost from the lake via the outlet; 433 pounds (84 percent) was lost to the sediments as the phosphorus that was attached to particles settled to the lake bottom. An estimated phosphorus budget for a normal year showed that of the total inputs (744 pounds), surface runoff contributed the largest amount; Powers Lake inlet contributed 45 percent and shoreline drainage contributed 35 percent. Precipitation contributed 9 percent; ground water, 1 percent; and septic systems, 10 percent.

The health of the lake was evaluated using Carlson's Trophic State Index and Vollenweider's model. Carlson's Trophic State Index showed that Powers Lake was moderately enriched and in the mesotrophic range. Comparison of guidelines from Vollenweider's model showed that the total phosphorus input for the study period and for an estimated average year would not cause eutrophic conditions.