Global Water Resources

Global warming is anticipated to have a significant negative effect on the global supply and quality of water. Potential factors contributing to this effect include warmer winters, changes in precipitation patterns, increasing evaporation, melting snow peaks, rising sea levels, saltwater intrusion into coastal aquifers, dry soil, and increasing storm-water runoff.

Background

A water resource is any surface or underground source of water that can be utilized by humans. Typical resources include water basins, aquifers, oceans, rivers, lakes, natural and artificial reservoirs, ice, snow, and wetlands. The hydrologic cycle renews freshwater resources, but these resources decrease when their withdrawal rate is faster than their recharge rate. Increases in population, industrialization, and other factors that increase demand can contribute to this depletion of resources. Environmental degradation and global warming also contribute to the decrease of available water. Water is an important factor in conflicts among stakeholders at the local, regional, national, and international levels. Water conflicts take many forms, but they almost always arise from the fact that the freshwater resources of the world are not partitioned to match political borders, nor are they evenly distributed in space and time.

Changes in Quantity of Water Resources

The amount of water on Earth undergoes continuous changes due to the hydrologic cycle. The amount of freshwater in an area might increase as a result of higher precipitation or melting ice or snow. However, for water supplies to increase, other requirements must be met that allow freshwater to be captured in aquifers and other stable, uncontaminated reservoirs. These requirements include longer rainy seasons that provide enough time for freshwater to infiltrate aquifers, limitations on the amount of runoff that flows into oceans and other saltwater bodies, and sufficient soil and aquifer permeability and thickness. Aquifer recharge is also increased by relatively cool air temperatures and relatively low rates of evaporation from the ground’s surface.

The overall temperature increases caused by greenhouse gases (GHGs) have significant effects on the environment and the quantity and quality of water resources. Higher temperatures increase the evaporation rate, which in turn increases the number and intensity of storms. Dry soil becomes difficult for water to infiltrate, and the violent precipitation of storms erodes soil, further decreasing the permeability of the ground’s surface layer. As a result, storm water is unlikely to recharge aquifers, and a high proportion of surface storm water is likely to run off into rivers and oceans. In addition, higher temperatures cause ice caps to melt and raise sea levels, which in some coastal regions can introduce saltwater intrusion into aquifers.

Changes in the Quality of Water Resources

Climate change can greatly affect water quality. Melting snow in the mountains increases river flow and erosion and transports large amounts of sediment, usually in winter or in rainy seasons. Such increased river flow can create flood hazards and heighten the potential for destruction of dams and levees. On the other hand, reduced river flow in summer can also have severe implications, because demand for water for irrigation, hydropower, and other uses is at its peak in summertime.

Evaporation rates increase with temperature. High evaporation rates in some areas reduce the ratio of water to its solutes, increasing the concentration of salts and chemicals and thereby decreasing water quality. The resulting increases in water salinity can cause changes in the rate of oxygen release into water. These changes in turn affect the plants in water reservoirs and the animals and plants in the surrounding environment. Furthermore, an increase in temperature and a consequent increase in the rate of ice melt can create a positive feedback loop. The melting away of ground ice, which reflects the Sun’s rays, decreases regional albedo, causing the ground to absorb more solar energy and raising the temperature of the region.

Water resources can be renewable or nonrenewable. Basins that are consistently recharged by the hydrologic cycle (primarily through precipitation) and thus are not generally affected by water withdrawals and use are renewable. Nonrenewable water resources, are recharged at a rate significantly slower than their rate of withdrawal. Human actions, such as modifying watersheds, cutting forests, or paving land, can affect hydrogeologic balance and reduce the recharge or flow characteristics of a given area, altering the availability and renewability of water. Thus, it is possible for human activity to transform renewable water resources into nonrenewable water resources.

Freshwater helps sustain humans, animals, and plants. Relatively large bodies of water, such as the Great Lakes, affect regional climate and weather conditions. Large-scale withdrawal or transfers may reduce the size or quality of a water body, significantly influencing both climate and ecosystem services. Changes to the timing of river flows may alter ecological conditions. Such changes have led to many ecological and human disasters. For example, changes to the Amu Dar’ya and Syr Dar’ya rivers in Central Asia caused the destruction of the Aral Sea ecosystem, the extinction of the sea’s endemic fish populations, shrinking of the sea, and local human health problems attributable to exposure to salt aerosols released into the air when the sea shrank.

Coral bleaching and other diseases of corals increased dramatically beginning in the late twentieth century. The increase of these diseases correlated with concurrent increases in sea surface temperatures. Thus, one consequence of global warming could be the mass destruction of coral reefs caused by emerging coral diseases and by the lack of epidemiological and biochemical information on even the known diseases.

Withdrawals of water from many rivers and streams in North America and Europe have led to reductions and extinctions of many fish populations (particularly anadromous fish). Decreased river flows have severely damaged river deltas and local communities, such as in the Sacramento/San Joaquin delta in California, the Nile River delta in Egypt, and the Colorado River delta in Mexico, where the local Cocopa tribal communities have been affected.

The world’s water is unevenly distributed. As a result, complex and expensive water systems have been built over centuries to capture water during wet periods for use during dry periods and to move water from water-rich regions to water-poor regions. Many markets for freshwater have been created by the growing demand generated by the industrial and agricultural sectors. These emerging markets led to the creation of various forms of international water trading and exchange. The continuing growth of water demand in the face of population growth, climate change, and other factors is motivating states and corporations to find new ways to transfer water, including pipelines, bottles, tankers, and giant bags.

The increasing demands placed on the global water supply threaten biodiversity, human food production, and other vital human needs. Water shortages already exist in many regions, and more than one billion people lack adequate drinking water. In addition, 90 percent of the infectious diseases in developing countries are transmitted by polluted water. Agriculture consumes about 70 percent of freshwater worldwide; for example, approximately 1,000 liters of water are required to produce 1 kilogram of cereal grain, and 43,000 liters are required to produce 1 kilogram of beef. New water supplies are likely to result from conservation, recycling, and improved water-use efficiency rather than from large development projects.

The Pacific Island developing countries (PIDCs), which are among the nations least responsible for contributing to the global warming problem, are among those that will most significantly suffer its consequences. These countries are responsible for less than one percent of the world’s carbon dioxide (CO₂) emissions; however, it is predicted that they will experience the earliest and the most severe effects of climate change over the next two centuries. In addition to rising sea levels and saltwater intrusion, climate change poses a long-term threat to freshwater quality and availability in these countries. Of the thirty thousand islands in the Pacific Ocean, one thousand are populated. Almost all islands rely on groundwater resources for their freshwater; some high islands, such as French Polynesia, Nauru, Palau, and the Cook Islands, rely on surface water, which does not require pumping, because it is transported by gravity. Rapid increases in population place a strain on limited water resources, and over-pumping of fragile groundwater aquifers will cause saltwater intrusion and the loss of supplies. Rapid urbanization, increases in pollutants, and inadequate sewage disposal infrastructure greatly affect Pacific Island water supply systems, because most such islands have thin and highly permeable soil zones.

Anthropogenic global warming is projected to result in the global temperature increasing by 1.4° Celsius to 5.8° Celsius by the end of the twenty-first century. Global mean sea level is projected to rise by 9 to 88 centimeters during the same period as a result of thermal expansion and loss of mass from melting glaciers and ice caps. Rising sea levels place freshwater resources at risk from saltwater intrusion. According to the World Meteorlogical Organizaton, by 2022, global climate change had continued to significantly impact the world's water resources. Environmental changes resulted in melting snow and glaciers, causing increases in floods and other water-related disasters. Additionally, climate change caused a significant increase in extreme weather patterns, such as powerful storms and droughts.

Context

Based on the predicted impacts of increased GHGs in the atmosphere and observed climate change, a host of national and international organizations has acted to preserve global water resources. The United Nations Framework Convention on Climate Change (UNFCCC), for example, is taking action to reduce GHG emissions and to develop adaptive protection strategies to save and conserve freshwater resources. The following measures are planned to be adapted by the PIDCs:

• • Comprehensive projections of future water demand
• • A sustainable research program to assess freshwater-resource availability
• • Electronic data logging and hydrological data-processing software
• • Rehabilitation and maintenance of water catchment and distribution systems to render them capable of surviving violent storms
• • Evaluation of proposed development projects and existing infrastructure in terms of their impact upon coastal freshwater resources
• • Implementation of leak-control measures in existing water systems

Public awareness and water conservation projects are also being developed in the Middle East to address the critical water issues specific to that region.

Many environmental stressors contribute to ecosystem degradation in general and water-resource depletion in particular. These include rapid population growth and industrialization, increased weather-related water shortages, soil salinization, and contamination by residual pesticides and heavy metals. All such stressors threaten human health. To understand the interdependencies of water, climate, and health, the United States Geological Survey (USGS) developed a program that documents environmental quality along the U.S.-Mexico border by integrating the data sets of both countries. The USGS has also implemented data collection and training programs in such Middle Eastern nations as Jordan and the United Arab Emirates. Such efforts are at the forefront of efforts to analyze, understand, and respond to the increasing need for and decreasing availability of water throughout the globe.

Key Concepts

• aquifer: a subsurface formation that is permeable and therefore allows water storage and flow
• concentration: the ratio, by weight, of a dissolved chemical compound to its solvent (such as water)
• desalination: distillation and of salt water to obtain freshwater
• evaporation: the transformation of a substance from its liquid to its gaseous state
• infiltration: the process of entering soil
• precipitation: the of atmospheric water, which then falls to Earth’s surface
• runoff: storm water flowing on Earth’s surface
• saltwater intrusion: a process along coastlines in which salt water flows inland into a freshwater aquifer
• sediment: that is transported by water and eventually deposited
• sediment load: the quantity of undissolved organic and inorganic particles carried by water
• water basin: an area of land where surface water and groundwater flow in the same direction
• watershed: an area of land where water flows into the same watercourse

Bibliography

Gleick, Peter, et al. The World’s Water, 2006-2007: The Biennial Report on the World’s Water Resources. Washington, D.C.: Island Press, 2006.

Grayman, Walter M. Water Resources and Environmental Issues in 2050. Reston, Va.: ASCE, 2008.

Raskin, P., et al. Comprehensive Assessment of Freshwater Resources of the World. Stockholm, Sweden: Stockholm Environment Institute, 1997.

"State of Global Water Resources 2022." World Meteorological Association, 12 Oct. 2023, wmo.int/publication-series/state-of-global-water-resources-2022. Accessed 10 Dec. 2024.

Vicuna, S., J. A. Dracup, and Larry Dale. Conjunctive Use and Other Adaptation Strategies to the Impacts of Climate Change in California Water Resources. Reston, Va.: ASCE, 2008.