An update on the theory and application of surface renewal for estimating ET

For well-watered crops and landscapes, energy-limited crop or landscape evapotranspiration (ET_c) is often estimated using crop coefficients (K_c = ET_c/ET_o), where ET_o is the standardized reference evapotranspiration (ET). Surface renewal (SR) is a method based on high-frequency temperature data to measure sensible heat flux density (H) that is used in conjuction with other energy flux data to estimate latent heat flux (LE) and then ET_c. In recent years, the surface renewal (SR) method has been widely used to measure crop ET (ET_c) to develop and improve K_c values. For deficit irrigated crops, there is no easy way to accurately estimate the impact of water stress on canopy growth or transpiration, so an in situ measurement is necessary to accurately determine the actual ET (ET_a) where ET_a≤ET_c. However, relatively low-cost SR H estimates are now used for direct measurement of ET_a for deficit irrigated vegetation. In most SR research, uncalibrated SR estimates of sensible heat flux (H’) are calibrated against sonic anemometer H, and the calibration factor is then used with SR H’ data to estimate H when the measurements are collected at the same height above a similar crop. More recently, two distinct methods to estimate SR H without the need for calibration against sonic anemometer H data were developed. In this paper, we will provide an update on recent theroretical and application advances in the use of SR methods for determining ET.