Global-scale analysis of river flow alterations due to water withdrawals and reservoirs

摘要

Global-scale information on natural river flows and anthropogenic river flowalterations is required to identify areas where aqueous ecosystems areexpected to be strongly degraded. Such information can support theidentification of environmental flow guidelines and a sustainable watermanagement that balances the water demands of humans and ecosystems. Thisstudy presents the first global assessment of the anthropogenic alterationof river flow regimes, in particular of flow variability, by waterwithdrawals and dams/reservoirs. Six ecologically relevant flow indicatorswere quantified using an improved version of the global water modelWaterGAP. WaterGAP simulated, with a spatial resolution of 0.5 degree, riverdischarge as affected by human water withdrawals and dams around the year2000, as well as naturalized discharge without this type of humaninterference. Compared to naturalized conditions, long-term average globaldischarge into oceans and internal sinks has decreased by 2.7% due towater withdrawals, and by 0.8% due to dams. Mainly due to irrigation,long-term average river discharge and statistical low flow Q₉₀ (monthlyriver discharge that is exceeded in 9 out of 10 months) have decreased bymore than 10% on one sixth and one quarter of the global land area(excluding Antarctica and Greenland), respectively. Q₉₀ has increasedsignificantly on only 5% of the land area, downstream of reservoirs. Dueto both water withdrawals and reservoirs, seasonal flow amplitude hasdecreased significantly on one sixth of the land area, while interannualvariability has increased on one quarter of the land area mainly due toirrigation. It has decreased on only 8% of the land area, in areasdownstream of reservoirs where consumptive water use is low. The impact ofreservoirs is likely underestimated by our study as small reservoirs are nottaken into account. Areas most affected by anthropogenic river flowalterations are the Western and Central USA, Mexico, the western coast ofSouth America, the Mediterranean rim, Southern Africa, the semi-arid andarid countries of the Near East and Western Asia, Pakistan and India,Northern China and the Australian Murray-Darling Basin, as well as someArctic rivers. Due to a large number of uncertainties related e.g. to theestimation of water use and reservoir operation rules, the analysis isexpected to provide only first estimates of river flow alterations thatshould be refined in the future.