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Discussion papers
https://doi.org/10.5194/hess-2019-233
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-233
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Jun 2019

Research article | 14 Jun 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Can we trust remote sensing ET products over Africa?

Imeshi Weerasinghe1, Ann van Griensven1,2, Wim Bastiaanssen2, Marloes Mul2, and Li Jia3,4 Imeshi Weerasinghe et al.
  • 1Vrije Universiteit Brussels, Brussels, Belgium
  • 2IHE Delft Institute for Water Education, Delft, Netherlands
  • 3Chinese Academy of Sciences, Beijing, China
  • 4Joint Center for Global Change Studies, Beijing, China

Abstract. Evapotranspiration (ET) is one of the most important components in the water cycle. However, there are relatively few direct measurements of ET (using flux towers), whereas various disciplines ranging from hydrology to agricultural and climate sciences, require information on the spatial and temporal distribution of ET at regional and global scale. Due to limited data availability, attention has turned toward satellite based products to fill observational gaps. Various remote sensing data products have been developed, providing a large range of ET estimations. Across Africa only a limited number of flux towers are available which are insufficient for systematic evaluation of remotely sensed (RS) derived ET products. Thus we propose a methodology for evaluating RS derived ET data at the basin scale using a general water balance (WB) approach, where ET is equal to precipitation minus discharge for long-term annual averages. Firstly, RS ET products are compared with WB inferred ET for basins without long-term trends present. The RS products are then assessed according to spatial characteristics through analysing two land cover elements across Africa, irrigated areas and water bodies. A cluster analysis is also conducted to identify similarities between individual ET products. Finally, the RS products are evaluated against the Budyko equation. The results show that CMRSET, SSEBop and WaPOR rank highest in terms of estimation of long-term annual average mean ET across basins with low biases. Along with ETMonitor, the same three products rank highest in spatial distribution of ET patterns across Africa. GLEAM and MOD16 consistently rank the lowest in most criteria evaluation. Many of the products analysed in this study can be trusted depending on the study under question, keeping in mind some of these products have large biases in magnitude estimation. However our recommendation would be the three highest ranked products being CMRSET, SSEBop and WaPOR.

Imeshi Weerasinghe et al.
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Imeshi Weerasinghe et al.
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Short summary
Water resource allocation to different sectors requires an understanding of the hydrological cycle, where evapotranspiration (ET) is a key component. Satellite derived products estimate ET but are hard to evaluate at large scales. This study presents an alternate evaluation methodology to point scale observations in Africa. The paper enables users to select an ET product based on their performance to selected criteria using a ranking system. The highest ranked products are WaPOR and CMRSET.
Water resource allocation to different sectors requires an understanding of the hydrological...
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