Water, the limiting resource
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CTA. 1995. Water, the limiting resource. Spore 57. CTA, Wageningen, The Netherlands.
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Water levels in many parts of the world are low and getting lower. Rivers have retreated from their banks, lakes are shrinking from their former shores and boreholes are pierced ever deeper to tap falling water tables. Almost universally water is...
Water levels in many parts of the world are low and getting lower. Rivers have retreated from their banks, lakes are shrinking from their former shores and boreholes are pierced ever deeper to tap falling water tables. Almost universally water is used as if the flow will never cease, and there is little or no appreciation that fresh water is a finite resource. If populations and living standards are to rise, demand for water will inevitably increase still further. Ways must be found not only of meeting that demand but also of maintaining the natural water cycle upon which future survival depends. World-wide there are some 80 countries, with 40% of the total world population, which are experiencing water shortages in some regions or at certain times of the year. Nearly one billion people in the world are without clean drinking water and have to rely on whatever other kinds of water they can get, from rivers, lakes, ditches and shallow wells. Many of these sources are frequently contaminated. Dirty water is the world's biggest health risk, accounting for as much as 80% of disease in developing countries with untreated sewage the major problem. An estimated 1.7 billion people are without adequate sanitation. About 95% of sewage is being poured straight into rivers and other water courses where it is joined by growing volumes of industrial waste. WHO estimates that, world-wide, 10 million people were dying annually from polluted drinking water at the beginning of the 1990s. Water scarcity in Africa now threatens two-thirds of the population, and some island nations, for example Cape Verde and Barbados, are running short of freshwater. In sub-Saharan Africa 265 million people have no access to safe water, and 344 million lack adequate sanitation. As a result, diseases such as guinea worm and cholera plague people of all ages and water-borne diseases are a primary source of infant deaths. Where 1,000 peop]e share one standpipe disease is inevitable, and yet bringing water into homes or backyards, while highly desirable from a health point of view, greatly increases water use, putting a further strain on scarce resources. While domestic demands for water for drinking and hygiene are set to increase as populations rise and become ever more concentrated in urban centres, so too are demands from industry and the food producing sector. Competition between agriculture, industry and cities for limited water supplies is already constraining development efforts in many countries. Ownership of water has become a political issue at international as well as at local level. Water often flows across national boundaries and its quantity and quality are affected by the activities of the upstream country or countries. Nine countries are dependent on water from the Blue and White Niles and Botswana, The Gambia, Mauritania, and Sudan all receive over 75% of their available water supplies from river flows of one or more upstream neighbours. Agriculture: thirst or thrift? Globally, agriculture consumes more than two-thirds of the water withdrawn from the earth's rivers, lakes and aquifers but, in low-income countries, agricultural water use is some 90% of the whole. Agriculture is not only the world's largest water user in terms of volume, it is also a relatively low-value, low-efficiency and highly subsidized water user. In contrast, poor families in some large cities are forced to spend up to 20% of their income on water. If there is to be enough water for all, on a sustainable basis, and if depleted groundwater resources are to be restored, it follows that agriculture, the prime consumer, must take prime responsibility for more efficient use. And yet farmers are being asked to produce more food on soils which are already overworked and degraded and where water may become the limiting resource. It seems that farmers are constantly being asked to achieve more with less. So how can this dilemma be resolved? Technical, social and political attitudes need to change. Equitable access will have to be achieved by policy-makers instituting measures to control demand, by engineers using technology to increase efficiency of supply and delivery systems, and by users who will have to guard against a profligate waste of a precious resource. But technical solutions have had a mixed press. World-wide, about 235 million hectares of farmland are currently irrigated, five times the area at the beginning of the century. During this period the population has tripled. Undoubtedly the growth of irrigation has made a major contribution to increased food production over the last 50 years. At present 2.4 billion people depend on irrigated agriculture for jobs, food and income and over the next 30 years an estimated 80% of the additional food supplies required to feed the world will depend on irrigation. But irrigation can be a demanding, costly and wasteful consumer. For example the amount of water required to irrigate one hectare of rice (15,000m ') for one season is sufficient for the needs of 100 nomads and 450 head of stock for three years. Nearly 30% of World Bank agricultural lending during the 1980s was spent on irrigation and spending commitments for irrigation by all aid agencies exceeded $2 billion per year in the past decade. And, as water becomes more scarce, delivery systems become more expensive to develop and maintain. Despite the huge costs involved, as much as 60% of the water diverted or pumped for irrigation is wasted through seepage and evaporation. Irrigation: little or large? Massive dam building projects have not proved to be the technical fix once hoped for. In Africa, investment costs for dam building and water distribution systems may reach as high as $20,000 per hectare of land irrigated. Even medium-sized irrigation construction costs in Africa can be US$7,200 per hectare. Many large scale projects have proved even more expensive than anticipated because reservoirs have silted up more quickly than predicted, thereby shortening the operating life of the dam. Environmental and social costs have also been high and economic gains disappointing. The Akosombo Dam that flooded 5% of Ghana when completed in 1966 not only failed to transform the country's economy but, behind the dam, Lake Volta has trapped so much of the River Volta's silt that coastal erosion in neighbouring Togo has washed away 10,000 homes in less than three decades. Yet, although the dam provides cheap electricity for smelting bauxite, the bauxite comes from abroad, the smelters are owned by foreign companies and there has never been any money for investment in irrigation to compensate for the drowned farmland. In southern Africa, the Kariba Dam powers Zambia's copper mines but at the cost of fisheries damaged and flood plain farming destroyed. Some people are still tempted to think big and seek funding for such prestigious projects such as plans to refill Lake Chad with water re-routed north from the River Zaire or to increase the flow of Nile water by completing the Jonglei Canal across the Sudd. But most grand designs have run aground on a sandbank of problems. Experience has shown that maintenance of machinery, dams and canals has proved too costly and too difficult to organize or there has not been the political will to do so. Furthermore too few rural people feel ownership of, and responsibility for, a system built by big forces outside their control; agencies who may have had more interest in gaining engineering and supply contracts than designing a system to suit the management capability of the country concerned. Small-scale irrigation is now considered to be a more appropriate option. It costs less to construct, takes less time to complete, causes less disruption to the population and to the environment and, provided they have been properly involved, is more likely to be supported by the local people. The three principal types of small-scale irrigation are flood plain cropping, stream diversion and lift irrigation but, at whatever scale, the use of irrigation has to be carefully managed if it is to avoid the twin problems of waterlogging and salinization. It is estimated that as much as one quarter of all irrigated land in developing countries suffers from varying degrees of salinity and that every year 1-1.5 million hectares are damaged in this way. Salt resistant crops are being developed but it is surely wiser to avoid increasing the need for them as far as possible. Simple techniques such as lining irrigation canals, or using surge irrigation (whereby water is released in two stages, the first to seal the soil, the second to irrigate crops), help prevent seepage. Diverting water from a stream, so that it is distributed along canals or ditches by gravity to where it is needed, is the basis of many traditional systems. New materials are being introduced to limit the huge amount of water that simply leaks out of the canals and reduces the area that can be watered from one source. Concrete, pvc or high density polyethylene pipes, corrugated iron sheets and steel water-control gates help to reduce water losses and the high labour costs for maintenance. Flood plain cropping mainly involves the construction of earthen banks or bunds around cultivated fields to retain or exclude water as required. However, many of the rivers that once inundated floodplains have been dammed for hydroelectric power and the silt they carried is trapped within the dam reservoir, threatening the efficacy of the turbines, instead of bringing fertility to the plains. In some former flood plains, for example the Sokoto Valley of Nigeria formed by the construction of the Bakolori dam, flood cropping has been replaced by lift irrigation from shallow groundwater using petrol driven pumps. Lift irrigation to extract water from underground sources has increased rapidly in recent years. Small motorized pumps can deliver 10-20 litres per second, enough to water a one hectare plot in about two hours of operation per day. These pumps are popular with farmers since they can be easily transported, require no human effort to operate and can be stopped and started as required. Schemes to provide credit for constructing tubewells and for purchasing pumps have boosted their popularity. But their success depends on availability of fuel, servicing, repair facilities and spare parts. In contrast, in Zimbabwe and Kenya human-powered rope washer and treadle pumps are replacing watering cans for irrigating vegetables with shallow groundwater. They allow larger areas to be irrigated for the same effort, a boon to the women who do most of the watering work. The treadle pump has also proved popular in Senegal where more than 600 farmers have invested what is only a fraction of the cost of a motorized pump. Up to eight cubic metres of water an hour can be drawn from depths of seven metres, saving many hours of labour compared to hand watering. Returning to tradition One of the drawbacks to the new technologies which make water extraction easier is that groundwater resources may be withdrawn at a rate faster than that at which they are naturally replenished. In effect the water is being mined. Water tables are lowered, more energy, and therefore more cost, is involved in extracting the water and, more seriously, aquifers may become contaminated with sea-water. If this happens the source of freshwater can never be replaced. Global warming resulting in rising sea levels, has increased this risk of saline incursion, which is of particular concern in coastal regions and to island nations. Modern engineers, attempting to find ways of making water available for irrigation on a sustainable basis, marvel at ancient water supply systems, many of which are still in use today. For 2,000 years gently sloping qanat tunnels have supplied water from underground sources from Southwest Asia to north Africa. Because no pumping is involved water cannot be extracted faster than it is replaced. When modern pump wells are installed water tables are lowered and traditional systems that used to work in balance with the environment run dry. The challenge is to find ways of making more water available for agriculture without exceeding natural refill rates. Farmers in areas where water conservation has always had to be a priority have developed systems that could act as a model in those regions where water scarcity is a recent threat. The most promising new technical solutions are those which have improved upon these traditional farming systems. When water is scarce, and rainfall low, people have the choice of moving to more congenial sites (now difficult to find) or devising means of capturing every drop of available water. Spectacular forms of terracing can be found in the African mountain regions such as the Mandara mountains in Cameroon and the Djebel Marra in Sudan. Necessity demanded such labour intensive activities, further examples of which can be seen on the Dogon plateau in Mali, where irrigated vegetable gardens have long been a feature of Dogon agriculture. A network of stone squares is constructed on the naked rock and then filled with silt and soil from the river bed to which manure is added. In the Yatenga region of Burkina Faso, farmers traditionally rehabilitated barren land by digging small pits called zay. At the end of the 1970s, a farmer from the village of Gourga improved the traditional pits by increasing their dimensions and by adding manure. The idea was picked up by the OXFAM-funded Projet Agro-Forestier which combined zay with stone bunds. Thousands of hectares of barren land have now been rehabilitated in this way. The procedure is that farmers break the surface crust, dig pits with a diameter of about 30cm and a depth of about 20cm. They put manure in the pits which attracts termites. These termites digest the manure, start digging holes and in this way increase soil fertility and water infiltration capacity. Yields of sorghum and millet can be over 1,000 kg/ha in a year of average rainfall, but even in years of below average rainfall zay give a reasonable yield because they collect and concentrate runoff. The idea has spread to the Illela district of Niger where hundreds of farmers have improved their traditional pits (tassa) using the same principle for conserving moisture. These traditional water harvesting techniques all require a heavy investment of time and labour in conditions that are hardly conducive to human toil. In the Ader Doutchi Maggia in Niger, stone lines have traditionally been constructed to slow runoff. Land users in the region have recently been introduced to 'modern' soil and water conservation techniques. It is interesting to note that the latter have not been expanded, nor even maintained, whereas the traditional stone lines except on the most marginal fields, continue to be maintained and even expanded. Waste not, want not! New technologies for conserving water will be useless unless consumers are urged to be less wasteful, even where water appears to be plentiful. All too often standpipes are left running and no-one seems to assume responsibility for turning off the tap. And, if water leaks from a pipe, it shows that there is water there, reinforcing the idea of plenty. That precious underground resources are flowing to waste seems a concept hard to understand. It is made harder when people are at the same time encouraged to use more water for washing and bathing and for watering vegetable plots to increase yields, and seldom is it suggested that waste domestic water can be used in gardens. The task of managing water resources and ensuring access to the supplies each sector requires is the responsibility of policy makers and planners. Whether a government decides to support, or not to support, the construction or maintenance of a large dam, flood control or a major irrigation project, its influence on water policy is there for all to see but many decisions are made which have a less obvious, but equally powerful, impact. If a government decides to promote high value, thirsty export crops, for example, in order to improve foreign exchange earnings and reduce debt, or to encourage irrigated rice for the domestic market in order to reduce imports, water consumption is affected. Since growing demand can be met only partially from developing new supplies, policy makers will have to devise means to influence the behaviour of users, in order to manage resources, reduce waste and reduce pollution. This would be difficult enough with a static population rate but as populations rise there will be an increasing strain on water resources, not only for food production but also for diluting and carrying away the waste products of the people themselves, and of their industrial activities. Furthermore, policies designed to meet the needs of humans must not conflict with the need to protect the environment. The mobility of water within the hydrological cycle means that activities in one part of the system inevitably have an impact on another. It is in the long-term interests of humanity to do as much as possible to retain water within that part of the cycle where it is of most use, i.e. as fresh water on land rather than as salt water in the sea. Many of man's activities have had the opposite effect. Deforestation, for example, and subsequent loss of vegetative cover on hillsides, hastens rainfall runoff and rivers flow much faster to the sea. Policy-makers need to establish a structure of incentives, regulations, permits, restrictions and penalties that will help guide, influence and coordinate how people use water while encouraging innovations in water-saving technologies. Sustainable agricultural development depends on sustainable water use and food security depends on water security. Policy-makers, water engineers and the end-user on-farm have to give greater thought to the wiser use of water. From the commercial farmer pumping water to irrigate high value export crops to the woman carrying water on her head, raised from a distant well in order to water the vegetable plot, water is too valuable to waste. For further reading: Runoff irrigation in the Sahel zone by W Tauer and G Humborg, CTA Wasting the rain by W M Adams, Earthscan Rainwater harvesting by Arnold Pacey with Adrian Collis, IT Publications State of food and agriculture 1993 - Water policies and agriculture, FAO The Dammed by Fred Pearce, The Bodley Head ** The poster depicted on the front page is distributed by ORCADES to schools in eight Sahelian countries. They are accompanied by a teacher s guide in English, French, Portuguese and Arabic. ORCADES, 12 rue des Carmelites, F-86000 Poitiers, FRANCE or PFI-CILLS-EEC-INSAH, BP 1530, Bamako, MALI.
- CTA Spore (English)