Advances in Geosciences www.adv-geosci.net 1680-7340 1680-7359 18 Advances and visions in large-scale hydrological modelling 2008 10.5194/adgeo-18-43-2008 http://www.adv-geosci.net/18/43/2008/ http://www.adv-geosci.net/18/43/2008/adgeo-18-43-2008.html http://www.adv-geosci.net/18/43/2008/adgeo-18-43-2008.pdf 43 50 2008-06-20 Human alterations of the terrestrial water cycle through land management S. Rost rost@pik-potsdam.de D. Gerten U. Heyder Potsdam Institute for Climate Impact Research, Telegrafenberg A62, 14473 Potsdam, Germany This study quantifies current and potential future changes in transpiration, evaporation, interception loss and river discharge in response to land use change, irrigation and climate change, by performing several distinct simulations within the consistent hydrology and biosphere modeling framework LPJmL (Lund-Potsdam-Jena managed Land). We distinguished two irrigation simulations: a water limited one in which irrigation was restricted by local renewable water resources (ILIM), and a potential one in which no such limitation was assumed but withdrawals from deep groundwater or remote rivers allowed (IPOT). We found that the effect of historical land use change as compared to potential natural vegetation was pronounced, including a reduction in interception loss and transpiration by 25.9% and 10.6%, respectively, whereas river discharge increased by 6.6% (climate conditions of 1991–2000). Furthermore, we estimated that about 1170 km³yr^−1 of irrigation water could be withdrawn from local renewable water resources (in ILIM), which resulted in a reduction of river discharge by 1.5%. However, up to 1660 km³yr^−1 of water withdrawals were required in addition under the assumption that optimal growth of irrigated crops was sustained (IPOT), which resulted in a slight net increase in global river discharge by 2.0% due to return flows. Under the HadCM3 A2 climate and emission scenario, climate change alone will decrease total evapotranspiration by 1.5% and river discharge by 0.9% in 2046–2055 compared to 1991–2000 average due to changes in precipitation patterns, a decrease in global precipitation amount, and the net effect of CO₂ fertilization. A doubling of agricultural land in 2046–2055 compared to 1991–2000 average as proposed by the IMAGE land use change scenario will result in a decrease in total evapotranspiration by 2.5% and in an increase in river discharge by 3.9%. That is, the effects of land use change in the future will be comparable in magnitude to the effects of climate change in this particular scenario. 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