Evapotranspiration rates at selected sites in the Powder River basin, Wyoming and Montana

Twelve sites were chosen for a study of evapotranspiration in the Powder River basin based on variations in topography and plant communities, geographic location, and the availability of groundwater data at the sites. Evapotranspiration rates were estimated from groundwater, meteorological, and vegetation data using the Blaney-Criddle method. Five of the sites were equipped with digital recorders that provided continuous groundwater level data at the sites for the 1978 growing season. Evapotranspiration was estimated monthly during the growing season and ranged from 0 to 3.7 inches per month. Total evapotranspiration rates for the growing season ranged from 8.3 to 14.9 inches. Discharge per mile of stream reach was estimated for three of the sites and ranged from 0.03 to 0.31 cubic foot per second. The well records for the remaining seven sites consisted of monthly, or less frequent, water-level measurements. Evapotranspiration rates estimated for those months for which water-level data were available ranged from 0 to 3.8 inches per month. Only one of these sites had monthly water-level measurements for the entire growing season; a total of 9.7 inches of evapotranspiration was estimated for the growing season at this site. (USGS)