Quantifying the uncertainty in estimates of surface–atmosphere fluxes through joint evaluation of the SEBS and SCOPE models

摘要

Accurate estimation of global evapotranspiration is considered to be of greatimportance due to its key role in the terrestrial and atmospheric waterbudget. Global estimation of evapotranspiration on the basis ofobservational data can only be achieved by using remote sensing. Severalalgorithms have been developed that are capable of estimating the dailyevapotranspiration from remote sensing data. Evaluation of remote sensingalgorithms in general is problematic because of differences in spatial andtemporal resolutions between remote sensing observations and fieldmeasurements. This problem can be solved in part by using soil-vegetation-atmosphere transfer (SVAT) models, because on the one hand these modelsprovide evapotranspiration estimations also under cloudy conditions and onthe other hand can scale between different temporal resolutions.In this paper, the Soil Canopy Observation, Photochemistry and Energy fluxes(SCOPE) model is used for the evaluation of the Surface Energy BalanceSystem (SEBS) model. The calibrated SCOPE model was employed to simulateremote sensing observations and to act as a validation tool. The advantagesof the SCOPE model in this validation are (a) the temporal continuity of thedata, and (b) the possibility of comparing different components of the energybalance. The SCOPE model was run using data from a whole growth season of amaize crop.It is shown that the original SEBS algorithm produces large uncertainties inthe turbulent flux estimations caused by parameterizations of the groundheat flux and sensible heat flux. In the original SEBS formulation thefractional vegetation cover is used to calculate the ground heat flux. Asthis variable saturates very fast for increasing leaf area index (LAI), the ground heat fluxis underestimated. It is shown that a parameterization based on LAI reducesthe estimation error over the season from RMSE = 25 W m?2 toRMSE = 18 W m?2. In the original SEBS formulation the roughnessheight for heat is only valid for short vegetation. An improvedparameterization was implemented in the SEBS algorithm for tall vegetation.This improved the correlation between the latent heat flux predicted by theSEBS and the SCOPE algorithm from ?0.05 to 0.69, and led to a decrease indifference from 123 to 94 W m?2 for the latent heatflux, with SEBS latent heat being consistently lower than the SCOPEreference. Lastly the diurnal stability of the evaporative fraction wasinvestigated.