Using surface renewal analysis to determine crop water use coefficients

The surface renewal (SR) method for estimating sensible heat flux from canopies provides a simple, portable, robust, and low-cost method to measure crop evapotranspiration (ETc) for determining crop coefficient (Kc) values, which are extensively used for irrigation management. Many experiments have been conducted to develop and refine the SR method using raw, high frequency data; however, it is impractical to set up systems with computers in irrigated fields to collect raw data for long periods of time. As a result, a data logger (Campbell Scientific, CR10x) program was written to collect raw data and calculate the statistical moments needed to analyze the temperature data for ramp characteristics using the Van Atta structure function method. The ramp characteristics are used to estimate sensible heat flux density (H). During recent years this method was used to measure sensible heat flux density over irrigated pasture, paddy rice, cotton, and a citrus orchard. On a half-hour basis, the latent heat flux density (LE) was determined as the residual of the energy balance equation using measured net radiation, soil heat flux density, and H from SR analysis. The half-hour LE values were converted to evapotranspiration in mm of water and totaled over the 24-h day to obtain estimates of daily ETc. Then the Kc values were calculated as Kc=ETc/ETo, where ETo was calculated using the 24-h sum of an hourly Penman-Monteith equation as recommended by the American Society of Civil Engineers. The results of these studies and experiences with the SR method will be reported.