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An irrigation study was conducted from 1991 to 2000 at the Konza Prairie Biological Station (KPBS) Long Term Ecological Research (LTER) site, near Manhattan, Kansas to provide a better understanding of how the prairie ecosystem uses and dispenses water over the growing season. The irrigation transect was established on an annually burned portion of the 3000 ha preserve of native tallgrass prairie in 1991. Irrigation transects created a water gradient over the topographically distinct upland and lowland areas of the experimental site. Inorder to calculate the plant water usage and soil moisture coefficients for the prairie ecosystem, the site was instrumented to measure soil moisture in 1994 and a water balance was performed on the irrigation transect to calculate actual evapotranspiration (AET). These values, along with potential evapotranspiration (PET) data calculated using the Penman Combination equation, were used to determine the plant water usage and soil moisture coefficients using the following relationships: AET = PET*Kc*Ksm, where Kc = plant water usage coefficient and Ksm = soil moisture coefficient. For fully watered sites, the plant water usage coefficient is the ratio of AET:PET since Ksm = 1.0 because of ample water. Results indicate maximum plant water usage coefficients of 1.46 in the fully watered sites, similar in magnitude to the crop coefficients of warm season agricultural crops. Over the season, tallgrass water usage appears to be roughly equal to the reference crop water use with an average coefficient of 1.0.