Snyder, Richard L. and Spano, Donatella Emma Ignazia and Duce, Pierpaolo and Paw U, Kyaw Tha and Rivera, Maria C. (2008) Surface renewal estimation of pasture evapotranspiration. Journal of Irrigation and Drainage Engineering, Vol. 134 (6), p. 716-721. eISSN 1943-4774. Article.
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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.
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