The ways in which freshwater resources are used, particularly for agriculture, leave much to be desired. In some places, these resources are overused in the sense that use exceeds renewable supply rates, and so cannot be indefinitely continued; elsewhere, wasteful overuse in one area deprives users in other areas, leading to falls in agricultural production and loss of jobs. Misuse occurs where clean water is abstracted and returned to the water system in an unusable state.
Abandoned boat on the dried-up sea floor of the Aral Sea. The Aral Sea is one of the planet's greatest environmental disasters. Prior to 1960 an average of 55000 million m3 of water flowed into the Aral Sea. Withdrawal for cotton irrigation and the construction of flood storage reservoirs resulted in a decline in average annual inflow to 7000 million m3 between 1981 and 1990. As a result, the sea level fell by 16 metres between 1962 and 1994 and the lake volume was reduced by three-quarters. Twenty of the 24 species of fish that used to be present in the sea have disappeared, and the fish catch that totalled 44000 tonnes a year in the 1950s and supported 60000 jobs has dropped to zero. Toxic dust-salt mixtures picked up from the dry seabed and deposited on surrounding farmland are harming and killing crops. The low river flows have concentrated salts and toxic chemicals, making water resources hazardous to drink and contributing to the high rate of many diseases in the area. Those who remain in the area have lost their main livelihood. Those who have left have become environmental refugees. |
Used irrigation water is often contaminated with salts, pesticides and herbicides. Industry and urban centres also return contaminated water to both surface and underground water resources.
One of the most conspicuous results of overuse is that some large rivers - including the HuangHe, the Colorado and the Shebelli - now dry up before reaching the sea. The Amu Darya River which feeds the Aral Sea (see box left) has been deprived of its entire water reserves for irrigating cotton plantations. The Yellow River in China did not complete its descent to the sea for a total of seven months during.
Dried-up rivers are a good example of the overuse of freshwater resources. Overuse in one place means deprivation in another. The flat fertile deltas of many rivers were once centres of high agricultural production. Where the rivers no longer flow, water for irrigation becomes unavailable, farmers go out of business and local production fails.
The causes are usually upstream development. Logging, road building and upstream agriculture often increase soil erosion, resulting in increased sedimentation. This leads to flooding in mid-stream areas and reduced water flows downstream. Sedimentation is also clogging the world's major water reservoirs, currently estimated to hold about 6000 km3 of water. About one percent of this - the equivalent of 60 km3 - is now being lost annually through sedimentation.
Irrigated agriculture has a significant impact on the environment. One positive impact is that high-productivity irrigation of a small area can often replace the use of a much larger area of marginal land for growing crops. However, abstraction of irrigation water from rivers and lakes can also jeopardize aquatic ecosystems such as wetlands, leading to losses in their productivity and biodiversity. This has important implications for human populations that once depended on the major inland fisheries that such areas previously supported and on the natural filtering action of wetlands which have historically been responsible for cleaning up much of the world's wastewater. Where wetlands have been eliminated in the name of irrigation, the results have usually been regretted.
The agricultural chemicals used in irrigated farming often contaminate surface runoff and groundwater. Potassium and nitrogen from fertilizer applications on both rainfed and irrigated land may be washed into groundwater or surface water where they can lead to algal blooms and eu trophication.
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Poplars being irrigated with raw sewage water in India.  |
Irrigation can also concentrate naturally-occurring salts in the water, which then accompany return flows to groundwater or to surface streams and rivers. Irrigation in arid regions can also leach naturally-occurring toxic elements such as selenium from soils and into surface water and groundwater. Overirrigation can lead to waterlogging which reduces yields substantially.
All these problems are amplified as water use intensifies. Furthermore, unconventional water sources have to be tapped as conventional supplies dry up: brackish water and sewage effluents may have to be used for irrigation, and risks to human health may result if not managed properly.
Many countries are already using more water than their renewable supply, and are in a water-deficit situation. Water deficits are created mainly by exploiting groundwater faster than it is replenished. This is in effect the mining of a natural resource, and some arid countries rely substantially on such mined resources, particularly for irrigation (see table). This is a non-sustainable use of resources which cannot be continued far into the future.
The overuse of groundwater as a resource for food production has serious implications. Aquifers have been overexploited in many countries. Estimates of annual depletion in the major water-deficit countries add up to about 160 km3. This suggests that about 180 million tonnes of grain, or some 10 percent of the global harvest, are being produced by depleting water resources. Ironically, an equal or greater amount of food production is under threat from rising groundwater tables in places where irrigation is used but drainage is inadequate.
Overuse of limited water resources is exacerbated by waste, which occurs at almost every point at which humans interfere with the natural water cycle. Irrigation is notoriously wasteful: water is wasted at almost every point in the cycle, from the leaking canals that are used to supply irrigation water to the huge volumes of water that fall uselessly on soil where there are no crops or which are in excess of the uptake required by the crop. Improving irrigation efficiency - currently less than 40 percent - is a key goal for the future.
Groundwater mining in selected countries | |
Country |
mining as % of total |
Kuwait |
46.5 |
Bahrein |
40.2 |
Malta |
32.2 |
United Arab Emirates |
70.9 |
Qatar |
14.9 |
Libyan Arab Jamahirija |
90.0 |
Jordan |
17.5 |
Saudi Arabia |
79.7 |
Source: Water Resources of the Near East Region: a review (FAO, Rome, 1997) | |