Technical Note: Development of Water Usage Coefficients for a Fully Watered Tallgrass Prairie

Abstract

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 3,500 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. In order to calculate the plant water coefficient (crop coefficient) for the prairie ecosystem, the site was instrumented to measure soil moisture in 1994, and a water balance was performed at the fully watered locations on the irrigation transect to calculate actual evapotranspiration (ETc). These values, along with reference evapotranspiration (ETr) data calculated using the modified Penman equation, were used to determine the plant water usage coefficient based on the following relationship: ETc = ETr Kc Ksm, where Kc is the plant water usage coefficient, and Ksm is the soil moisture coefficient. For fully watered sites, the plant water usage coefficient is the ratio of ETc/ETr, since Ksm = 1.0 because of ample water. Results indicated maximum plant water usage coefficients of 1.32 in the fully watered locations, similar in magnitude to the crop coefficients of warm-season agricultural crops (ETr is alfalfa based). Over the season (June 1 to September 30), tallgrass water usage was below the reference crop water use, with an average growing season coefficient of 0.90.