Surface renewal estimates of evapotranspiration. Theory

Traces of high frequency temperature data are known to exhibit ramp-like shapes, and the mean ramp amplitude (a) and inverse ramp frequency (l+s) during a time interval can be used to estimate sensible heat flux density (H) using the surface renewal (SR) method. Structure functions are used to determine the mean ramp characteristics, and then H is estimated using a basic conservation of energy equation. The SR estimates of H have been tested against H estimated with a sonic anemometer over several crop canopies. The method provides good estimates of H without the need for temperature profile and wind speed data over a wide range of agronomic and horticultural crops. When combined with measured net radiation and soil heat flux density, the SR method provides an inexpensive, accurate method to estimate latent heat flux density (lambda E) from a vegetative surface.