By Tom Trout, USDA-ARS-Water Management Research
Tom Trout checking out the CoAgMet weather station instrumentation. Credit: Peggy Greb
Water rights transfers in Colorado are based on consumptive use. A city or company that purchases water from a farmer can only use the amount of water that the farmer has historically consumed—that is, the water that actually evaporated and transpired from the crop and soil. Thus, they must estimate the evapotranspiration, or ET, for the fields that had been irrigated. The method to estimate ET commonly used in Colorado is an old method developed over 50 years ago called the Blaney-Criddle method. The method is based only on temperature and is simple, but not very accurate.
Much more accurate ET calculation methods are now available. ET weather station networks in Colorado—Colorado Agricultural Meteorological Network (CoAgMet), and Northern Water’s network—have been collecting detailed weather data for over 20…
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