Changing precipitation patterns are already affecting water supplies. Increasingly heavy rain and snow are falling on the mid- and high latitudes of the Northern Hemisphere, while rains have decreased in the tropics and subtropics in both hemispheres. In large parts of eastern Europe, western Russia, central Canada and California, peak stream flows have shifted from spring to winter as more precipitation falls as rain rather than snow, therefore reaching the rivers more rapidly. Meanwhile, in Africa’s large basins of the Niger, Lake Chad and Senegal, total available water has decreased by 40 – 60%.

Climate change will lead to more precipitation – but also to more evaporation. In general, this acceleration of the hydrological cycle will result in a wetter world. The question is, how much of this wetness will end up where it is needed?

Precipitation will probably increase in some areas and decline in others. Making regional predictions is complicated by the extreme complexity of the hydrological cycle: a change in precipitation may affect surface wetness, reflectivity, and vegetation, which then affect evapo-transpiration and cloud formation, which in turn affect precipitation. In addition, the hydrological system is responding not only to changes in climate and precipitation but also to human activities such as deforestation, urbanization, and the over-use of water supplies.

Changing precipitation patterns will affect how much water can be captured. Many climate models suggest that downpours will in general become more intense. This would increase runoff and floods while reducing the ability of water to infiltrate the soil. Changes in seasonal patterns may affect the regional distribution of both ground and surface water supplies. At the local level, the vegetation and physical properties of the catchment area will further influence how much water is retained.

The drier the climate, the more sensitive is the local hydrology. In dry climates, relatively small changes in temperature and precipitation could cause relatively large changes in runoff. Arid and semi-arid regions will therefore be particularly sensitive to reduced rainfall and to increased evaporation and plant transpiration. Many climate models project declining mean precipitation in the already-dry regions of central Asia, the Mediterranean, southern Africa and Australia.

High-latitude regions may see more runoff due to greater precipitation. Runoff would also be affected by a reduction in snowfall, deep snow, and glacier ice, particularly in the spring and summertime when it is traditionally used for hydroelectricity and agriculture. All climate change models show increased wintertime soil moisture in the high northern latitudes. Most models produce less soil moisture in summer in northern mid latitudes, including some important grain producing areas; these projections are more consistent for Europe than for North America.

The effects on the tropics are harder to predict. Different climate models produce different results for the future intensity and distribution of tropical rainfall. South Asia, however, is expected to see increased precipitation from June through August whereas Central America is expected to see less rain during these months.

New patterns of runoff and evaporation will affect natural ecosystems. Freshwater ecosystems will respond to altered flood regimes and water levels. Changes in water temperatures and in the thermal structure of fresh waters could affect the survival and growth of certain organisms, and the diversity and productivity of ecosystems. Changes in runoff, groundwater flows, and precipitation directly over lakes and streams would affect nutrients and dissolved oxygen, and therefore the quality and clarity of the water.

Reservoirs and wells would be also affected. Surface water storage could decline as extreme rainfalls and landslides encourage siltation and thus reduced reservoir capacity. An increase in extreme rainfalls and flooding could also lead to more water being lost as run-off. In the longer term this could also affect aquifers. Water quality may also respond to changes in the amount and timing of precipitation.

Rising seas could invade coastal freshwater supplies. Coastal freshwater aquifers may be polluted by saline intrusion as salty groundwater rises. The movement of the saltwater-front up estuaries would affect upriver freshwater-pumping plants, brackish-water fisheries, and agriculture.

Reduced water supplies would place additional stress on people, agriculture, and the environment. Already, some 1.7 billion people – a third of the world population – live in water-stressed countries, a figure expected to rise to 5 billion by 2025. Climate change will exacerbate the stresses caused by pollution and by growing populations and economies. The most vulnerable regions are arid and semi-arid areas, some low-lying coasts, deltas, and small islands.

Tensions could rise due to the additional pressures. The links among climate change, water availability, food production, population growth, and economic growth are many and complex. But climate change is likely to add to economic and political tensions, particularly in regions that already have scarce water resources. A number of important water systems are shared by two or more nations, and in several cases there have already been international conflicts.

Improved water resource management can help to reduce vulnerabilities. New supplies must be developed and existing supplies used more efficiently. Long-term strategies for supply and demand management could include: regulations and technologies for directly controlling land and water use, incentives and taxes for indirectly affecting behavior, the construction of new reservoirs and pipelines to boost supplies, improvements in water-management operations and institutions, and the encouragement of local or traditional solutions. Other adaptation measures can include protecting waterside vegetation, restoring river channels to their natural form, and reducing water pollution.