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

Abstract

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 ( H SR ) values. These values were compared against eddy covariance sensible heat flux ( H EC ) 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, MSE s ) showed that the agreement between H SR and H EC 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 ( LE SR ) and the eddy covariance ( LE EC ) methods, using H SR and H EC , respectively, and measurements of net radiation and soil heat flux. For the calibration period, error analysis statistics for LE SR were quite similar to those for H SR , although errors were mostly at random. LE SR 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 H SR and LE SR 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.