Influence of Hillslope Flow on Hydroclimatic Evolution Under Climate Change

We analyzed the influence of hillslope flow on projections of climate change by comparing two transient climate simulations with the IPSL climate model between 1980 and 2100. Hillslope flow induces a reorganization and increment of soil moisture (+10%), which increases evapotranspiration (+4%) and precipitation (+1%) and decreases total runoff (-3%) and air temperature (-0.1 degrees C) on an annual average over land for 1980-2010 when compared to simulation not representing hillslope flow. These changes in land/atmosphere fluxes are not homogenous and depend on regional climate and surface conditions. Hillslope flow also influences climate change projections. On average over land, it amplifies the positive trend of soil moisture (+23%), evapotranspiration (+50%), and precipitation (+7%) and slightly attenuates global warming (-1%), especially for daily maximum air temperature. The role of hillslope flow in supporting surface/atmosphere fluxes is more evident at a regional scale. Where precipitation is projected to decrease, hillslope flow is shown to attenuate the related declines in evapotranspiration, precipitation, and total runoff, regardless of aridity conditions and mean air temperature. Where precipitation is projected to increase, hillslope flow amplifies evapotranspiration enhancement but attenuates the increase in precipitation and total runoff. Warming is generally attenuated, especially in semiarid and cold areas, and humid and warm/temperate regions, but the signal is weak. These results demonstrate the role of hillslope flow in enhancing water and energy fluxes between the surface and the atmosphere. They also suggest that including hillslope flow in climate models would weaken the projected intensification of hydrological extreme events.