Abstract

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 (ET_c) 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 K_c=1.00 is appropriate for estimating well-watered pasture ET_c. In the Twitchell Island study, surface renewal was used to determine the actual evapotranspiration (ET_a) from a commercial pasture. A stress coefficient of Ks=ET_a/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 K_s=1.0. Thus, assuming no future changes in irrigation management, using ET₀ from Twitchell Island, a K_c=1.00, and K_s=1.00 will provide good estimates of ET_a during low to moderate ET periods and K_s≈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 ET_a.

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