By Martin Ager*
ADDIS ABEBA – One common observation in most countries of East Africa is that there are intense rainstorms followed by long dry periods. Long-term average rainfall in the region varies from around 1,200mm/year in the wetter countries, such as Rwanda and Burundi, to 220mm/year in Djibouti.
Even these figures do not tell the whole story, as there are variations within countries and from one season to the next.
The rain that falls runs off rapidly, in some cases leading to flooding, but once it is gone, farmers face long periods of drought. Even in the wetter countries, there are seasons when rainfed crops cannot survive while in the drier areas only extensive livestock farming is possible.
A dry season can spell disaster to the livelihoods of farmers across the region as there is not enough infrastructure to capture and store water. Rainwater harvesting techniques to capture the water when there is too much for use has been identified as a possible solution to this problem that has been tried in many countries.
Rainwater harvesting requires the building of structures where water can be captured and stored. A range of relatively low cost and simple techniques have been developed ranging from simple tanks collecting rainwater from the roofs of houses to larger ponds collecting surface runoff or small dams to capture the flow in intermittent streams.
The design of rainwater harvesting structures involves making a balance between the level of rainfall, catchment size, water storage size and the amount of water that is needed. Factors such as the cost of the work and the land available have to be taken into account since on smallholder farms the owner may not be able to give up a significant part of their land for water storage.
In many cases, the size of water storage is the limiting factor and some part of the intense storms still runs to waste while there is simply not enough water stored to meet the needs of crops, livestock or people throughout the year.
While people often think of the drier climates as having the most potential for rainwater harvesting, the opposite may be true. While the need for water in dry areas, such as Djibouti, is the greatest, much larger water storage structures are needed to retain water through the long dry season and this is often unrealistic. As the needs are so great that they will not last for many months after the end of the rainy season. In contrast, however, in wetter climates such as in parts of Kenya, the water storage will be frequently replenished and can provide water for a much greater part of the year.
Another technique is water harvesting from the roofs of houses, into plastic or concrete tanks of perhaps 5,000 liters which can only realistically be used for domestic water supply with maybe a very small vegetable garden or a few goats and chickens. This water is under the direct control of the household concerned to make decisions on its use.
While water coming directly from rain is relatively clean compared to water from rivers or shallow wells it may still not meet the standards for drinking water quality due to dust and animal or bird droppings from the roof or contamination within the tank. Various techniques exist for diverting the dirtier first flow at the beginning of the rainy season, filtration and disinfection; but in reality, these are rarely used in East Africa.
Larger ponds of around 3,000,000 liters may also be built on smallholder farms to collect surface water runoff. Again, these fall under the management of one household but they can be used for a hectare or more of irrigation or a larger number of livestock.
Significantly larger water ponds or small dams are more likely to be built by governments or development partners for communal use, either for irrigation or for livestock. Such shared resources should preferably be managed by a water users’ association or committee to make decisions about the allocation of water and the maintenance of the structures.
In agriculture, there are further rainwater harvesting techniques that can be used to store water directly in the soil for use by growing crops and these do not require the construction of any tanks or ponds. Run-off rainwater harvesting techniques involve capturing rainwater directly where it falls in the field by a variety of bunds or pits that keep the water in place for long enough to infiltrate into the soil. In some circumstances, this can be augmented by directing water to run on to the field, perhaps from a nearby hillside.
Efforts to increase the use of rainwater harvesting
The problem of the loss of water from intense storms followed by water shortages has been observed in many countries and many efforts have been made to harvest the rainwater by governments, the Food and Agriculture Organization of the United Nations (FAO) and a variety of development partners and individuals.
In Ethiopia a decade ago, the Government had a major initiative to support the construction of household rainwater harvesting ponds. Where the soils are impervious, no lining was needed but in some areas, they were lined with concrete (which sometimes cracked) or with plastic sheets that did not last long leading to excessive water losses through seepage. Some of these ponds are still in use but in other cases they have filled with silt due to soil erosion in the catchment area.
A similar project is now underway in Kenya where the main focus of the government is to dig the water ponds. More effort is needed to ensure that the catchments are of appropriate size to fill the ponds and work is now underway to ensure more sustainable designs and develop include relevant training and equipment to maximize the agricultural productivity of the ponds.
Meanwhile, in Djibouti, an FAO project is building rainwater harvesting structures of various scales for livestock including large open ponds to be filled by water diverted from watercourses during the occasional storms. Smaller structures capture overland flow into concrete tanks from which water can be taken by local residents using buckets to water their livestock.
Subsurface concrete dams have been built in Uganda to preserve water underground in sandy dry river beds. These allow water to be drawn from shallow hand-dug wells to be given directly to animals.
In order to alleviate the problem of water shortages for households, smallholder farms and pastoralist farmers it is recommended that governments and other development partners consider a significant scaling up of all types of rainwater harvesting.
It is clear that additional water storage is needed to bridge the dry periods. Relevant technicians and extension workers should be trained and funded to build well-designed rainwater harvesting systems to meet the specific needs of different localities and users. Depending on the country, these may be located in the Ministries responsible for water resources, irrigation, agriculture or livestock.
*Martin Ager works as a Land and Water Officer in the FAO Subregional Office for Eastern Africa