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https://doi.org/10.5194/bg-2020-275
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-275
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 Aug 2020

14 Aug 2020

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A revised version of this preprint is currently under review for the journal BG.

What determines the sign of the evapotranspiration response to afforestation in the European summer?

Marcus Breil1, Edouard L. Davin2, and Diana Rechid3 Marcus Breil et al.
  • 1Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 2Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland
  • 3Climate Service Center Germany, Helmholtz-Zentrum Geesthacht, Hamburg, Germany

Abstract. Uncertainties in the evapotranspiration response to afforestation constitute a major source of disagreement between model-based studies of the potential climate benefits of forests. Forests typically have higher evapotranspiration rates than grassland in the tropics, but whether this is also the case in the mid-latitudes is still debated. To explore this question and the underlying physical processes behind these varying evapotranspiration rates of forests and grasslands in more detail, a regional model study with idealized afforestation scenarios was performed for Europe. In the first experiment Europe was maximally forested and in the second one, all forests were turned into grassland.

The results of this modelling study exhibit the same contradicting evapotranspiration characteristics of forests and grasslands as documented in observational studies. But by means of an additional sensitivity simulation, in which the surface roughness of forest was reduced to grassland, the mechanisms behind these varying evapotranspiration rates could be revealed. Due to the higher surface roughness of a forest, solar radiation is more efficiently transformed into turbulent sensible heat fluxes, leading to lower surface temperatures (top of vegetation) than in grassland. The saturation deficit between the vegetation and the atmosphere, which depends on the surface temperature, is consequently reduced over forests. This reduced saturation deficit counteracts the transpiration facilitating characteristics of a forest (deeper roots, a higher LAI and lower albedo values than grassland). If the impact of the reduced saturation deficit exceeds the effects of the transpiration facilitating characteristics of a forest, evapotranspiration is reduced compared to grassland. If not, evapotranspiration rates of forests are higher. The interplay of these two counteracting factors depends on the latitude and the prevailing forest type in a region.

Marcus Breil et al.

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Marcus Breil et al.

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Short summary
The physical processes behind varying evapotranspiration rates of forests and grasslands in Europe are investigated in a regional model study with idealized afforestation scenarios. The results show that the evapotranspiration response to afforestation depends on the interplay of two counteracting factors; the transpiration facilitating characteristics of a forest and the reduced saturation deficits of forests caused by an increased surface roughness and associated lower surface temperatures.
The physical processes behind varying evapotranspiration rates of forests and grasslands in...
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