University of California Water Resources Center

This project considers the micrometeorologicel processes that govern the use of water by plants and the availability of soil moisture. Evapotranspiration (ET) was obtained from the residual of the energy budget terms, including a new method, that of surface renewal, to estimate one of the energy budget terms. This method appears to offer some alternative to somewhat more cumbersome methods presently used for ET estimates and crop coefficients. Evapotranspiration estimates are considered very important to the state because evapotranspiration represents the major component of agricultural water usage.

Data from experiments carried out in 1988 through 1994 were analyzed to validate the new method. The plant canopies considered in these experiments were an English walnut orchard, a maize canopy, a grass canopy, and other short. Sensible heat was estimated using the eddycovariance and other methods, and then compared to the new method. Project results show that regression of sensible heat flux density obtained from the surface renewal method against eddy correlation/covariance yields reasonable fits under stable conditions (standard errors of less than 20 W m-2), when data were filtered or when the structure function was used. Analysis also showed the method could be used over grass and other short crop canopies, if a height greater than the canopy height was used. Some preliminary comparisons with ET data were made for some canopies.