The 120-acre Nevin wetland at the south edge of Madison, Wis., is a discharge area of the local ground-water system. A hydrologic unit composed of drift and the upper part of an underlying sandstone sequence provides ground-water inflow. Ground water enters as springflow and as leakage upward through the organic wetland soils.

The average annual water budget for the wetland was based on the 3 years 1974 through 1976. Although 1976 and 1975 were years of more than average precipitation, and 1976 was a year of less than average precipitation, the 3-year average precipitation was only slightly less than the long-term average. Inflow was composed of direct precipitation (7 percent), surfacewater inflow from the adjacent uplands (h percent), and ground-water inflow (89 percent). Ground-water inflow included springflow, upward leakage, and flow from artesian wells used in the Nevin State Fish Hatchery and discharged as effluent. Outflow was composed of streamflow (92 percent) and evapotranspiration (8 percent). The amount of water stored in the organic soils varied by only 1 inch during the study. Some ground water flowed beneath the wetland but did not discharge within the study area.

Nutrients were contained in ground-water inflow, surface-water inflow, and precipitation. Ninety-five percent of the nitrogen that entered the wetland (1974 through 1976) entered in ground-water inflow, which included the hatchery discharge. Two percent was from precipitation, and 3 percent entered in surface-water inflow. Sixty-six percent of the phosphorus input was from ground water, 31 percent from surface water, and 3 percent from precipitation. Some nutrients were retained because only 79 percent as much nitrogen and 93 percent as much phosphorus left the wetland in streamflow as entered it.

Sediment was also retained. Only 20 percent as much sediment (1974 through 1976) left the wetland in streamflow as entered it. Sediment was deposited where surface-water inflow from upland areas flowed onto the wetland surface and along stream channels.

Inflows and outflows are quantified, and nutrient and sediment sources are identified, with the result that the effect of proposed land-use changes in the basin can be evaluated. Land-use practices that affect the wetland include drainage, urbanization, and water-supply development. Additional ground-water supplies can be obtained in the basin, but springflow and upward leakage of ground water through the organic soils will decrease.