Published July 2, 2015
symposium: water security task force
WorldWaterResourcesandAchievingWaterSecurity
RattanLal*
aBstract
Water is the elixir of life and a principal indicator of sustainable development. It is important to understand the magnitude of water
resources, the hydrological cycle, and the impact of land use and management. Large area of Earth’s surface is covered by water, but
renewable fresh water resources are finite (~2.5% of the total), unequally distributed geographically, and prone to eutrophication and
pollution. Conceptually, water resources comprise of four categories: (i) blue water is the fresh surface and ground water (e.g., water in
lakes, rivers, and aquifers), (ii) green water is the precipitation on land that is stored in the soil for plant use, (iii) gray water is contaminated by human use, and (iv) virtual water is embedded in agricultural and industrial produce. Water scarcity, when demand exceeds
supply, occurs wherever the per capita availability of renewable freshwater is <1700 m3/yr. In contrast, water stress occurs when the per
capita availability of renewable freshwater is <1000 m3/yr. Therefore, water security exists when all people at all times have physical
and economic access to sufficient, safe, and clean water that meets their basic needs for an active and healthy life. Water footprint, the
amount of water required to produce goods and services for human consumption, is increasing with increase in world population and
its growing affluence. Thus, sustainable management of water involves technologies to increase the green water storage in soil, purify
the gray water, reduce export of virtual water, increase water use efficiency, and desalinize brackish water.
TotalamountofwateronEarthis estimated at
1.26 × 1021L. Of this, 96.54% is in the ocean which cover
70% of the Earth’s surface. Saline water also exists in ground
water (0.93%) and in lakes (0.007%). The saline water is about
97.437%% of the Earth’s total water reserves (Table 1). Of
the fresh water, 1.74% is contained in the icecaps, glaciers,
and permanent snow and 1.69% in ground water (Table 1).
The fresh water, usable by plants and animals, is ~2.5321% (or
3.19 × 1019L). From agricultural perspective, soil moisture
reserve (0.0021% of 15.7 × 1015L or 15.7km3) is the green
water on which all plants depend. Thus major reservoirs of
water on Earth are ocean, lakes, rivers, land (ground water),
soil, plants, and animals, and the atmosphere. Because most of
the water is saline (brackish),renewable fresh water resources
are finite, and unequally distributed geographically.Thus, sustainable management of green water is of a critical importance
to the wellbeing of flora and fauna and nature conservancy.
Therefore, the objective of this paper is to describe the global
use of fresh water resources, deliberate growing water demand,
and outline the challenges to achieving water security.
Carbon Management and Sequestration Center, Ohio State Univ.,
Columbus, OH 43210. Received 23 Jan. 2015. Accepted 8 Apr. 2015.
*Corresponding author ([email protected]).
Published in Agron. J. 107:1526–1532 (2015)
doi:10.2134/agronj15.0045
Available freely online through the author-supported open access option.
Copyright © 2015 by the American Society of Agronomy, 5585 Guilford
Road, Madison, WI 53711. All rights reserved. No part of this periodical
may be reproduced or transmitted in any form or by any means, electronic or
mechanical, including photocopying, recording, or any information storage
and retrieval system, without permission in writing from the publisher.
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tYPes of water
As is stated above, ~97.5% of Earth’s water resources comprise of saline water (Table 1). The remaining 2.5% of the water
is called fresh water which falls as precipitation. Of the freshwater, 69.5% is contained in ice caps, glaciers, permanent snow,
ground ice, and permafrost (Table 1). Only 1.2% of all fresh
water is surface water and other fresh water, and it is this water
which can be used for all living organisms (animals including
human, plants etc.) (Fig. 1).
The fresh water (415 km3, Fig. 1) can be conceptually
grouped under two categories, liquid water in lakes, rivers,
ground water; and atmospheric water which falls as precipitation. This water is called blue water. Thus, the blue water
amounts to ~85.9% of all fresh water. Soil water, which plants
can use for uptake and transpiration, is estimated at ~14.1% or
15.78 km3 and is called “ green water”. The strategy is to augment the green water with supplemental use of blue or gray water
so that plant growth and agronomic productivity can be increased.
The water used for urban and industrial purposes and contaminated by human waste is called gray water. Thus, for safe
use, gray water must be treated to denature contaminants and
fi ltered to remove all organic and inorganic pollutants. Use of
gray water to supplement blue and green water is a high priority.
The water imbedded in agricultural products (e.g., grains,
meat, cheese, cloth) and industrial materials, and thus can be
traded across regions or international boundaries is called virtual water. Thus, virtual water is imported and exported along
with other produce.
Abbreviations: MRT, mean residence time; WF, water footprint.
A g ro n o myJ o u r n a l • Vo l u m e107,I s s u e 4 • 2 015
Table 1. World water resources (adapted from Speidel and Agnew, 1982; Shiklomanov, 1993; ICA, 2012).
Percent of
Reserve
Amount
Total
Freshwater
––––––––––––––––––––– % –––––––––––––––––––––
106 km3
Ocean
1338.00
96.54
–
Ice caps, glaciers, permanent snow
24.06
1.74
68.6
Ground water
23.40
1.69
–
i. Fresh
10.53
0.76
30.1
ii. Saline
12.87
0.93
–
Ground ice and permafrost
0.300
0.022
0.86
Soil moisture
0.0165
0.001
0.05
Lakes
0.1764
0.013
–
i. Fresh
0.091
0.007
0.026
0.0854
0.007
–
ii. Saline
Atmosphere
0.0129
0.001
0.04
Swamp water
0.01147
0.0008
0.03
Rivers
0.00212
0.0002
0.006
Biological water
0.00112
0.0001
0.003
Fig. 1. World water resources (redrawn from data sources listed in Table 1).
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Sustainable intensification of agro ecosystems, to produce more
from the resources already allocated for agricultural production,
implies judicious use of all types of water (blue, green, gray, and
virtual),and to maximize the use efficiency of green water.
Global Water Cycle
Water on Earth exists primarily in three physical forms: solid
(ice), liquid, and vapor. Some water is also bound or absorbed
through chemical and physical processes. Water changes from
one form into another depending on the temperature, but also
from one reservoir into another depending on its use. This
process of change in water from one form into another, and
exchange in water among principal reservoirs (e.g., land, ocean,
atmosphere and plants) is called the hydrologic cycle. Simply
put, hydrologic cycle refers to the movement of water in, on,
and above the Earth in the form of liquid, vapor, and solid
phases. Water is recharged because of continuous recycling,
thus flow of water among reservoirs is important to determine
the availability of water resources for human consumption.
Readers are referred to other reviews (e.g., Trenberth et al.,
2007) for detailed understanding of the hydrologic cycle, different pools/reservoirs and fluxes among them.
The principle mode of exchange between land, ocean,
and atmosphere is precipitation and evaporation (Fig. 2).
Precipitation involves condensation of water vapor as rain,
snow, hail, fog etc. Cumulative annual precipitation is estimated at 486 × 103 km3, of which 373 × 103 km3 is over the
ocean. If all the water in the atmosphere condensed and rained
at once on the land, it will be ~2.5cm (1 in.) of rain.
Evaporation involves conversion of water from liquid to
vapor phase and transfer into the atmosphere. Specifically,
evaporation from plant surfaces is called transpiration. Total
annual evapotranspiration, equal to that of precipitation is
486 × 103km3. Of this, 73 × 103 km3 occurs from land and
413 × 103 km3 from ocean (Fig. 2). These values are averages, and estimates of precipitation over the land and ocean
vary widely (Malinin, 1994; Klige, 1985; UNESCO, 1978).
In comparison with the data on precipitation and evaporation presented in Fig. 2, Speidel and Agnew (1982) estimated
annual precipitation and evaporation at 111 × 103 km3 and
71 × 103 km3 on land vs. 385 × 103 and 425 × 103 km3 on
ocean, respectively. Other sources of global data on the hydrological cycle include those by Berner and Berner (1987), and
Van der Leeden et al. (1990) among others.
The mean residence time (MRT) of water, computed as pool
divided by flux, varies widely among pools. From the data in
Table 1 and Fig. 2, MRT of water in the atmosphere is 9.5 d
(12,900 km3/486 × 103 km3/yr = 9.5 d). Accordingly, MRT
is 1 to 2 mo for water in soil, 2 to 6 mo in rivers, 50 to 100 yr
in shallow ground water and 10,000 yr in deep ground water
(deep), 3200 yr in oceans and 20,000 yr in Antarctica (Physical
Geography, 2006; Jouzel et al., 2007; USGS, 2008). The projected climate change will accelerate the global water cycle
(Huntington, 2005) and, the MRT of different reservoirs will
change. The climate system determines the upper limit on the
circulation rate of renewable freshwater resources (Oki and
Kanae, 2006).
Global Water Use
Water is the principal indicator for sustainable development and eradication of poverty (Zalewski, 2010). Expectedly,
human activities are strongly affecting the global water cycle.
Directly, humans affect the water cycle by withdrawing it from
different reservoirs for agricultural, urban, and industrial uses
(Table 2). Globally, water withdrawal for these uses is estimated
at 70% for agriculture, 11% for urban, and 19% for industrial
uses (FAO, 2015). Because of the large variation among countries, some withdrawing more than others, the statistics based
Fig. 2. The global hydrological cycle depicting exchange of water between land, atmosphere, and the ocean (the data on precipitation and
evaporation is from Trenberth et al. 2007).
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Table 2. Estimates of global freshwater use withdrawn from rivers, lakes, and ground water (adapted from ICA, 2012).
Activity
Percent use
%
Agriculture
68
Domestic and other industrial
19
Power
10
Evaporation from reservoirs
3
on averaging the ratio of each country is 59, 23, and 18 for agriculture, urban, and industrial uses, respectively (FAO, 2015).
Only ~1% of the world’s fresh water is accessible for direct
human use, and need to be renewed by precipitation. Indirectly,
humans also impact water through land use change and alterations in climate through fossil fuel combustion (Rost et al.,
2008). Within agricultural activities, the largest withdrawal
of blue water (e.g., rivers, lakes, ground water) is estimated at
~2500 km3/yr. In the United States, 40% of the freshwater
withdrawal is for irrigated agriculture which contributes more
than US$50 billion to the national economy (Rost et al., 2008;
Kabat et al., 2002; Vörösmarty and Sahagian, 2000; L’Vovich
and White, 1990). Yet, the green water (soil water) is used on
the rainfed cropland producing 60 to 70% of the global food
(Falkenmark and Rockström, 2004). With increase in food
demand, use of both blue and green water is likely to increase
(Röckstrom et al., 2008, 2007, 1999; Röckstrom and Gordon,
2001). Total consumption of blue and green water for global
food production is ~5000 km3/yr. Similar estimates of global
water use provided by ICA are shown in Table 2.
Water Scarcity
Water is essential for all life, and the demand for this essential
commodity is increasing rapidly at regional and global scales. The
world population is growing at the rate of ~73 million per year
(UNU, 2013). The freshwater withdrawal, which has already
tripled since 1965, is increasing at the rate of 64 km3/yr (FAO,
2015). In addition to the demand for food, increase in water
demand is attributed to change in dietary preferences toward
animal-based diet and growing production of biofuels.
Consumptive water use refers to the water which is not
returned into surface runoff or stream flow, and enters the atmospheric pool through evaporation and transpiration. In contrast,
non-consumptive use involves returns to the surface runoff as
contaminated water. In addition to blue and green water, the
water contaminated after domestic use is called, “gray water.” The
processes of water contamination by agricultural fertilizers and
plant nutrients are called “eutrophication” and “nonpoint-source
pollution”. Human use of gray and contaminated water can be a
serious health hazard. Indeed, 780 million people lack access to
clean water (WHO, 2008; UN Water, 2008). More than 2 billion people presently live under highly water-stressed areas (Oki
and Kanae, 2006). By 2050, 67% of the world population may
live in regions where water is scarce (Wallace, 2000).
It is widely believed that demand of renewable freshwater
may drastically exceed its supply, especially in arid and semiarid
regions with high and increasing population. Water scarcity is
defined as per capita supplies of <1700 m3/yr (IPCC, 2007).
In some cases, water scarcity is also defined when a country
or region’s water supply is <1000 m3/capita/yr (ICA, 2012).
Fig. 3. The 3 I’s of water security as proposed by Hall et al. (2014).
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Table 3. Key aspects of water security.
Aspect
1. Access:
Explanation
To safe and sufficient drinking water at an affordable cost to meet all basic needs.
2. Protection:
Of livelihoods, human rights, and cultural and recreational.
3. Preservation and protection:
Of ecosystems in water allocation and values management systems to maintain and enhance
ecosystems goods and services.
For socioeconomic development and activities (e.g., energy, transport, industry, tourism).
4. Water supplies:
6. Collaborative approaches:
Of gray, contaminated and other types of used water to protect human life and the environment,
and recycle it for safe reuse.
National and international levels for equitable and just use of trans-boundary water resources.
7. Ability:
To cope with risks and uncertainties of water-related hazards (e.g., floods, drought, pollution).
8. Effective governance and
accountability:
With due consideration of interests of all stakeholders.
5. Collection and treatment:
About 1 billion people live in areas of physical scarcity and
other 1.6 billion live in developing countries that lack the basic
infrastructure to withdraw blue water from rivers and aquifers
(Climate Institute, 2006). Several regions characterized by
agricultural intensification are prone to rapid groundwater
depletion (Konikow and Kendy, 2005). The vulnerability to
climate change may aggravate the water supply (Arnell, 1999;
Frederick and Major, 1997; Vörösmarty, 2009; Vörösmarty et
al., 2000; Loáiciga, 2003).
Rather than the supply per se, it is also the question of proper
management of existing water resources. For example, the
region receiving the highest rainfall in the world, Mawsynram
in Assam province of India with annual rainfall of ~1150 cm,
suffers from drought during the months of low or no rainfall.
Similarly, if all the freshwater resources on Earth are equally
divided among the world population, the per capita supply of
5000 to 6000 m3/yr (Vörösmarty et al., 2000) is more than
three times the required amount of 1700 m3/person/year. The
water scarcity is caused by an uneven distribution of renewable
freshwater in time and space. Climate change may accelerate
the water cycle, and increase the available renewable freshwater
supply (Oki and Kanae, 2006). Thus, judicious or sustainable
management of water by reducing the current vulnerability is
crucial to achieving water security.
Water Security
The variable and unpredictable climate is strongly impacting
the hydrologic cycle. Thus, components of the hydrologic cycle
(rainfall, runoff, soil moisture, snowmelt) are highly variable
and difficult to predict (Hall et al., 2014). Extreme floods and
drought, heat waves and cold air are the obvious ramifications
of changing climate. Thus, water security is an increasingly
growing issue regionally and globally, and is likely to be exacerbated by climate change.
In view of the water scarcity and health concerns in a changing climate, there is also a growing emphasis on ensuring water
security. It is defined as “the capacity of a population to safeguard sustainable access to adequate quantities and acceptable
quality of water for sustaining livelihoods, human wellbeing,
and socioeconomic development, for sustaining protection
against water-borne pollution and water-related disasters, and
for persevering ecosystems in a climate of peace and political
stability” (UNU, 2013). Simply put, water security implies
management of the risks associated with extreme variability,
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and to minimize the stress on the society and the economy
(Grey et al., 2013; Hall et al., 2014; IPCC, 2012). Economic
(and agronomic) productivity is strongly affected by the
hydrologic cycle (Brown and Lall, 2006). Rather than a focus
on military risks and conflicts, the term “security” now also
encompasses human wellbeing in the context of environmental
quality. It is in this context that the terms water security and
food security are closely intertwined. Therefore, access to safe
and adequate amount of water is important to all inhabitants
of planet Earth.
In accord with the concept of food security (FAO, 2006),
“water security exists when all people, at all times, have physical and economic access to sufficient, safe, and clean water that
meets their basic needs for an active and healthy life.” Thus,
four basic tenants of water security are:
i. Water availability: The availability of sufficient quantities of
water of appropriate quality,
ii. Water access: Access by individuals and communities to
sufficient water (surface and aquifer) through legal, political, economic and social arrangements at local, regional,
national and international level.
iii. Utilization and Retention: Utilization of clean water for
personal consumption, sanitation, healthcare, food production and processing, and recreational uses.
iv. Stability: To be water secure, a community, household or
individual must have access to sufficient and clean water at
all times, and not be vulnerable to losing access to water as
a consequence of sudden shocks (e.g., an economic, political or climatic crisis) or cyclical events (dry season). The
stability encompasses both the availability and access.
Hall et al. (2014) proposed the 3 I’s of water security (e.g.,
institutions, infrastructure, and information) and suggested
that achieving security involves addressing these 3 I’s simultaneously rather than in isolation (Fig. 3). Thus, water security
is now an important component of the U.N. Sustainable
Development Goals (UNU, 2013). In this context, key aspects
of water security are shown in Table 3.
Effective governance and accountability for attaining water
security also involves protection and enhancement of natural
infrastructure and ecosystems on the basis of a conceptual
framework as outlined by UNU (2013). Among three components of water security (Fig. 4), the first and foremost involves
fulfilling the physiological needs for drinking and sanitation.
The second component is the risk management such as that
Agronomy Journal • Volume 107, Issue 4 • 2015
Fig. 4. Critical issues to management and improvement of natural infrastructure.
against drought, floods, and contamination or eutrophication
(i.e., the problem of algal bloom experienced in Toledo, OH,
in summer of 2014). The third aspect of water security involves
fulfilling all basic needs such as for production of food, energy,
recreation etc. (Fig. 4).
Virtual Water
As discussed above, virtual water is also known as the
embedded or embodied water in the produce (e.g., food, biofuel). Thus, virtual water trade refers to the hidden flow of
water along with the trade of other commodities from one
place (region, country) to another. On average, for example, it
takes 1.6 × 106 L of water to produce 1 Mg of wheat (Triticum
aestivum L.). Thus, trade of wheat, rice (Oryza sativa L.), soybean [Glycine max (L.) Merr.], biofuel, and industrial products
involves trade in virtual water which must be accounted for.
Water-scarce countries import virtual water to meet their basic
needs of food, feed, and industrial raw materials.
Water Footprint
Water footprint (WF) refers to the amount of water required
to produce the goods and services for human consumption.
It can be computed at person, community, state, country, or
global scale. Hoekstra and Mekonnen (2012) assessed global
WF (total of green, blue, and gray water) for the decade between
1960 and 2005 at 7404 Gm3/yr for crop production, 913 Gm3/
yr for pasture, 46 Gm3/yr for animal raising, 400 Gm3/yr for
industrial production, and 324 Gm3 for domestic supply. Total
WF of all production was estimated at 9087 Gm3/yr. Similarly,
Hoekstra and Mekonnen (2012) also computed the gross international virtual water flow. Of the total virtual water flow for
the decade between 1996 and 2005 at 2320 Gm3/yr, 2038 Gm3/
yr was related to trade in agricultural products and 282 Gm3/yr
in industrial products. On average, about one-fifth of the global
WF in the period from1996 to 2005 was for export rather than
domestic consumption. Evidently, increase in population and the
standard of living will increase the WF of all produce and also
that of the virtual water. Increase in the trade of virtual water is
indicative of the water scarcity at global scale.
Conclusions
Water scarcity and insecurity are likely to be exacerbated by
the climate change, growth in population and affluent lifestyle,
and competing uses (e.g., agriculture, urbanization, industry,
recreation). Trade in virtual water, involving food and biofuel
along with industrial produce, can also adversely impact water
security of the exporting countries.
Thus, there are important researchable priorities to address
issues pertaining to water security during the 21st century.
Important among researchable topics can be grouped under the
following categories: (i) food and agriculture, (ii) energy and
biofuel, (iii) innovative and non-conventional sources, including the wastewater, (iv) climate change adaptation and management, (v) risk management and governance including water
quality, and (vi) measurement, monitoring, and modeling.
Similar to soils, Earth’s water resources are adequate to meet
the present and future demands. However, water resources
must also be never taken for granted. They must be used,
recycled, purified, and restored through science-based policy
interventions and governance.
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Agronomy Journal • Volume 107, Issue 4 • 2015