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Estimation of crop evapotranspiration (ETc) <br /> <br />where <br /> <br />ETrs = reference crop ET (tall reference like alfalfa) <br /> = the ET rate from a uniform surface of dense, actively <br />growing vegetation (hypothetical crop) having <br />specified height (50 cm or 20 inches for alfalfa) and <br />surface resistance (to vapor transport), not short of <br />soil water, and representing an expanse of at least <br />100 m (328 ft) of the same or similar vegetation <br />(ASCE-Standardized Reference ET equation) <br />Kcr = crop coefficient based on tall reference <br /> <br /> rs <br />c <br />ET <br />ET <br />ETc = ETrs x Kcr