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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 24 Jan 2018

Research article | 24 Jan 2018

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

Global 5-km resolution estimates of secondary evaporation including irrigation through satellite data assimilation

Albert I. J. M. van Dijk1, Jaap Schellekens2,3, Marta Yebra1, Hylke E. Beck4, Luigi J. Renzullo1, Albrecht Weerts2,5, and Gennadii Donchyts2 Albert I. J. M. van Dijk et al.
  • 1Fenner School of Environment & Society, Australian National University, Canberra, ACT, Australia
  • 2Deltares, Delft, The Netherlands
  • 3Vandersat B.V., Haarlem, The Netherlands
  • 4Princeton University, Princeton, NJ, USA
  • 5Wageningen University & Research, Wageningen, The Netherlands

Abstract. A portion of globally generated surface and groundwater resources evaporates from wetlands, water bodies and irrigated areas. This secondary evaporation of blue water directly affects the remaining water resources available for ecosystems and human use. At the global scale, a lack of detailed water balance studies and direct observations limits our understanding of the magnitude and spatial and temporal distribution of secondary evaporation. Here, we propose a methodology to assimilate satellite-derived information into the landscape hydrological model W3 at an unprecedented 0.05° or c. 5 km resolution globally. The assimilated data are all derived from MODIS observations, including surface water extent, surface albedo, vegetation cover, leaf area index, canopy conductance, and land surface temperature (LST). The information from these products is imparted on the model in a simple but efficient manner, through a combination of direct insertion of surface water extent, evaporation flux adjustment based on LST, and parameter nudging for the other observations. The resulting water balance estimates were evaluated against river basin discharge records and the water balance of closed basins and demonstrably improved water balance estimates compared to ignoring secondary evaporation (e.g., bias improved from +38mm/d to +2mm/d). The evaporation estimates derived from assimilation were combined with global mapping of irrigation crops to derive a minimum estimate of irrigation water requirements (I0), representative of optimal irrigation efficiency. Our I0 estimates were lower than published country-level estimates of irrigation water use produced by alternative estimation methods, for reasons that are discussed. We estimate that 16% of globally generated water resources evaporate before reaching the oceans, enhancing total terrestrial evaporation by 6.11012m3y−1 or 8.8%. Of this volume, 5% is evaporated from irrigation areas, 58% from terrestrial water bodies and 37% from other surfaces. Model-data assimilation at even higher spatial resolutions can achieve a further reduction in uncertainty but will require more accurate and detailed mapping of surface water dynamics and areas equipped for irrigation.

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Albert I. J. M. van Dijk et al.
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Albert I. J. M. van Dijk et al.
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Short summary
Evaporation from wetlands, lakes and irrigation areas need to be measured to understand water scarcity. So far this has only been possible for small regions. Here, we develop a solution that can be applied at very high resolution globally, by making use of satellite observations. Our results show that 16 % of global water resources evaporate before reaching the ocean, most of it from surface water. Irrigation water use is less than 1 % globally but is a very large water user in several dry basins.
Evaporation from wetlands, lakes and irrigation areas need to be measured to understand water...