The surface renewal (SR) method for estimating sensible heat flux from canopies provides a
simple, portable, robust, and low-cost method to estimate crop evapotranspiration (ETc).
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 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 (?E) 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 ?E 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. Because of its relatively low cost and portability, the surface renewal
method provides an excellent technique to refine crop coefficients for to help farmers and
urban irrigators to improve efficient water usage. Experiences with the surface renewal
method will be discussed.
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