Crop coefficients for rangeland

Abstract

Crop coefficients (Kc) provide a means of relating evapotranspiration (ET) to standard references such as pan evaporation or other climatic based reference ET’s (ET = Kc*reference ET). They have been used extensively in irrigated agriculture but only limitedly on rangelands. This study used lysimeter-measured ET to determine Kc’s for conditions where water was nonlimiting for both transpiration (T) and soil water evaporation (E) and transpiration coefficients (Tc) for conditions where E was minimal and water was nonlimiting for T. Growing season ET was measured daily with hydraulic lysimeters from mixed grass, shortgrass, and sagebrush-grass plant communities near Newell, South Dakota; Gillette, Wyoming; and Reynolds, Idaho, respectively. From seasonal plots of daily ET/reference ET, lysimeter-measured ET, and daily precipitation, time periods were identified, following periods of precipitation, that met the conditions for determining Kc and Tc values. The Kc values were relatively constant among the 3 study sites and over most of the growing season ranging from 0.75 to 0.90. Maximum Tc varied among years with most of the values occurring within the 0.40 to 0.60 range.