Evaluation of the Surface Renewal Method for Estimating Cotton Evapotranspiration in South Carolina

Abstract

HighlightsThe Surface Renewal (SR), Eddy Covariance (EC), and FAO56 methods were used to estimate crop evapotranspiration.An independently calibrated SR approach was used with a low-cost cup anemometer and assumed energy balance closure.All three approaches (SR, EC, and FAO56) resulted in similar crop ET values during later stages of growth (R2 = 0.87).SR has the potential to lower field-scale ET measurement costs and equipment requirements.Abstract. Evapotranspiration (ET) is important for many agricultural and hydrological applications, but its measurement normally requires expensive instrumentation. The Surface Renewal method (SR) has been proposed as a relatively low-cost alternative to measure ET, but the need for local calibration has been cited as a shortcoming. Castellví (2004)(2004) proposed a similarity-based SR approach to address this shortcoming, but field comparisons evaluating the accuracy of the approach using low-cost anemometry are currently lacking. The objective of this study was to evaluate the reliability of the similarity-based SR approach using low-cost anemometry (cup anemometer) for estimating cotton ET in comparison to the Eddy Covariance (EC) and FAO56 methods (Allen et al., 1998). Concurrent estimates of cotton ET were taken in 2019 at two adjacent cotton fields in South Carolina using the SR, EC, and FAO56 methods. SR and EC measurements of sensible heat flux were replicated using two in-field tower locations in the north field and ET was derived using the energy balance closure approach. An analysis was made to ensure =90% of the EC and SR seasonal data came from the field area (Kljun et al., 2015), with additional filtering to ensure that the higher-mounted EC had >90% footprint from the field area for each half-hour of analysis. In general, the results showed very good agreement between the daily crop ET values measured by SR and EC (R2north = 0.93, R2south = 0.97) for all of the season, and good agreement between SR and FAO56 measurements when crop height was more developed (R2north = 0.92, R2south = 0.87), near or above a canopy height of 0.75 m. There was disagreement between the non-continuous north tower SR measurements and FAO56 measurements in earlier stages of growth (June and July R2 = 0.09), which may have been due to uncertainty with the FAO56 local single crop coefficients or with a lack of energy balance closure. The results suggest that the use of low-cost anemometry with the independently calibrated SR approach is a useful alternative to measure crop ET under the conditions of this study, but further research may be necessary to evaluate the use of low-cost anemometry for direct SR ET measurements. Keywords: Cotton irrigation, Crop coefficients, Eddy covariance, Evapotranspiration, ET, Surface renewal.