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 $\left({\mathrm{ET}}_c\right)$ over short grass, and the results were compared with the ASCE-EWRI standardized reference evapotranspiration $\left({\mathrm{ET}}_0\right)$ for a short canopy to establish that a crop coefficient $K_c=1.00$ is appropriate for estimating well-watered pasture ${\mathrm{ET}}_c$ . In the Twitchell Island study, surface renewal was used to determine the actual evapotranspiration $\left({\mathrm{ET}}_a\right)$ from a commercial pasture. A stress coefficient of $K_s={\mathrm{ET}}_a∕{\mathrm{ET}}_0\approx 0.90$ was observed during the high ET period $({\mathrm{ET}}_0>7\mathrm{mm}{\text{day}}^{-1})$ 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 ${\mathrm{ET}}_0$ from Twitchell Island, a $K_c=1.00$ , and $K_s=1.00$ will provide good estimates of ${\mathrm{ET}}_a$ during low to moderate ET periods and $K_s\approx 0.90$ should be used when ${\mathrm{ET}}_0>7.0\mathrm{mm}{\text{day}}^{-1}$ . 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 ${\mathrm{ET}}_a$ .