Estimation of sensible and latent heat flux from natural sparse vegetation surfaces using surface renewal

This paper reports a study undertaken to evaluate the feasibility of the surface renewal method to accurately estimate long-term evaporation from the playa and margins of an endorreic salty lagoon (Gallocanta lagoon, Spain) under semiarid conditions. High-frequency temperature readings were taken for two time lags (r) and three measurement heights (z) in order to get surface renewal sensible heat flux (HSR) values. These values were compared against eddy covariance sensible heat flux (HEC) values for a calibration period (25–30 July 2000). Error analysis statistics (index of agreement, IA; root mean square error, RMSE; and systematic mean square error, MSEs) showed that the agreement between HSR and HEC improved as measurement height decreased and time lag increased. Calibration factors α were obtained for all analyzed cases. The best results were obtained for the z=0.9 m (r=0.75 s) case for which α=1.0 was observed. In this case, uncertainty was about 10% in terms of relative error (RE). Latent heat flux values were obtained by solving the energy balance equation for both the surface renewal (LESR) and the eddy covariance (LEEC) methods, using HSR and HEC, respectively, and measurements of net radiation and soil heat flux. For the calibration period, error analysis statistics for LESR were quite similar to those for HSR, although errors were mostly at random. LESR uncertainty was less than 9%. Calibration factors were applied for a validation data subset (30 July–4 August 2000) for which meteorological conditions were somewhat different (higher temperatures and wind speed and lower solar and net radiation). Error analysis statistics for both HSR and LESR were quite good for all cases showing the goodness of the calibration factors. Nevertheless, the results obtained for the z=0.9 m (r=0.75 s) case were still the best ones.