Water harvesting: Reviving ancient techniques
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CTA. 1988. Water harvesting: Reviving ancient techniques. Spore 13. CTA, Wageningen, The Netherlands.
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The recent recurrent droughts and famine in parts of Africa have revived iriterest in an ancient technology, that of water harvesting. This system of water conservation, involving the collection and storage of run-off from infrequent storms and...
The recent recurrent droughts and famine in parts of Africa have revived iriterest in an ancient technology, that of water harvesting. This system of water conservation, involving the collection and storage of run-off from infrequent storms and flash floods, is known to have supported a flourishing agriculture in the Negev Desert more than 5,000 years ago. Modern experimental systems suggest there is considerable scope for extending the traditional schemes still in limited use in arid areas all over the world to this day. Aerial photographs and careful fieldwork have allowed researchers to piece together and, in some cases, reconstruct the ancient water harvesting systems of the Negev Desert. The simplest system involved the building of low stone walls across the dry desert valleys, or wadis. In this way a series of terraces was formed, so that during the occasional flash floods water was retained for long enough behind each wall to allow it to soak into the terrace soil. This infrequent soaking provided a sufficient reserve of water in the root zone for a variety of crops to be grown. In larger valleys,. where torrential floodwater would have destroyed the terrace walls, a dam was built part of the way across the valley to divert some of the stormwater onto the adjoining terraced fields. In small tributary valleys, elaborate systems of stone barriers and channels were used to check sheet runoff from the hills surrounding the farmsteads and direct it onto the terraced fields below. Complex systems of gates and stone steps, or spillways, between terraces allowed the farmer considerable control over how much water each terraced plot received Trapping water and silt Although the ancient systems of the Negev Desert are the best known and most researched examples of water harvesting, similar systems are known to have existed in arid areas elsewhere. There is evidence that the Mexican Indians created garden plots by building loose rock dams across gullies and valleys more than 1,000 years ago. These not only trapped water but silt as well so that over a period of time a deep and fertile alluvial soil accumulated behind each stone barrier. The North American Indians in Arizona and Colorado were also familiar with water harvesting techniques over 500 years ago. The ancient Persians dug pits in gently sloping land to trap water for their crops, and water harvesting techniques have a long tradition in many other parts of the Middle East and in parts of North Africa such as Algeria and Tunisia as well. Limited use is still made of traditional water harvesting methods in many areas to this day. The Bedouin exploit the ancient terraced wadis in the Negev by sowing a crop of barley as the floodwaters retreat. A similarly opportunistic form of water harvesting is practised in north-west Kenya, where sorghum is grown in naturally occurring depress~ons where the rain water collects. In Sudan, high retaining walls are built on the gentle slopes to the east of the Nile to hold back floodwater for long enough to saturate the soil and allow a quick maturing crop such as millet to be grown as the soil water recedes. Floodwater from the mounts in North and South Yemen is diverted and trapped in ponds made by building up high earth banks, and similar methods of water harvesting are to be found in India and Pakistan. Impervious surfaces The two features common to all water harvesting systems, whether ancient, traditional or modern, are the catchment area and the storage area, or reservoir. Ideally the surface of the catchment area should be impervious to water to allow maximum run-off to occur. In some cases such catchments occur naturally, but in others the catchment surface is treated to improve run-off or an artificial surface mav be used. Granite or sandstone rock outcrops, and soils such as the loess of the Negev Desert which crust over as soon as rain falls, need little modification, although clearing rubble, filling cracks and removing vegetation can all help improve run-off. In less favourable situations run-off can be increased by smoothing and compacting the soil surface. An example is the roaded catchment system, widely used to collect water for livestock in Western Australia. Here road construction equipment is used to compact and camber the soil surface, to form a series of parallel roads measuring between 5 and 12 m from crest to crest, on a stretch of sloping ground between 50 and 300m long. Storm water collects in the furrows between the roads and flows into a channel at the lower end of the catchment, from where it is led into a reservoir. Soil surfaces which have been compacted for other reasons, such as roads, airstrips and school playgrounds, can sometimes be exploited to provide run-off water for crops or livestock. Careful design and regular maintenance is necessary to avoid soil erosion with compacted catchments, although in some systems the washing down of soil can be beneficial by filling up small depressions in the catchment surface and building up soil fertility in the area where the crop is grown. Chemical treatment has also been used to improve surface run-off mainly in experimental systems. In Arizona, clay soils have been treated with sodium salts; these cause the clay particles to disperse so blocking the pores in between. Treatment with water repellents such as paraffin and silicon wax has also been tried. Surface sprays of asphalt have been tested on the sand dunes of Rajasthan in India and the Sahara in Libya and might be a feasible option for wealthy countries with oil. Other examples of high tech water harvesting have been tried in Arizona, Utah and Hawaii, where butyl rubber sheeting has been used to line both the catchment and the reservoir to provide water for livestock or domestic purposes. Although there have been problems as the rubber deteriorates and as a result of rodent damage, these schemes have generally been succesful Run-off water once collected has to be stored. In most of the traditional systems described, where the water is used for growing crops, the storage area is the soil itself. Deep water-retentive soils are therefore ideal. But if the soil is very freedraining, evaporative losses are high, run-off is excessive or if the water is to be used for cattle or domestic purposes, then a surface reservoir is needed for storage. Such reservoirs may range from simple pits dug in the ground, sealed by puddling with clay, to more substantial tanks and cisterns. Water losses as a result of evaporation can be reduced by covering the surface of the reservoir with materials such as wax, polystyrene or foamed rubber sheeting. Ensuring minimum yields At the Desert Runoff Farm Unit in the Negev Desert, some of the ancient run-off farming systems have been carefully re-created and now serve as experimental farms. Here, where the scanty and unpredictable rainfall is less than 200 mm a year, a tremendous range of crops has been successfully grown using harvested water alone. These include grasses and cereals (such as wheat, barley, sorghum and millet), legumes (lentils, chickpeas and novel varieties of beans), grape vines, soft fruit (logan berries and fruit, nut and fuelwood trees plums, apricots, peaches, pomegranates, cherries apples, figs, olives, carobs, pistachios, Acacia, Prosopis, Leucaena and Eucalyptus). In good years yields are comparable with crops grown in irrigated conditions but even in drought years there is usually some yield. Cereals, almonds, pistachios and olives do particularly well. Annual crops which are quick maturing (such as millet) or deep rooting (such as sorghum) and perennials which can tolerate both drought and occasional inundation are those which do best. Dr Pedro Berliner, head of the Desert Farms Research Unit, says 'We believe this type of agriculture can help solve some of the more urgent needs of the sub-Saharan belt and other similarly drought-stricken parts of the world'. Others are more guarded in their optimism. Kutsch, a German researcher who has identified a number of areas in Africa likely to be suitable for run-off farming, points out that only in northern Africa, where rain occurs in the cooler winter months. are conditions directly comparable with those in the Negev Desert. In the dry areas of tropical Africa, where temperatures are higher and any rain usually falls in the hot summer months, rapid water loss from the crop and the soil may mean that only schemes where the annual rainfall exceeds 300 mm will succeed. Mathematical models which take account of weather data and soil conditions can now help in the selection of areas suitable for rainwater harvesting and in the design of appropriate systems. But where few records exist site selection is likely to depend, as in the past, on local knowledge and intuition, with schemes being modified or abandoned in the light of experience. Farmers in the Turkana District of northern Kenya use the growth of native plants to guide their choice of suitable sites. The dwarf shrub Duosperma indicates a deep loamy soil, whereas a particular Acacia indicates a site liable to waterlogging. For pasture improvement, semi-circular barriers of earth or stone can be constructed at intervals down sloping land. Grass seed is then broadcast and as rainwater washes over the surface, both water and seed collect behind each barrier allowing clumps of grass to establish which then soon spread. To be successful, new schemes need to take account not only of physical conditions but of local social and cultural traditions as well. For example nomadic people such as the Turkana may be reluctant to be tied to one area in order to keep elaborate systems maintained. However existing schemes show that water harvesting can do much to improve conditions in drought stricken areas. They not only provide people with water but also help control the soil erosion which occurs during sudden heavy storms. Harvesting relies only on local water and systems can be constructed using locally available materials and skills. Micro-catchments are particularly simple to lay out and maintain. In relatively wet areas water harvesting can allow crops to be planted or provide extra water for small irrigation schemes. In arid areas it can allow other sources of water to be reserved for emergencies, and so make the difference between life and death.
- CTA Spore (English)