Water and total-phosphorus budgets were determined for Balsam Lake in northwestern Wisconsin. All significant components of the lake's water budget were determined independently. The lake's trophic state was evaluated in relation to total-phosphorus loading from December 1, 1987 through November 30, 1989. The information obtained in the study can be used by local and State agencies to develop and assess lake- and watershed-management alternatives for maintaining or improving the water quality of the lake.

The three-basin, 1,900-acre drainage lake receives flow from two main tributary streams, Harder and Rice Creeks. Precipitation, ground-water levels, and streamflow were below normal during the study period and the year preceding the study. Precipitation was 6.09 inches below normal the first year of the study and 8.71 inches the second year.

Precipitation, the dominant water-budget inflow component, was followed in decreasing order by inflows from Rice Creek, ground water, Harder Creek, and near-lake drainage. Total inflows in the first and second years of the study were 11,040 and 11,650 acre-feet, respectively. Surface inflow was 23 percent greater in the second year of the study than in the first year.

The largest outflow from Balsam Lake was outflow to Balsam Branch, and the second largest outflow was evaporation. These outflows accounted for 98 percent of outflow from the lake. Recharge to ground water accounted for 2 percent.

Total-phosphorus input to the lake was 692 and 1,144 pounds in the first and second years of the study. Rice Creek and near-lake drainage accounted for 80 percent of phosphorus entering the lake. Outflow to Balsam Branch removed 30 percent of the phosphorus that entered the lake.

The main basin of the lake was mesotrophic, whereas the loading rates determined for the study period would have expected to result in oligotrophic conditions. Internal loading and below-normal external loading during the study are the likely explanation for this discrepancy. The northwest basin of the lake (locally called "Little Balsam Lake") received 66 percent of all external phosphorus that entered Balsam Lake. The trophic state of Little Balsam Lake ranged from upper mesotrophic to lower eutrophic. Phosphorus that entered Little Balsam Lake during the study period would have been expected to result in eutrophic conditions.