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Discussion papers | Copyright
https://doi.org/10.5194/hess-2017-573
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Nov 2017

Research article | 06 Nov 2017

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

Local tower-based merging of two land evaporation products

Carlos Jiménez1,2, Brecht Martens3, Diego M. Miralles3, Joshua B. Fisher4, Hylke E. Beck5, and Diego Fernández-Prieto6 Carlos Jiménez et al.
  • 1Estellus, Paris, France
  • 2LERMA, Paris Observatory, Paris, France
  • 3Laboratory of Hydrology and Water Management, Ghent University, Ghent, Belgium
  • 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
  • 5Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA
  • 6ESRIN, European Space Agency, Frascati, Italy

Abstract. An inverse-variance weighting of two terrestrial evaporation (ET) products from the WACMOS-ET project based on FLUXNET sites is presented. The two ET models, PT-JPL and GLEAM, share a common modeling framework based on the Priestley–Taylor evaporation formulation, although the stress factors to convert from potential to actual evaporation are calculated differently. Both data sets are daily at 25km resolution and use common input data when possible. The weights are based on the variance of the difference between the tower ET and the modeled ET, and are made dynamic by estimating them using a 61-day running window centered on each day. Seasonal variability in these weights is observed over some stations, but the deviations from the 0.5 value assumed by the arithmetic mean of both products is typically small. Seasonal averaged statistics for three land cover groups confirm that the performance of the weighted product is very close to the arithmetic mean product. This implies that at a large number of stations the tower data was not informative enough to identify distinct error patterns between GLEAM and PT-JPL likely to result in more varying weights. Different factors can be responsible for this behavior. Concerning the model estimates, some of the shared modeling assumptions and the common inputs make the ET estimates from both models rather dependent, thus their errors are expected to be correlated. Regarding the tower data, the 84 selected tower sites are mostly at temperate regions. In these regions the models ET differences are mainly reflecting the variability in Priestley and Taylor estimates of potential ET, which translates in model ET differences smaller than in other places. The large mismatch between the model spatial resolution and the tower footprint is also likely to result again in common error patterns for the model estimates. Overall, this study suggests that merging tower observations and ET products at the time and spatial scales of this study is not straightforward, and that care should be taken regarding the dependence of the products to be merged, the tower spatial representativeness of their measurements at the products resolution, and the nature of the error in both towers and gridded data sets.

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Short summary
Observing the amount of water evaporated in nature is not easy, and we need to combine accurate local measurements with estimates from satellites, more uncertain but covering larger areas. This is the main topic of our paper, where local observations are compared with global land evaporation estimates, followed by a weighting of the global observations based on this comparison to derive a more accurate evaporation product.
Observing the amount of water evaporated in nature is not easy, and we need to combine accurate...
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