Abstract
Abstract. 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 Furthermore, we estimated that about 1170 km3yr−1 of irrigation water could be withdrawn from local renewable water resources (in ILIM), which resulted in a reduction of river discharge by 1.5%.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. On present irrigated areas future water withdrawal will increase especially in regions where climate changes towards warmer and dryer conditions will be pronounced.
Highlights
There is growing evidence that humans are altering the global water cycle to an unprecedented and globally visible scale (Vorosmarty et al, 1997; Vorosmarty and Sahagian, 2000; Nilsson et al, 2005; Hanasaki et al, 2006; Haddeland et al, 2006; Shiklomanov and Rodda, 2003)
We found that land cover change alone (INO minus PNV) decreased interception loss globally by 25.9%, which reflects the decrease in canopy closure and in growing season as compared to the forest vegetation in the absence of human land use
The effect of deforestation resulted in an increase of river discharge of 2349 km3yr−1 in the INO simulation compared to PNV (Table 1)
Summary
There is growing evidence that humans are altering the global water cycle to an unprecedented and globally visible scale (Vorosmarty et al, 1997; Vorosmarty and Sahagian, 2000; Nilsson et al, 2005; Hanasaki et al, 2006; Haddeland et al, 2006; Shiklomanov and Rodda, 2003). One reason for these anthropogenic disturbances is the production of food and other commodities through irrigation; agriculture represents the sector with the largest water demand.
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