Particular uncertainties encountered in using a pre-packaged SEBS model to derive evapotranspiration in a heterogeneous study area in South Africa

Abstract

Abstract. The focus of this paper is on the pre-packaged version of SEBS in ILWIS and the sensitivity of SEBS to some parameters over which the user has some control when using this version of the model, in order to make informed choices to limit uncertainties. The sensitivities of SEBS to input parameters are related to daily ET rather than energy flux results since this is of interest to water managers and other users of the results of the SEBS model. This paper describes some of the uncertainties introduced by the sensitivity of the SEBS model to (a) land surface temperature and air temperature gradient, (b) the choice of fractional vegetation formula, (c) displacement height and the height at which wind speed is measured, and (d) study area heterogeneity. It was shown that SEBS is sensitive to land surface temperature and air temperature gradient and the magnitude of this sensitivity depended on the land cover and whether or not the wetlimit had been reached. The choice of fractional vegetation cover formula was shown to influence the daily ET results by up to 0.7 mm. It was shown that the height of the vegetation canopy should be considered in relation to the weather station reference height to avoid the sensible heat flux from becoming unsolvable due to a negative ln calculation. Finally the study area was shown to be heterogeneous although the resolution at which fluxes were calculated did not significantly impact on energy partitioning results. The differences in the upscaling from evaporative fraction to daily ET at varying resolutions observed implies that the heterogeneity may play the biggest role in the upscaling and the influence of albedo on this calculation should be studied.

Reconnoitering the effect of shallow groundwater on land surface temperature and surface energy balance using MODIS and SEBS

“…The limitedness of both topographic relief and vegetation cover in the study area invalidated possible factors of uncertainty in SEBS estimates of energy fluxes (Gibson et al, 2011). Actually, the agreement between the SEBS estimated evaporation and soil moisture values and the field measured values led to the conclusion that the utilization of SEBS in this area under such limitedness was a success.…”

“…It has been extensively applied and validated with a variety of methods in different regions and climates (Su, 2002;Jia et al, 2003Jia et al, , 2009Su et al, 2005;McCabe and Wood, 2006;McCabe et al, 2008;Badola, 2009;van der Kwast et al, 2009;Gibson et al, 2011).…”

Surface Energy Balance System (Sebs) and Satellite Data for Monitoring Water Consumption of Irrigated Sugarcane

“…Since SEBS model was proposed, it has been used and tested by several authors such as ELHAG et al (2011), GIBSON et al (2011) andALKHAIER et al (2012), and applications have been made for databases with different spatial and temporal resolutions for various regions and crops. However, papers dealing with the operationalization of SEBS model and full methodologies that could be effectively applied for monitoring irrigated agriculture are limited.…”

Review: Estimating evapotranspiration using remote sensing and the Surface Energy Balance System – A South African perspective

“…Gibson et al (2011a) reported an underestimation of the sensible heat flux when compared with measurements from an eddy covariance system (Jarmain and Mengistu, 2011), and put forward 4 points of advice when using the SEBS model pre-packaged in ILWIS. These pertained to: (i) the uncertainties associated with land surface temperature retrievals and the sensitivity of SEBS to this parameter; (ii) the choice of fractional vegetation cover formula; (iii) the choice of weather station relating to the height at which wind speed is measured; and (iv) landscape heterogeneity and pixel resolution.…”

“…Taking a catchment scale approach, coarse resolution MODIS TERRA and AQUA data has been used by Gibson et al (2009), Gibson et al (2011a) and Gibson et al (2011b). Gibson et al (2011a) reported an underestimation of the sensible heat flux when compared with measurements from an eddy covariance system (Jarmain and Mengistu, 2011), and put forward 4 points of advice when using the SEBS model pre-packaged in ILWIS.…”