Global Threats to Human Water Security and River Biodiversity

Abstract

Protecting the world’s freshwater resources requires diagnosing threats over a broad range of scales, from global to local. Here we present the first worldwide synthesis to jointly consider human and biodiversity perspectives on water security using a spatial framework that quantifies multiple stressors and accounts for downstream impacts. We find that nearly 80% of the world’s population is exposed to high levels of threat to water security. Massive investment in water technology enables rich nations to offset high stressor levels without remedying their underlying causes, whereas less wealthy nations remain vulnerable. A similar lack of precautionary investment jeopardizes biodiversity, with habitats associated with 65% of continental discharge classified as moderately to highly threatened. The cumulative threat framework offers a tool for prioritizing policy and management responses to this crisis, and underscores the necessity of limiting threats at their source instead of through costly remediation of symptoms in order to assure global water security for both humans and freshwater biodiversity.

Full text

ARTICLE doi:10.1038/nature09440
Global threats to human water security
and river biodiversity
C. J. Vo
¨ro
¨smarty
1
*, P. B. McIntyre
2
*{, M. O. Gessner
3
, D. Dudgeon
4
, A. Prusevich
5
, P. Green
1
, S. Glidden
5
, S. E. Bunn
6
,
C. A. Sullivan
7
, C. Reidy Liermann
8
& P. M. Davies
9
Protecting the world’s freshwater resources requires diagnosing threats overa broad range of scales, from global to local.
Here we present the first worldwide synthesis to jointly consider human and biodiversity perspectives on water security
using a spatial framework that quantifies multiple stressors and accounts for downstream impacts. We find that nearly
80% of the world’s population is exposed to high levels of threat to water security. Massive investment in water
technology enables rich nations to offset high stressor levels without remedying their underlying causes, whereas
less wealthy nations remain vulnerable. A similar lack of precautionary investment jeopardizes biodiversity, with
habitats associated with 65% of continental discharge classified as moderately to highly threatened. The cumulative
threat framework offers a tool for prioritizing policy and management responses to this crisis, and underscores the
necessity of limiting threats at their source instead of through costly remediation of symptoms in order to assure global
water security for both humans and freshwater biodiversity.
Water is widely regarded as the most essential of natural resources, yet
freshwater systems are directly threatened by human activities
1–3
and
stand to be further affected by anthropogenic climate change
4
.Water
systems are transformed through widespread land cover change, urb-
anization, industrialization and engineering schemes like reservoirs,
irrigation and interbasin transfers that maximize human access to
water
1,5
. The benefits of water provision to economic productivity
2,6
are
often accompanied by impairment to ecosystems and biodiversity, with
potentially serious but unquantified costs
3,7,8
. Devising interventions to
reverse these trends, including conventions
9
and scientific assessments
10
to protect aquatic biodiversity and ensure the sustainability of water
delivery systems
11
, requires frameworks to diagnose the primary threats
to water security at a range of spatial scales from local to global.
Water issues feature prominently in assessments of economic
development
6
, ecosystem services
3
, and their combination
12–14
.
However, worldwide assessments of water resources
2
rely heavily on
fragmented data often expressed as country-level statistics, seriously
limiting efforts to prioritize their protection and rehabilitation
15
.
High-resolution spatial analyses have taken understanding of the
human impact on the world’s oceans
16,17
and the human footprint on
land
18
to a new level, but have yet to be applied to the formal assessment
process for freshwater resources
2
despite a recognized need
19,20
.
The success of integrated water management strategies depends on
striking a balance between human resource use and ecosystem pro-
tection
2,9,10,21
. To test the degree to which this objective has been
advanced globally, and to assess its potential value in the future,
requires systematic accounting. An important first step is to develop
a spatial picture of contemporary incident threats to human water
security and biodiversity, where the term ‘incident’ refers to exposure
to a diverse array of stressors at a given location. Many stressors
threaten human water security and biodiversity through similar
pathways, as for pollution, but they also influence water systems in
distinct ways. Reservoirs, for example, convey few negative effects on
human water supply, but substantially impact on aquatic biodiversity
by impeding the movement of organisms, changing flow regimes and
altering habitat. Similarly, non-native species threaten biodiversity
but are typically inconsequential to human water security.
Here we report the results of a global-scale analysisof threats to fresh
water that, for the first time, considers human water security and
biodiversity perspectives simultaneously within a spatial accounting
framework. Our focus is on rivers, which serve as the chief source of
renewable water supply for humans and freshwater ecosystems
2,3
.We
use river networks to redistribute the distinctive impacts of stressors on
human water security and biodiversity along a continuum from head-
waters to ocean, capturing spatial legacy effects ignored by earlier
studies. Our framework incorporates all major classes of anthro-
pogenic drivers of stress and enables an assessment of their aggregate
impact under often divergent value systems for biodiversity and
human water security. Enhancing the spatial resolution by orders-of-
magnitude over previous studies (using 309latitude/longitude grids)
allows us to more rigorously test previous assertions on the state of the
world’s rivers and to identify key sources of threat at sub-national
spatial scales that are useful for environmental management. Finally,
we make the first spatial assessment of the benefits accrued from tech-
nological investments aimedat reducing threats to human water secur-
ity, revealing previously unrecognized, global-scale consequences of
local water management practices that are used extensively worldwide.
Global patterns of incident threat
Using a global geospatial framework
22
, we merged a broad suite of
individual stressors to produce two cumulative incident threat indices,
one for human water security and one for biodiversity. The resulting
1
The Environmental CrossRoads Initiative, City University of New York, The City College of New York, New York, New York 10035, USA.
2
School of Natural Resources and Environment, University of Michigan,
Ann Arbor, Michigan 48109, USA.
3
Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, and Institute ofIntegrative Biology (IBZ), ETH Zurich,8600 Du
¨bendorf,
Switzerland and Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), 16775 Stechlin, Germany.
4
Division of Ecology and Biodiversity, School of Biological Sciences, The University of Hong
Kong, Hong Kong SAR, China.
5
Water Systems Analysis Group, University of New Hampshire, Durham, New Hampshire 03824, USA.
6
Australian Rivers Institute, Griffith University, Nathan, Queensland
4111, Australia.
7
School of Environmental Science and Management, Southern Cross University, New South Wales 2480, Australia.
8
School of Aquatic and Fishery Sciences, University of Washington,
Seattle, Washington 98195, USA.
9
Centre of Excellence in Natural Resource Management, The University of Western Australia, Albany 6330, Australia. {Present address: Center for Limnology, University of
Wisconsin, Madison, Wisconsin 53706, USA. *These authors contributed equally to this work.
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maps reflect the central role of hydrology in spatially configuring
environmental impacts, with local stressor loads routed downstream
through digital river networks
23
and adjusted for new sources and
dilution (Supplementary Methods and Supplementary Fig. 1).
Similar to an approach used for marine systems
16,17
, multiple stressors
were combined using relative weights to derive cumulative threat
indices. Stressors were expressed as 23 geospatial drivers organized
under four themes (catchment disturbance, pollution, water resource
development and biotic factors). Expert assessment of stressor impacts
on human water security and biodiversity produced two distinct
weighting sets, which in turn yielded separate maps of incident threat
reflecting each perspective.
We find that nearly 80% (4.8 billion) of the world’s population (for
2000) lives in areas where either incident human water security or
biodiversity threat exceeds the 75th percentile. Regions of intensive
agriculture and dense settlement show high incident threat (Fig. 1), as
exemplified by much of the United States, virtually all of Europe
(excluding Scandinavia and northern Russia), and large portions of
central Asia, the Middle East, the Indian subcontinent and eastern
China. Smaller contiguous areas of high incident threat appear in
central Mexico, Cuba, North Africa, Nigeria, South Africa, Korea
and Japan. The impact of water scarcity accentuates threat to dry-
lands, as is apparent in the desert belt transition zones across all
continents (for example, Argentina, Sahel, Central Asia, Australian
Murray–Darling basin).
Spatial differentiation of incident threat also arises from the inter-
action of multiple factors. China’s arid western provinces would be
expected to show high threat due to minimal dilution potential, but
sparse population and limited economic activity combine to keep
indices low. In contrast, heavily populated and developed eastern
China shows substantially higher threat, despite greater rainfall and
dilution capacity, especially within the Yangtze basin. Other large
rivers are incapable of fully attenuating the impacts of concentrated
development. Over 30 of the 47 largest rivers, which collectively dis-
charge half of global runoff to the oceans, show at least moderate
threat levels (.0.5) at river mouth, with eight rivers (for human water
security) and fourteen (for biodiversity) showing very high threat
(.0.75).
A strikingly small fraction of the world’s rivers remain unaffected
by humans. Remote areas of the world including the high north
(Siberia, Canada, Alaska) and unsettled parts of the tropical zone
(Amazonia, northern Australia) show the lowest threat levels.
Across remote areas (Fig. 1), incident threat arises largely from
trans-boundary atmospheric pollution. A mere 0.16% of the Earth’s
area experiences low scores for every contributing stressor (that is,
lowest decile globally).
Upstream–downstream transects of incident threat yield signatures
of human water security or biodiversity conditions unique to each
river that arise from the action of hydrology and networked flow paths
(Fig. 2). Such transects highlight the diversity of stressors in river
No appreciable ow
No appreciable ow
0.2 0.6
0.4 0.80
1
Incident human water
security threat
0.2 0.6
0.4 0.80
1
Incident biodiversity
threat
Figure 1
|
Global geography of incident threat to human water security and
biodiversity. The maps demonstrate pandemic impacts on both human water
security and biodiversity and are highly coherent, although not identical
(biodiversity threat 50.964 3human water security threat 10.018; r50.97,
P,0.001). Spatial correlations among input drivers (stressors) varied, but were
generally moderate (mean
|
r
|
50.34; n5253 comparisons). Regional maps
exemplify main classes of human water security threat (see main text and
Supplementary Fig. 4). Spatial patterns proved robust in a varietyof sensitivity
tests (Supplementary Methods and Supplementary Discussion). Threat indices
are relative and normalized over discharging landmass.
RESEARCH ARTICLE
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systems, combining the accumulation of diffuse non-point source
pollutants with dilution by less impacted tributaries, often punctuated
by point sources from large urbanized areas. Levels of threat often
grow in the downstream direction (for example, the Huang He and
Nile rivers), indicating the accumulation of residual stressor impacts
generated upstream and augmented by dense development along
major river corridors. The Amazon shows the reverse, with impacts
from human-dominated source areas in Peru and Bolivia persisting
but progressively diluted downstream. Even sparsely settled basins
like the Lena in Siberia with generally low threat can show the impact
of development near the river mouth. The proliferation of densely
settled areas in the coastal zone including mega-cities means that its
many rivers show high threat over virtually their entire length (for
example, Paraı
´ba do Sul (Sa
˜o Paulo state), Pasig (Manila), Ogun
(Lagos)).
Our results agree with recent field surveys, underscoring the dire
state of river health. Recent sampling of rivers across the United States
showed impairment across 750,000 km (50%) of sampled river length
and demonstrated the coincidence of multiple stressors, with agricul-
tural factors predominant
24
. In China, 45% of major river reaches
surveyed in 2008 were moderately to badly polluted
25
. Reviews of
global pollution based on water monitoring
26
and modelling studies
27
have shown broadly similar patterns to our threat maps. Our results
are also congruent with previous threat assessments conducted at the
coarser catchment and ecoregional scales
7,28
(Supplementary
Discussion), yet provide the much greater levels of spatial detail
needed for environmental planning and management.
Despite the variety of stressors that we considered, our study and all
previous assessments
7,28
of anthropogenic impacts are conservative
owing to insufficient information on pharmaceutical and other syn-
thetic compounds, mining, interbasin water transfers, and other com-
monplace stressors
1,3
. Our current inability to account for in-stream
transformations, stressor synergies
21
, concentrated impacts during
low flow periods, and threats to smaller streams (#Strahler order 5;
1:62,500 scale)
23
are additional limitations. Finally, uncertainties in
stressor data are inevitable, but our standardization procedures lim-
ited their influence on our results (Supplementary Information).
Chief determinants of global threat
Globally, the catchment disturbance, pollution, and water resource
development themes are spatially well correlated (r$0.75 for human
water security, P,0.001; r$0.62 for biodiversity, P,0.001;
n546,517 grid cells), reflecting congruent gradients of human activ-
ities and their impacts (Supplementary Table 3). Biotic factors are less
strongly correlated with other themes (r#0.37 for human water
security, P,0.001; r#0.44 for biodiversity, P,0.001), reflecting
the spatial decoupling of fish species introductions from human
population density (Supplementary Table 3) and the broad distri-
bution of inland fisheries. Incident threats to human water security
and biodiversity are themselves well correlated (Fig. 1), with the high-
est levels in heavily settled regions.
In areas of high incident threat (.0.75), water resource develop-
ment and pollution are dominant contributing themes for both
human water security and biodiversity (Fig. 3), and they typically
occur together. Their combined importance derives from the water-
borne nature of the stressors: water pollution distributed throughout
the world’s rivers is broadly coincident with the widespread presence
of engineering works that enable the overuse and mismanagement of
water in many locations. Catchment disturbance and biotic factors
have a secondary role in high incident threat areas as their stressors
often represent more localized effects.
High levels of incident human water security and biodiversity
threat emerge only from the spatial concordance of high scores for
many stressors (Fig. 3). Stressors within the catchment disturbance
and pollution themes generally act in unison across human water
security and biodiversity, highlighting shared sources of impact, with
cropland the predominant catchment stressor and nutrient, pesticide
and organic loads dominating pollution sources. For the remaining
themes, stressors act more independently, reflecting distinctions
between human water security and biodiversity perspectives.
Stressors associated with impoundments and flow depletion are the
clearest sources of biodiversity threat by directly degrading habitat,
while negligibly affecting human water security. These results high-
light the diverse and unique sets of stressor impacts confronting
rehabilitation efforts in high impact areas, and argue for replacing
current fragmentary approaches to management with integrative
strategies that deliberately alleviate multiple sources of threat
29
.
Reducing threats to human water security
Our incident threat maps do not reflect technological investments
that can improve human water security. To capture this effect, we
derived an ‘investment benefits factor’, depicting supply stabilization,
improved water services and access to waterways, then used it to
calculate an ‘adjusted human water security threat’. Comparison of
incident and adjusted human water security threats reveals that tech-
nological investments produce globally significant, positive impacts
on human water security and substantially reconfigure exposure to
threat (Fig. 4 and Supplementary Information). Highly developed
Distance to ocean (km)
Huang He
3,500 3,000 2,500 2,000 1,500 1,000 500 0
0
0.2
0.4
0.6
0.8
1
Amazon
4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000
500 0
0
0.2
0.4
0.6
0.8
1
Lena
4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0
Uganda
Mixture of
White + Blue
Nile waters
Ethiopia
Nile
Lake
Victoria
headwaters Settlements
on mainstem Aswan
High Dam
Cairo
Egypt
Natl
parks
Sudan
Atbara R.
4,000 3,000 2,000 1,000 05,000
Biodiversity threat index
Figure 2
|
Incident biodiversity threat transects from headwaters to ocean.
Distinctive patterns characterize each river system resulting from complex
spatial patterns of stressor loadings across the catchment plus mixing of higher
and lower concentration tributary waters through river networks. Transects
represent the collective impact of stressors operating within particular
development settings, and thus serve to diagnose the chief factorsgiving rise to
threat or to identify critical areas at risk, as shown for the Nile (Natl, National).
Threat indices depict conditions over the full basin at set distances from river
mouth, but can be reconfigured to track individual reaches or tributary sub-
basins.
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regions with high incident threat (for example, United States, Western
Europe) often show much lower adjusted threat indices, gaining bene-
fit from massive investments in water infrastructure, the total value of
which is in the trillions of US dollars
2,3,30
. Investments by high-income
countries benefit 850 million people, lowering their exposure to high
incident threat by 95%, with corresponding values for upper middle-
income countries of 140 million and 23% (Table 1). Minimal invest-
ment in developing countries means that their vulnerability remains
high, with 3.4 billion people in these regions residing in areas showing
the highest adjusted threat category.
Our analysis is a spatial expression of the many water security
challenges facing the world’s poor, as identified in case studies, docu-
mentary evidence and global, although fragmentary, data
2,6,12
(Fig. 4).
Most of Africa, large areas in central Asia and countries including
China, India, Peru, or Bolivia struggle with establishing basic water
services like clean drinking water and sanitation
31
, and emerge here as
regions of greatest adjusted human water security threat. Lack of
water infrastructure yields direct economic impacts. Drought- and
famine-prone Ethiopia, for example, has 150 times less reservoir stor-
age per capita than North America
32
and its climate and hydrological
3,000
2,000
1,000
0
Pollution Watershed
disturbance
Water resource
development
Human water security
Biodiversity
Biotic
factors
0
1,000 2,000 3,000
Wetland
disconnectivity
Dam density
River fragmentation
Consumptive water loss
Human water stress
Agricultural water stress
Flow disruption
Soil salinization
Nitrogen loading
Phosphorus loading
Mercury deposition
Pesticide loading
Sediment loading
Organic loading
Potential acidication
Thermal alteration
Non-native shes (%)
Non-native shes (no.)
Fishing pressure
Aquaculture pressure
Cropland
Livestock density
Impervious surfaces
Water resource developmentWatershed disturbance
Biotic factors
Pollution
0
1,000 2,000 3,000
0
1,000 2,000 3,000
0
1,000 2,000 3,000
Aggregate score
A
gg
re
g
ate score
Aggregate score
Aggregate score
A
gg
re
g
ate score
Figure 3
|
Theme and driver contributions in areas where incident threat
exceeds the 75th percentile. High incident threat typically arises from the
spatial coincidenceof multiple themes and/or drivers of stress acting in concert.
Each aggregate score represents the number of grid cells exceeding the 75th
percentile for each individual theme or driver over the high incident threat
areas. Influence of each of the four themes (left)is relative to its contribution to
overall incident threat. For the individual drivers (right), scores are relative to
other drivers in the same theme. Bars summarize results over the entire
discharging landmass.
Relative
re-scaling
No appreciable ow
Incident
HWS threat
Investment
benets factor
Reduced
HWS threat
Adjusted
HWS threat
Adjusted HWS threat
×
0.2 0.6
0.4 0.80
1 0.2 0.6
0.4 0.80
10.2 0.6
0.4 0.80
1 0.2 0.6
0.4 0.80
1
0.2 0.6
0.4 0.80
1
Figure 4
|
Shifts in spatial patterns of relative human water security threat
after accounting for water technology benefits. Inset maps illustrate the
analytical approach and net impact of investment over a north–south transect
(top). Incident human water security (HWS) threat is converted to reduced
threat (inset maps), which is then globally re-scaled into adjusted human water
security threat. The final map shows relative units: areas with substantial
technology investments have effectively limited exposure to threat whereas
regions with little or no investment become the most vulnerable in a global
context. Colour spectra depictthree measures of threat (increasing, blue to red)
and investment benefits (increasing, light to dark).
RESEARCH ARTICLE
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variability takes a 38% toll on gross domestic product (GDP)
2
. The
number of people under chronically high water scarcity, many of
whom are poor, is 1.7 billion or more globally
2,3,15
, with 1.0 billion
of these living in areas with high adjusted human water security threat
(.0.75).
Contrasts between incident and adjusted human water security
threat are striking when considered relative to national wealth.
Incident human water security threat is a rising but saturating func-
tion of per capita GDP, whereas adjusted human water security threat
declines sharply in affluent countries in response to technological
investments (Fig. 5). The latter constitutes a unique expression of
the environmental Kuznets curve
33
, which describes rising ambient
stressor loads during early-to-middle stages of economic growth
followed by reduced loading through environmental controls insti-
tuted as development proceeds. The concept applies well to air pollu-
tants that directly expose humans to health risks, and which can be
regulated at their source
33
. The global investment strategy for human
water security shows a distinctly different pattern. Rich countries
tolerate relatively high levels of ambient stressors, then reduce their
negative impacts by treating symptoms instead of underlying causes
of incident threat.
The biodiversity dilemma
We find that 65% of global river discharge, and the aquatic habitat
supported by this water, is under moderate to high threat (.0.5). Yet,
we were unable to compute a globally meaningful estimate of adjusted
biodiversity threat due to the paucity of relevant data but also the
reality that much less comprehensive investment has been directed
to biodiversity conservation than to human water security
34,35
.
Limited global investment in environmental protection and rehab-
ilitation means that stresses on biodiversity for many locations go
unabated. In addition, the substantial reductions in incident human
water security threat through point-of-service strategies emphasizing
water supply stabilization and delivery incorporate some of the very
factors that negatively impact biodiversity through flow distortion
and habitat loss. This helps to explain why environmental Kuznets
curve benefits that typically rise with increasing levels of affluence do
not necessarily hold for fish biodiversity
36
or water quality
33
, and why
river restoration efforts often fail
29
. Indeed, Europe still suffers sig-
nificant biodiversity threat despite concerted, high-level efforts aimed
at achieving the contrary
35,37
.
The worldwide pattern of river threats documented here offers the
most comprehensive explanation so far of why freshwater biodiversity
is considered to be in a state of crisis
38–41
. Estimates suggest that at least
10,000–20,000 freshwater species are extinct or at risk
8,42
, with loss
rates rivalling those of previous transitions between geological epochs
like the Pleistocene-to-Holocene
43
. Although we have not established
causality, our results establish a precursor to future studies that could
link the role of stressors to biodiversity loss more directly.
Rising to a dual challenge
Given escalating trends in species extinction, human population, cli-
mate change, water use and development pressures
44
, freshwater sys-
tems will remain under threat well into the future. Without major
policy and financial commitments, stark contrasts in human water
security will continue to separate rich from poor. We remain off-pace
for meeting the Millennium Development Goals for basic sanitation
services
31
, a testament to the lack of societal resolve, when one con-
siders that a century of engineering know-how is available and returns
on investment in facilities are high
2
. For Organisation for Economic
Co-operation and Development (OECD) and BRIC (Brazil, Russia,
India and China) countries alone, 800 billion US dollars per year will
be required in 2015 to cover investments in water infrastructure, a
target likely to go unmet
30
. The situation is even more daunting for
biodiversity. International goals for its protection lag well behind
Table 1
|
Reconfiguring global exposure to incident human water security threat through technology investments
Income level*GDP (PPP){
(10
3
US dollars per capita)
Global population
by income level{(%)
Fraction of population within each income level{where HWS threat .0.75
Incident HWS threat (%) Adjusted HWS threat (%)
Low ,1 7 43 96
Lower middle 1–5 61 85 88
Upper middle 5–10 14 79 61
High .10 18 90 5
Percentages were determined by summing populations within national-scale designations of income that were exposed initially to high levels of incident human water security (HWS) threat and then residual
adjusted human water security threat, after benefits were tabulated and results re-scaled globally. Differences in the last two columns indicate a major global-scale realignment of relative risk, with human water
security most assured for wealthy nations and least so for the world’s poor. Investments are represented by existing infrastructure comprising water supply, use and delivery services, plus access to waterways
(specific driver data sets and calculation procedures used are given in Supplementary Methods ‘Overview’).
*Approximated from World Bank categories
50
.
{Classifications are for 2008
50
.
{Computed over the discharging landmass.
GDP (PPP) 103 US dollars per capita
Incident threat
Reduced threat
Benecial effect
of investments
< 0.5
1.0–
2
.5
2.5–
5
.0
0
.5
–
1.
0
5.0
–
7
.5
7.
5
–1
0
1
0
–2
5
>2
5
Incident
Adjusted
Relative threat to water security
1.0
0.5
0.0
0.0 <0.25 <0.5 <0.75 <1.0
Null expectation
Fraction of global population
Threat to human water security
1.0
0.5
0.0
Figure 5
|
Globally aggregatedhuman water security threat indices linked to
population and level of economic development. Investments in engineering
infrastructure and services improve water security, with their value expressed
here in reduced threat units. Net benefits accrue to only a fraction of global
population (top). Technology investments greatly benefit wealthy nations,
shifting them from most to least threatened (bottom). The fraction of global
population is over the discharging landmass. GDP (PPP) refers to annual gross
domestic product in 2008 at purchasing power parity exchange
50
, with
associated grid-cell means of incident human water security threat (red bars)
and reduced threat (yellow; see Fig. 4). Vertical lines represent ranges.
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expectation and global investments are poorly enumerated but likely
to be orders of magnitude lower than those for human water secur-
ity
35,45
, leaving at risk animal and plant populations, critical habitat
and ecosystem services that directly underpin the livelihoods of many
of the world’s poor
46
. Left unaddressed, these linked human water
security–biodiversity water challenges are forecast to generate social
instability of growing concern to civil and military planners
47
.
Our threat maps enable spatial planning to enhance water security
for humans and nature
16
. Although our intent is not to develop formal
priorities to mitigate risk, we present a final analysis that is instructive
in considering options. Comparing adjusted human water security to
incident biodiversity threats highlights regions where either human
water security or biodiversity challenges, or their conjunction, pre-
dominate (Fig. 6). Such patterns are important to identify because the
main stressors determining human water security and biodiversity
threat are sometimes distinct, thus requiring different and potentially
conflicting management solutions (Fig. 3).
In remote areas with low indices of both human water security and
biodiversity threat, preserving critical habitat and ecosystem pro-
cesses may be the single best strategy to contain future risk, yet the
issue of who will pay for such protection is unresolved
34,45
. Solutions
for densely settled regions will be more elusive. Although there may be
easy consensus on controlling factors that lead to both human water
security and biodiversity threat (for example, pollution), the decision
to construct large-scale dams is a prime example of how development
pressure is often at odds with biodiversity conservation and thus more
contentious
11,48
. In populated regions of the developed world, existing
human water security infrastructure will require re-engineering to
protect biodiversity while retaining human water services. Across
the developing world, establishing human water security for the first
time while preserving biodiversity constitutes a dual challenge, best
met through integrated water resource management
2
that expressly
balances the needs of humans and nature. Although our results offer
prima facie evidence that society has failed to institute this principle
broadly, there are promising, cost-effective approaches to preserve
and rehabilitate ecosystems
29
. Engineers, for instance, can re-work
dam operating rules to maintain economic benefits while simulta-
neously conveying adaptive environmental flows for biodiversity
49
.
Protecting catchments reduces costs for drinking water treatment,
whereas preserving river floodplains sustains valuable flood protec-
tion and rural livelihoods
3
. Such options offer developing nations the
opportunity to avoid the high environmental, economic and social
costs that heavily engineered water development systems have pro-
duced elsewhere
11
.
The need to mobilize financial resources to support integrated
approaches remains urgent, lest further deterioration of fresh water
becomes the accepted norm
2,34
. Habitat monitoring
24–26
and spatially
explicit species inventories
7
are essential in evaluating the success of
investments
31,34
and detecting the emergence of new challenges. Trade-
offs and difficult choices involving competing stakeholders are already
commonplace
2,3,48
and resolving these dilemmas more effectively
requires high-resolution spatial approaches that engage policymakers
and water managers at scales relevant to their decisions, including sub-
national administrative units, river basins and individual stream
reaches. Uniting our current approach with ocean-based assess-
ments
16,17
will identify areas where improved freshwater and land
management would benefit the world’s impaired coastal zones. If cli-
mate mitigation is any guide, a generational timeframe may be neces-
sary to stimulate sufficient political willpower to address the global
river health challenge. In the meantime, a substantial fraction of the
world’s populationand countless freshwater species remainimperilled.
METHODS SUMMARY
Maps of incident threat to river systems were based on spatially explicit data
depicting 23 stressors (drivers), grouped into four major themes representing
environmental impact. We chose drivers based on their documented role in
degrading river systems and the availability of global-scale information with
sufficient fidelity and spatial resolution. Conceptual and computational details
are given in Supplementary Methods. Briefly, impacts of individual drivers ori-
ginated from the spatial distribution of loadings onto 309(latitude 3longitude)
grid cells covering the actively discharging portion of global landmass bearing
local runoff or major river corridor flow (46,517 cells representing 99.2 million
km
2
). Driver loadings were routed down digital river networks
23
, accounting for
new stressor inputs, and dilution or concentration from tributary mixing, based
on spatial changes in river discharge determined from net precipitation and
abstraction, where appropriate. Global, high-resolution maps of each driver were
then standardized using a cumulative density function that ranked all grid cells,
yielding final driver scores between 0 and 1 that reflect the relative stressor level
on each cell across the globe. The re-scaled driver scores were combined into
overall incident threat indices using a two-tiered relative weight matrix derived
from expert opinion (first among drivers within each theme, then among
themes). We used separate weights to capture differences between human water
security and biodiversity perspectives on each driver and theme (Supplementary
Table 1). Separately, we applied the same procedure to an additional set of five
drivers to derive an index of the beneficial effects of water-related capital and
engineering investments
2,3,6,31
in alleviating threats to human water security. By
applying this investment benefits factor to the incident human water security
threat index and re-scaling the global results, we produced the map of relative
adjusted human water security threat (Fig. 4). There is insufficient information to
map corresponding adjustments to incident biodiversity threat.
Biodiversity threat
Low High
Human water
security threat
High Low
Figure 6
|
Prevailing patterns of threat to human water security and
biodiversity. Adjusted human water security threat is contrasted against
incident biodiversitythreat. Much of the developed world faces the challenge of
reducing biodiversity threat and protecting biodiversity, while maintaining
established water services. The developing world shows tandem threats to
human water security and biodiversity, posing an arguably more significant
challenge. Large, contiguous areas of low threat to biodiversity and human
water security remain where dense population and agriculture are absent.
These contrasts help to identify target regions and investment strategies to
enhance water stewardship and biodiversity protection
34,45
. In this Figure, a
breakpoint of 0.5 delineates low from high threat.
RESEARCH ARTICLE
560 | NATURE | VOL 467 | 30 SEPTEMBER 2010
Macmillan Publishers Limited. All rights reserved
©2010

Received 21 January; accepted 19 August 2010.
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www.nature.com/nature.
Acknowledgements We thank A. DeSherbinin, L. Poff, C. Revenga, J. Melillo and O.
Young for comments on the manuscript; D. Allan, R. Abell, J. Bogardi, M. Meybeck, W.
Wollheim, R. F. Wright, D. Boswell, R. Lacey, N. Schneider and D. Vo
¨ro
¨smarty for advice;
and D. Dube and B. Fekete for technical support. Grant support for database and tool
developmentwas from NASA Inter-Disciplinary ScienceProgram Grant NNX07AF28G,
with additional support from the NSF Division of EarthSciences (Hydrologic Sciences
Program Award #0854957) and Global Environment Facility (UPI 00345306). P.B.M.
was supported by a D.H. Smith Fellowship. Financial and logistical support for expert
group meetings and communications was from the Global Water System Project
(Bonn), DIVERSITAS-freshwaterBIODIVERSITY (Paris), NSF BestNet, and Australian
Agency forInternational Development (AusAID) throughthe Australian Water Research
Facility. Conference facilities were provided by the Swiss Federal Institute of Science &
Technology (Eawag) and The City College of New York/CUNY.
Author Contributions All authors contributed to project conceptualization during
workshops led by C.J.V. C.J.V. designed the global analysis, and P.B.M., A.P., P.G. and
M.O.G. designed and implemented the analytical approach with essential input from
S.E.B., D.D., C.A.S., P.M.D. and C.R.L. A.P., P.G. and S.G. developed the database and
mapping tools.Several authors led a separatecomponent of data set developmentand
all providedquality assurance.C.J.V., P.B.M. andM.O.G. wrote the manuscriptwith input
from all authors.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. The authors declare no competing financial interests.
Readers are welcome to comment on the online version of this article at
www.nature.com/nature. Correspondence and requests for materials should be
addressed to C.J.V. (contact@riverthreat.net).
ARTICLE RESEARCH
30 SEPTEMBER 2010 | VOL 467 | NATURE | 561
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CORRECTIONS & AMENDMENTS
ERRATUM
doi:10.1038/nature09549
Global threats to human water
security and river biodiversity
C. J. Vo
¨ro
¨smarty, P. B. McIntyre, M. O. Gessner, D. Dudgeon,
A. Prusevich, P. Green, S. Glidden, S. E. Bunn, C. A. Sullivan,
C. Reidy Liermann & P. M. Davies
Nature 467, 555–561 (2010)
In this Article, the full present address for author P. B. McIntyre was
inadvertently missing from the bottom of the page. The correct pre-
sent address is: Center for Limnology, University of Wisconsin,
Madison, Wisconsin 53706, USA. This has been corrected in the
online PDF.
334|NATURE|VOL468|11NOVEMBER2010
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©2010