Surface Renewal Estimation of Pasture Evapotranspiration

Experiments to measure the evapotranspiration of an improved, irrigated pasture were conducted at the University of California, Davis, CA field station and over a commercial irrigated pasture on Twitchell Island in the Sacramento-San Joaquin River Delta using the surface renewal (SR) method. In Davis, the SR method was used to determine well-watered crop evapotranspiration ( ETc ) over short grass, and the results were compared with the ASCE-EWRI standardized reference evapotranspiration ( ET0 ) for a short canopy to establish that a crop coefficient Kc =1.00 is appropriate for estimating well-watered pasture ETc . In the Twitchell Island study, surface renewal was used to determine the actual evapotranspiration ( ETa ) from a commercial pasture. A stress coefficient of Ks = ETa ∕ ET0 ≈0.90 was observed during the high ET period ( ET0 >7 mm day−1 ) from about mid-June through mid-July for the Twitchell Island pasture. Otherwise, the pasture was mainly unstressed, so the Ks =1.0 . Thus, assuming no future changes in irrigation management, using ET0 from Twitchell Island, a Kc =1.00 , and Ks =1.00 will provide good estimates of ETa during low to moderate ET periods and Ks ≈0.90 should be used when ET0 >7.0 mm day−1 . In general, a thermocouple for SR measurements costs about $100, whereas the price for a sonic anemometer varies between $3,000 and $20,000, so the SR method provides a low-cost method to measure ETa .