Natural and human induced changes in the water cycle: Relative magnitudes and trends

Abstract:

Humans have manipulated much of the global land surface, the signature of which accelerated rapidly with the growth of global population over the last ~100 years. Only a modest part of the global land surface is now free of human effects, much of which have been for agriculture and pasture, and to a lesser extent human habitation in urban areas. These land surface changes have in turn affected the land surface water cycle, mostly via enhanced evapotranspiration from that portion of the agricultural area that is irrigated, and from storage of river runoff in reservoirs that provide most of the irrigation water. I evaluate the signature of these perturbations on the land surface water balance on a continent by continent basis, and for selected major river basins. I further evaluate the effect of anthropogenic perturbations in terms of their interseasonal and interannual magnitudes of variation relative to other terms in the land surface water cycle, including precipitation, evapotranspiration, runoff, and storage change associated with snow and soil moisture. I also evaluate the anthropogenic changes to date relative to projected changes over the next century associated with a warming climate as inferred from IPCC AR5 model output for selected large river basins.