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A water‐budget model was developed to aid the study of transport of nutrients in surface and subsurface flows across a marsh. The model is based on field studies of Wingra marsh adjacent to Lake Wingra in Madison, Wis., and was used to calculate monthly totals of actual evapotranspiration, surface runoff, and subsurface drainage from April 1970 through August 1976. About 90% of the water entering the marsh comes from urban storm runoff. The remainder is precipitation (8%) and deep ground‐water inflow (2%). Water leaves the marsh through evapotranspiration (7%), shallow ground‐water discharge (8%), and surface outflow (85%). The model was used to estimate the water‐table elevation at an observation well where direct measurements were taken (November 1975 through October 1976). The observed depth to the water table ranged from 0 to 150 cm below the surface and the simulated value usually was within ± 10 cm. This suggests that the model adequately computes the water budget for the marsh and provides a sound basis for calculating dissolved and suspended material loads transported through the marsh into Lake Wingra.