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Transpiration-use efficiency, the ratio of biomass (Y) produced per unit of water transpired (T) by a crop, depends on crop characteristics and on the environment in which crops develop. Transpiration-use efficiency has been described as Y/T=k_c/D_a, where k_c is a crop dependent constant and D_a is the daytime air vapor pressure deficit. Our objectives were to determine Y/T and k_c of barley grown in Pullman, WA, and to analyze the variation in Y/T and k_c of barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) reported in the literature. Transpiration and biomass accumulation of barley crops were measured in the years 2000 and 2001. The coefficient k_c was estimated as the slope of the regression between cumulative values of biomass and T/D_a. It ranged from 6.6±0.4 to 6.9±0.2Pa. These figures are greater than 5.8Pa obtained by applying equations developed by Tanner and Sinclair [Tanner, C.B., Sinclair …