Waiting for the snow to melt…
The widely used Penman-Monteith equation to estimate crop evapotranspiration (ET) has limited utility in many areas of the world because of its requirement for full meteorological data. Legal and engineering water agencies commonly use the original Blaney-Criddle method in their efforts to manage competing water demands in mountain basins, both for its longtime familiarity and minimal data requirements. The original Blaney-Criddle equation predicts crop ET based solely on readily available mean monthly air temperature, t, and percentage of daylight hours. However, in semiarid, high-elevation environments, Blaney-Criddle underestimates crop ET. Subsequent modifications have not fully corrected this underestimation. Low nighttime temperatures at high elevations incorrectly weight the estimate, resulting in significant variation between computed crop ET and lysimeter measurements. Our objective was to evaluate three modifications of the Blaney-Criddle temperature expression against the original equation with mean t, and another temperature method, Hargreaves, using lysimeter measurements from nine irrigated grass meadow sites in the upper Gunnison River basin of Colorado (1999–2003). Two of the modified temperature expressions resulted in improved correlation of Blaney-Criddle estimated crop ET with lysimeter ET. Similar improvements were observed when estimating with Hargreaves, which incorporates an additional term, Tdiff, the difference between maximum and minimum daily temperature. Correlations of solar radiation (Rs, the primary energy input to ET) with alternative temperature expressions and Tdiff were improved over correlations of Rs with mean t, supporting the improved prediction performance of alternative temperature expressions. These modifications to the original Blaney-Criddle can be applied successfully throughout Colorado mountain basins and may be globally applicable to high-elevation areas.
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