The impact of global land-cover change on the terrestrial water cycle

Floods and droughts cause perhaps the most human suffering of all climate-related events; a major goal is to understand how humans alter the incidence and severity of these events by changing the terrestrial water cycle. Here we use over 1,500 estimates of annual evapotranspiration and a database of global land-cover change(1) to project alterations of global scale terrestrial evapotranspiration (TET) from current anthropogenic land-cover change. Geographic modelling reveals that land-cover change reduces annual TET by approximately 3,500 km(3) yr(-1) (5%) and that the largest changes in evapotranspiration are associated with wetlands and reservoirs. Land surface model simulations support these evapotranspiration changes, and project increased runoff (7.6%) as a result of land-cover changes. Next we create a synthesis of the major anthropogenic impacts on annual runoff and find that the net result is an increase in annual runoff, although this is uncertain. The results demonstrate that land-cover change alters annual global runoff to a similar or greater extent than other major drivers, affirming the important role of land-cover change in the Earth System(2-4). Last, we identify which major anthropogenic drivers to runoff change have a mean global change statistic that masks large regional increases and decreases: land-cover change, changes in meteorological forcing, and direct CO2 effects on plants.