Research Article

Estimating evapotranspiration in semi-arid rangelands: connecting reference to actual evapotranspiration and the role of soil evaporation

Published in: African Journal of Range & Forage Science
Volume 36, issue 1, 2019 , pages: 17–25
DOI: 10.2989/10220119.2018.1505779
Author(s): Onalenna GwateInstitute for Water Research, Rhodes University, South Africa, Sukhmani K MantelInstitute for Water Research, Rhodes University, South Africa, Andiswa FincaAgricultural Research Council–Animal Production Institute, South Africa, Lesley A GibsonSchool of Engineering, UK, Zahn MunchDepartment of Geography and Environmental Studies, South Africa, Anthony R PalmerInstitute for Water Research, Rhodes University, South Africa


In a context of water scarcity, efforts to increase landscape production should focus on improving water productivity. This requires an appreciation of the various components of evapotranspiration (ET), including soil evaporation (Es) because the latter reflects ‘unproductive’ water loss. Both complex and simple algorithms have been developed to determine ET. In data scarce areas, developing and testing parsimonious algorithms is useful. This study sought to improve a simple single layer ET model by incorporating an Es component. Empirical methods were also explored to predict ET from vegetation indices (VIs), leaf area index (LAI) and reference evapotranspiration (ET0). A large aperture scintillometer and an eddy covariance (EC) system were used to validate the proposed algorithm at three sites over Grasslands and Albany Thicket biomes in the Eastern Cape, South Africa. There was good agreement between the observed and predicted ET with RMSE of 0.30–0.58 mm d−1 when average daily observed ET was 0.43–3.24 mm. The VIs had moderate correlations with the observed data due to the significant role played by Es (65%–84%) across the sites and stomatal conductance at the Albany Thicket site. The simple algorithms developed would make determining ET easier in data scarce regions.

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