Irrigation and Flood Control
For thousands of years, Africans have sought to manage the flow of water through their landscape. The continent's unreliable rainfall and frequent droughts make irrigation an essential tool for agriculture. In addition, various rivers flood frequently, and many people live in the floodplains surrounding them. Farmers and engineers have devised a variety of irrigation and flood control systems to make the best use of available water resources.
Traditional Irrigation Schemes
The earliest use of irrigation probably took place in ancient EGYPT along the banks of the NILE RIVER. The Nile floods every summer, leaving behind a layer of rich silt when the water recedes in early fall. Egyptian farmers developed a system called basin irrigation that involved dividing the land along the river into large basins with low walls. During the flood, water filled the basins. When the river fell, the farmers allowed the water to drain away and then planted crops in the wet soil left behind. Although basin irrigation supplied water for planting crops, it could not control the size of the flood, and farmers still faced crop failures during dry years. In addition, farmers using basin irrigation could only plant one crop a year on their fields.
Floodwaters also support crops in MALI, where the NIGER RIVER forms an inland delta. Farmers in the region practice a technique known as flood cropping. They plant rice near the river in July and August. The river floods in the fall, covering the land and helping the rice to grow. When the water recedes between December and February, the farmers harvest the rice. Like that of the Nile, the size of the Niger flood is unpredictable. The farmers therefore plant several varieties of rice, some adapted to drier and some to wetter conditions. In this way, a crop will survive regardless of the extent of the flood.
Livestock herders take advantage of river flooding. In Mali herders graze their flocks in the Niger floodplain during the dry season, moving away when the waters begin to rise. The annual flood allows the delta to support about one-fifth of Mali's cattle, sheep, and goats. In addition, the flood provides excellent conditions for fishing. Nutrients brought to the fields by floodwaters encourage the growth of vegetation. The vegetation attracts small creatures that provide food for fish, which migrate into the floodplain to breed. For this reason droughts that reduce the extent of the flood affect fishing.
Basin irrigation and flood cropping both depend on river flooding, which can vary from year to year. To overcome this problem, farmers developed technology that allowed them to draw water from rivers for irrigation and to store water during the flood season for use at other times of the year. These new systems have made it much easier to provide crops with water during droughts.
Some basic irrigation systems found in Africa use simple pumps or animal power to move quantities of water. Another device, the shadoof, has a long pole set on a pivot with a bucket at one end and a counterweight on the other. The bucket is filled with water and raised using the counterweight. The pole is then rotated to swing the bucket and empty the water into irrigation canals that carry it to the fields.
More advanced irrigation systems such as barrages and dams are designed to store water. A barrage is a barrier built across a river that blocks the flow of water during flood season, creating a reservoir. The water is stored in the reservoir until the dry season, when it is released to irrigate fields downstream. Muhammed Ali Pasha, the viceroy of Egypt, built the first two barrages on the Nile between 1833 and 1843. Located about 70 miles north of CAIRO, they allowed the Egyptians to control the flow of water to the Nile delta and increase the amount of land under irrigation.
Dams also block the flow of river water, often creating a lake behind the barrier. Africa's first large dams were constructed in the 1900s. In 1925 the Sennar Dam was built on the Blue Nile in SUDAN. The dam provided water to irrigate the Gezira plain south of KHARTOUM. Because of its success, the Gezira project became a model for large-scale irrigation programs throughout Africa. In the 1960s Egypt built the Aswan High Dam on the Nile, which created Lake Nasser, the largest artifical lake in the world. Dams supply water for irrigation and produce hydroelectric power and new fishing industries. However, they also disrupt the local ecology by changing the flow of the river and submerging dry land under water.
Modern Irrigation Programs
Since independence many African nations have concentrated on building or improving irrigation systems as a way to increase crop yields. During the 1970s and 1980s, NIGERIA set up a number of River Basin Development Authorities to build dams, control water supplies, and provide services and supplies such as seeds to farmers. The governments of Mali, MAURITANIA, and SENEGAL jointly created the Senegal River Development Organization to manage water resources in their region. However, some government irrigation programs have fallen short of expectations due to the high cost of construction and maintenance, poor planning and management, and lack of knowledge of the region's environment and economy. In many instances modern systems have failed to improve on indigenous methods of irrigation because they were unsuited to the local conditions or crops.
In recent years African countries have increasingly turned to farmermanaged irrigation systems. Under these plans local farmers control water supplies, plan irrigation schedules, and maintain irrigation equipment. By taking control of irrigation away from the government and placing it in the hands of local farmers, African nations hope to benefit from the farmers' knowledge and skill while controlling costs. These programs also recognize the value of indigenous irrigation systems, which are often the most efficient methods of watering crops.
It is hoped that all of these changes will improve the quality of irrigation systems in Africa. Irrigation policy is becoming more diverse, with more options available to farmers at all economic levels. In addition, government leaders are paying more attention to indigenous methods and are not forcing farmers to use systems that might use more technology, but actually produce poorer results. The skills and needs of small farmers now play an important part in irrigation policy, although it will take time for new systems to gain acceptance. In addition, there is hope that large-scale agriculture businesses will also benefit from new policies. (See also Agriculture, Climate, Deserts and Drought, Ecosystems, Fishing.)