Abstract
Abstract. Global-scale information on natural river flows and anthropogenic river flow alterations is required to identify areas where aqueous ecosystems are expected to be strongly degraded. Such information can support the identification of environmental flow guidelines and a sustainable water management that balances the water demands of humans and ecosystems. This study presents the first global assessment of the anthropogenic alteration of river flow regimes, in particular of flow variability, by water withdrawals and dams/reservoirs. Six ecologically relevant flow indicators were quantified using an improved version of the global water model WaterGAP. WaterGAP simulated, with a spatial resolution of 0.5 degree, river discharge as affected by human water withdrawals and dams around the year 2000, as well as naturalized discharge without this type of human interference. Compared to naturalized conditions, long-term average global discharge into oceans and internal sinks has decreased by 2.7% due to water withdrawals, and by 0.8% due to dams. Mainly due to irrigation, long-term average river discharge and statistical low flow Q90 (monthly river discharge that is exceeded in 9 out of 10 months) have decreased by more than 10% on one sixth and one quarter of the global land area (excluding Antarctica and Greenland), respectively. Q90 has increased significantly on only 5% of the land area, downstream of reservoirs. Due to both water withdrawals and reservoirs, seasonal flow amplitude has decreased significantly on one sixth of the land area, while interannual variability has increased on one quarter of the land area mainly due to irrigation. It has decreased on only 8% of the land area, in areas downstream of reservoirs where consumptive water use is low. The impact of reservoirs is likely underestimated by our study as small reservoirs are not taken into account. Areas most affected by anthropogenic river flow alterations are the Western and Central USA, Mexico, the western coast of South America, the Mediterranean rim, Southern Africa, the semi-arid and arid countries of the Near East and Western Asia, Pakistan and India, Northern China and the Australian Murray-Darling Basin, as well as some Arctic rivers. Due to a large number of uncertainties related e.g. to the estimation of water use and reservoir operation rules, the analysis is expected to provide only first estimates of river flow alterations that should be refined in the future.
Highlights
Freshwater ecosystems, which include rivers, wetlands and lakes as well as their floodplains, have been deteriorating faster than other ecosystems
According to WGHM, the damming of a few lakes (e.g. Lake Baikal in Russia) has caused a small increase of discharge compared to the situation without a dam, i.e. if they are modeled like natural lakes (Fig. 4c)
The question is: how well can we estimate river flow alterations due to human water use and dams with WGHM? As four of the six indicators presented here are based on mean monthly river discharges, we looked at mean monthly river discharge at four gauging stations for which observations and partly independent estimates of naturalized flows were available
Summary
Freshwater ecosystems, which include rivers, wetlands and lakes as well as their floodplains, have been deteriorating faster than other ecosystems. 20% of the world’s 10 000 described freshwater fish species have been listed as threatened, endangered, or extinct in the last few decades. Over one third (37%) of the freshwaterdependent mammal species assessed for the IUCN Red List were globally threatened, including manatees, river dolphins and porpoises (Millenium Ecosystem Assessment, 2005). About one half of the 200 species of freshwater turtles were classified as globally threatened in the IUCN Red List. Threats to global freshwater biodiversity can be grouped under five categories: overexploitation, invasion by exotic species, water pollution, destruction or degradation of habitat (e.g. related to changes in fluvial morphology), and alteration of flow regimes (Dudgeon et al, 2006). While the first two relate to the biotic component of freshwater ecosystems, the latter affect the abiotic component
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