Understanding uncertainties in future evapotranspiration projections to study the impact of climate change on hydrology over France.

Air temperature increase due to climate change can lead to an increase in evapotranspiration, depending on whether the atmospheric demand is water or energy limited. This results in the intensification of the transfer from the continental hydrological compartment to the atmosphere. Investigating the uncertainties in the representation of potential evapotranspiration (PE) can provide a better understanding of the sensitivity of hydrological projections to atmospheric demand. This work summarizes our findings on different aspects about PE evolution under climate change and the uncertainties associated. First, we explored the relative importance of the contribution of PE formulations to the total uncertainty in PE projections, compared to the other steps in the modeling chain. Second, we evaluated the importance of the negative feedback of atmospheric CO₂ concentration on evapotranspiration for hydrological projections by modifying the stomatal resistance equation in the Penman-Monteith equation according to three formulations proposed in the literature to evaluate the sensitivity of hydrological projections to vegetation responses.

Regarding the importance of PE formulations on PE projections, it appears that the relative role of formulations is minor compared to other sources of uncertainty (namely climate models and greenhouse gases emission scenarios). Regarding the impact of CO₂, results highlight a zone of uncertain change from decreasing to increasing average annual runoff, depending on the emission scenario and stomatal resistance equation. Thus, in the north of France, where PE fluxes are energy-limited, various levels of runoff response are showed with three tested formulations. The use of a PE formulation with stomatal resistance as a function of atmospheric CO₂ could be used as forcing of a conceptual hydrological model, to include vegetation responses.