The design of wetlands to replace those lost to development requires quantitative understanding of the wetland water budget in order to estimate the amount of water available to the wetland over time. Many methods exist to estimate each component of the wetland water budget. In this study, monthly values of the water budget components namely, precipitation, runoff, evapotranspiration, and groundwater seepage were calculated using a water budget model and compared to on-site field measurements for a wetland in Manassas, Prince William County, VA. The monthly precipitation estimated from a weather station 32.18 km from the site differed from the on-site values by as much as 2.9 times. Runoff estimates calculated by the Soil Conservation Service (SCS) method using antecedent moisture condition (AMC) II underpredicted runoff for every month by as high as 100 percent compared to the on-site measured runoff. The choice of AMC greatly affected the SCS runoff estimates. Runoff was the dominant water budget component at the Manassas wetland. The evapotranspiration (ET) estimates using the Thornthwaite method either over or underestimated ET when compared to ET calculated from diurnal cycles of the water table in the wetland. Groundwater seepage losses were calculated using Darcy's equation with an assumed hydraulic gradient of one, and with gradients measured with nested piezometers. Seepage losses at the Manassas wetland were negligible. Overall, the water budget model provided conservative estimates of the available water in the wetland during the 10-month period of observation.