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 °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. Plain Language Summary: We analyze how the flows caused by topography, called hillslope flow, affect the evolution of climate using simulations from a climate model. Results show that hillslope flow increases soil moisture in the valleys. More soil moisture enhances evapotranspiration and precipitation and decreases total runoff and air temperature for the period 1980–2010. But the increase in water exchanges between land and atmosphere is not homogenous. In the future, hillslope flow amplifies positive trends of climate change for soil moisture, evapotranspiration and precipitation, while global warming is minimally slowed. The role of hillslope flow in sustaining exchanges of water and energy in the future is most evident in the regions. Where precipitation decreases in the future, evapotranspiration and precipitation declines are less intense when hillslope flow is included, and that is the case for the decline in total runoff as well. In regions where precipitation increases in the future, evapotranspiration increases faster, but precipitation and runoff increases slower. Effect of hillslope flow on warming is weak, but in general, the air warms up more slowly, especially in both semiarid/cold regions, and humid and warm/temperate regions. The results highlight the role of hillslope flows in increasing water exchange between the surface and atmosphere Key Points: Hillslope flow sustains higher soil moisture, enhancing evapotranspiration and precipitation rates and cooling down the air temperatureHillslope flow slightly attenuates global warmingAt regional scale, hillslope flow attenuates climate change trends of all hydrological variables except for evapotranspiration increases [ABSTRACT FROM AUTHOR]