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Climate Change, Hydrology & Water Resources

Introduction

Climate change will lead to an intensification of the global hydrological (water) cycle through increases in surface temperature and rates of evapo-transpiration, and in some regions, increases in precipitation. Changes in the total amount of precipitation and its frequency and intensity directly affect the magnitude and timing of run-off and the intensity of floods and droughts. Such changes will have significant impacts on regional water resources. This fact sheet investigates the impacts of climate change on the hydrological cycle, and by implication, the management of water resources.

Hydrological Cycle


Impacts of Climate Change on the Hydrological Cycle

The effects of future climate change on hydrological patterns can be estimated by combining hydrological models (which simulate water catchments and run-off) and climate models (which simulate the climatic effects of increasing atmospheric concentrations of greenhouse gases). As with all computer models simulating the environment, these are subject to a considerable amount of uncertainty. Nevertheless, a number of conclusions have been proposed.

An increase in air temperature would increase potential evapo-transpiration, but the magnitude of increase also depends on changes in sunlight, humidity, wind speed, rainfall and vegetation characteristics. Actual evapo-transpiration may increase or decrease according to the availability of soil moisture.

It is not certain how individual catchment areas will respond to changing evapo-transpiration rates and precipitation. It is likely, however, that drier hydrological regimes will be more sensitive to changes in climate. Relatively small changes in temperature and precipitation could cause relatively large changes in run-off. Arid and semi-arid regions will therefore be particularly sensitive to reduced rainfall and to increased evaporation and plant transpiration.

An increase in the duration of dry spells will not necessarily lead to an increased likelihood of low river flows and groundwater levels, since increases in precipitation may be experienced during other seasons. More probably, increased rainfall will lead to an increased likelihood of river flooding. Changes in seasonal patterns of rainfall may affect the regional distribution of both ground and surface water supplies.

Hydrological regimes in high latitude or mountain areas are often determined by winter snowfall and spring snowmelt. Most climate models predict that global warming will reduce the amount of precipitation falling as snow in these regions, increasing the rate of water run-off and enhancing the likelihood of flooding. Climatic effects on tropical hydrological regimes are harder to predict. In the mid-latitudes, including the UK, wintertime soil moisture is expected to increase whilst summertime soil moisture may decrease. There will, however, be regional variations.

Freshwater ecosystems, including lakes, streams and non-coastal wetlands will be influenced by changes to the hydrological cycle as a result of climate change. These influences will interact with other man-made changes in land use, waste disposal and water extraction. Freshwater ecosystems will respond to altered flood regimes and water levels. Changes in water temperatures could affect the survival and growth of certain organisms, and the diversity and productivity of freshwater ecosystems. Changes in run-off, groundwater flows, and precipitation directly over lakes and streams would affect nutrients and dissolved organic oxygen, and therefore the quality and clarity of the water. Assemblages of organisms will tend to move poleward with warming, with extinctions occurring at the lower latitudes.

Impacts of Climate Change on Water Resources Management

Water availability is an essential component of human welfare and productivity. Much of the world's agriculture, hydroelectric power production, water needs and water pollution control is dependent upon the hydrological cycle, and the natural recharching of surface and groundwater resources. Changes in the natural water availability, as a result of global warming, would result in impacts which are generally most detrimental in regions already under existing climatic stresses. The ability to manage water resources effectively in these, as well as more benign climates, will receive increasing attention as climate change increases the level of competition between potential users for water.

Changes in surface water availability and run-off will influence the recharging of groundwater supplies and, in the longer term, aquifers. Water quality may also respond to changes in the amount and timing of precipitation. Rising seas could invade coastal freshwater supplies. Coastal aquifers may be damaged by saline intrusion as salty groundwater rises. Reduced water supplies would place additional stress on people, agriculture, and the environment. Regional water supplies, particularly in developing countries, will come under many stresses in the 21st century. 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.

As for impacts on the hydrological cycle, predicting where water resource problems due to climate change will occur is an uncertain business, and can at present only be realised at a subcontinental scale. Uncertainties regarding future climate variability, water demand and the socio-economic and environmental effects of response measures all confound projections for future impacts on water resource management. Nevertheless, it is likely that demand for water will increase over the next few decades, particularly for municipal water supplies in rapidly urbanising areas, for energy production, and for agriculture. Water resource management will consequently need to focus on demand management and the implementation of regulatory controls, and legal and economic instruments to minimise stresses resulting from the increased demand. In addition, new supplies must be developed and existing supplies used more efficiently. Long-term management strategies will also need to consider the construction of new reservoirs and pipelines to boost supplies.

Conclusion

Climate change, through increasing surface temperatures, and changing rates of precipitation and evapo-transpiration, will influence the hydrological (water) cycle. Whilst some areas may become wetter with increased soil moisture availability, others, particularly in the sub-tropics, will be exposed to the increased incidence of drought and desiccation. Changing patterns of snowfall in higher latitudes and mountain regions may increase the likelihood of springtime flooding. At present it is uncertain how these impacts to the hydrological cycle will affect river flows, run-off and groundwater levels. Whatever the changes, however, it is probable that freshwater ecosystems, including lakes, streams and non-coastal wetlands will be significantly affected. The effects of climate change on water availability are likely to place further stresses on the management of water resources which, due to ever increasing demand, are already causing serious problems in many regions of the world.