1
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Rivers in Crisis: Global Water Insecurity for 5
Humans and Biodiversity 6
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C. J. Vörösmarty
1
*, P. B. McIntyre
2
*, M. O. Gessner
3
, D. Dudgeon
4
, A. Prusevich
5
,12
P. Green
1
, S. Glidden
5
, S. E. Bunn
6
, C. A. Sullivan
7
, C. Reidy Liermann
8
, P. M. Davies
9
13
14
15
16
Submitted as an Article to: Nature 17
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13 June 2010 20
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Corresponding Author: 23
C. Vörösmarty <cvorosmarty@ccny.cuny.edu>24
Tel: +1-212-650-7042 Fax: -809725
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PRESENT ADDRESS INFORMATION 29
* These authors contributed equally to this work30
1
The Environmental Cross-Roads Initiative, City College of New York, City University of New York, 31
New York, NY 10035 (USA) 32
2
Center for Limnology & Department of Zoology, University of Wisconsin, Madison, WI 53706, and 33
School of Natural Resources & Environment, University of Michigan, Ann Arbor, MI 48109 (USA) 34
3
Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science & Technology, and 35
Institute of Integrative Biology (IBZ), ETH Zurich, 8600 Dübendorf (Switzerland) 36
4
Division of Ecology & Biodiversity, School of Biological Sciences, The University of Hong Kong, 37
Hong Kong SAR (China) 38
5
Water Systems Analysis Group, University of New Hampshire, Durham, NH 03834 (USA) 39
6
Australian Rivers Institute, Griffith University, Nathan, Queensland 4111 (Australia) 40
7
School of Environmental Science and Management, Southern Cross University, NSW 2480 (Australia) 41
8
School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA 98195 (USA) 42
9
Centre of Excellence in Natural Resource Management, The University of Western Australia, Albany 43
6330 (Australia) 44
45
This document is the accepted manuscript version of the following article:
Vörösmarty, C. J., McIntyre, P. B., Gessner, M. O., Dudgeon, D., Prusevich,
A., Green, P., … Davies, P. M. (2010). Global threats to human water security
and river biodiversity. Nature, 467, 555-561.
https://doi.org/10.1038/nature09440
1
SUMMARY 1
2
Protecting the world's surface water resources requires a diagnosis of threat over a broad range of 3
scales, from global to local. We present the first global synthesis to unite human and biodiversity 4
perspectives on water security using a spatial framework that quantifies multiple stressors and 5
accounts for downstream impacts. We find nearly 80% of world population is exposed to high 6
threat. Huge investments in water technology enable wealthy nations to offset high stressor 7
levels but without remedying their underlying causes and leaving the poor vulnerable. A similar 8
lack of precautionary investment jeopardises biodiversity, with habitats representing 65% of 9
continental discharge classified as moderately to highly threatened. The cumulative threat 10
framework offers a tool for prioritising policy and management responses to this crisis, and 11
demonstrates that limiting threats at their source rather than through costly remediation is an 12
effective strategy to assure global water security for both humans and aquatic biodiversity. 13
14
2
Water is widely regarded as the world’s most essential natural resource, yet freshwater systems 1
are directly threatened by human activities
1,2,3
and stand to be further impacted by anthropogenic 2
climate change
4
. Direct stressors include widespread land cover change, urbanisation, 3
industrialisation, and engineering schemes like reservoirs, irrigation, and interbasin transfers that 4
maximise human access to water
1,5
. The benefits of water provision on economic productivity
2,6
5
are often accompanied by impairment to ecosystems and biodiversity, with potentially grave but 6
unquantified costs
3,7,8
. Devising interventions to reverse these trends, such as conventions to 7
protect aquatic biodiversity
9,10
and ensure the sustainability of water delivery systems
11
, requires 8
frameworks to diagnose the primary threats to water security at a range of spatial scales from 9
local to global. 10
11
Water issues figure prominently in assessments of the state of human development
6
, ecosystem 12
services
3
, and their combination
12,13,14
. Yet global assessments of water resources
2
are 13
fragmented and generally confined to country-level statistics, seriously limiting efforts to 14
prioritise their protection and rehabilitation
15
. Spatially explicit analyses have taken 15
understanding of human impacts on the world’s oceans
16,17
and the human footprint on land
18
to 16
a new level, but have yet to be applied to the formal assessment process for freshwater resources 17
despite a recognised need
19,20
. 18
19
The success of integrated water management strategies depends on striking a balance between 20
human resource use and ecosystem protection
2,9,10,21
. To test whether this objective can be 21
advanced globally, we map Incident Threats to human water security (HWS) and biodiversity 22
(BD), where the term Incident refers to the exposure to a diverse array of stressors at a given 23
3
location. Many stressors threaten HWS and BD through similar pathways, as for pollution, but 1
they also influence water systems in distinct ways. Reservoirs, for example, convey few negative 2
effects on human water supply, but substantially impact aquatic biodiversity by impeding faunal 3
migration and changing flow regimes. Similarly, non-native species threaten BD but are typically 4
inconsequential to HWS. 5
6
We report here on a global-scale analysis of threats to freshwater that, for the first time, 7
considers human water security and biodiversity perspectives simultaneously within a spatial 8
accounting framework. Our focus is on rivers, which serve as the chief source of renewable 9
water supply for humans and freshwater ecosystems
2,3
. We use river networks to redistribute 10
distinctive HWS and BD stressors along a continuum from headwaters to ocean, capturing spatial 11
legacy effects ignored by earlier studies. Our framework incorporates all major classes of 12
anthropogenic drivers of stress and enables an assessment of their net impact under alternative 13
value systems for BD and HWS. Enhancing the spatial resolution by orders-of-magnitude over 14
prior studies (using 30' latitude/longitude grids) allows us to rigorously test prior assertions on 15
the state of world rivers and to identify key sources of threat at sub-national spatial scales that 16
are relevant to environmental management. Finally, we make the first spatial assessment of the 17
global benefits accrued from technological investments aimed at reducing threats to HWS, 18
revealing previously unrecognized consequences on people and biodiversity associated with 19
traditional water management approaches that are employed extensively over the global domain. 20
21
22
23
4
Global Patterns of Incident Threat 1
Using a global geospatial framework
22
, we aggregated the relative strength of individual 2
stressors to produce a cumulative Incident Threat index. The resulting maps reflect the central 3
role of hydrology in spatially configuring environmental impacts, with local stressor loads routed 4
downstream through digital river networks
23
and adjusted for new sources and dilution 5
(Supplementary Methods, SI Figure 1). Similar to the approach of Halpern et al.
16,17
for 6
marine systems, multiple stressors (expressed as 23 geospatial drivers under four themes) were 7
combined with relative weights to determine cumulative threat indices.
Expert assessment of 8
stressor impacts on HWS and BD produced two distinct weighting sets, which in turn yielded 9
separate maps of Incident Threat reflecting each perspective. 10
11
We find that nearly 80% (4.8Bn) of the world's population lives in areas where either Incident 12
HWS or BD Threat exceeds the 75
th
percentile. Regions of intensive agriculture and dense 13
settlement show high Incident Threat (Figure 1), as exemplified by much of the United States, 14
virtually all of Europe (excluding Scandinavia and northern Russia), and large portions of 15
Central Asia, the Middle East, the Indian subcontinent, and eastern China. Smaller contiguous 16
areas of high Incident Threat appear in central Mexico, Cuba, North Africa, Nigeria, South 17
Africa, Korea, and Japan. The impact of water scarcity accentuates threat to drylands, as is 18
apparent in the desert belt transition zones across all continents (e.g., Argentina, Sahel, Central 19
Asia, Australian Murray-Darling basin). Within the broad regions separating intensively settled 20
basins and remote areas, as in North America and northern Asia (Figure 1), Incident Threat 21
arises largely from transboundary atmospheric pollution. 22
23