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 (ETo) 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/ETo approximate to 0.90 was observed during the high ET period (ETo>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 ETo 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 approximate to 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.