The sensitivity of wetland saturated hydraulic heads and water budgets to evapotranspiration (ET) was examined using a simplified hydrologic model and eight representations of ET. Estimates of ET that created the most reliable wetland saturated hydraulic heads and water budgets employed vegetation coefficients to correct potential ET, calculated by the Priestley-Taylor equation, to actual ET. The accuracy of simulated hydraulic heads generally improved by < 1 cm, however, when using the most reliable ET estimates based on vegetation coefficients. An ET estimate that used a regression-defined extinction depth created substantial errors in simulated water budgets. Specifically, the extinction-depth ET overestimated the annual actual ET by about 40 (400 mm). An ET approximation that overestimates actual ET by 400 mm annually applied in a regional hydrologic model over the 5400 km2 area of Everglades National Park would underestimate the annual volume of water available for ground-water recharge and surface-water runoff to coastal estuaries by 2.3 billion m3. For comparison, this underestimation is about two thirds of the mean volume of water in Lake Okeechobee (3.8 billion m3), the largest lake in Florida, and clearly demonstrates unbiased estimates of ET are necessary for reliably simulating wetland water budgets. ?? 2008 The Society of Wetland Scientists.