Benefits from upstream irrigation projects cannot replace those derived from natural floodplains
in semi-arid Africa, the natural floodplains (or wetlands) inundated by seasonal river flows are of strategic economic importance. In many instances, the use of wetlands -- for agriculture, grazing, water supply and other benefits -- are integrated with those of the surrounding drylands. The floodwater is rarely wasted.
Thus, when irrigation schemes divert floodwater away from these wetlands, they have a considerable impact on the local people. Assessment of these impacts is generally not undertaken. There is, however, a growing awareness that these impacts should be studied before proceeding with such projects.
Suppose an upstream irrigation project in a semi-arid river basin diverts water from an existing floodplain, then any resulting loss in wetland benefits downstream should be included as part of the overall cost of the project. Using this rationale, Edward B Barbier of the University of York, the uk, and Julian R Thompson of the University London College, the uk, analysed the Hadejia and Jama'are river basin in northern Nigeria ( Ambio , Vol 27, No 6).
The two rivers converge to form the Yobe River, which subsequently drains into Lake Chad. Although referred to as wetlands, much of the floodplain is dry for most of the year, barring August and September when extensive areas are flooded. During this time, the floodplain provides a source of income and nutrition to the local population in the form of agriculture, non-timber forest produce, fuelwood and fishing. The wetlands also provide dry-season grazing, agricultural surpluses for the states in which they lie and serve as "insurance" in times of drought.
However, in recent times, the floodplain has come under increasing pressure from upstream projects. The maximum extent of flooding has declined from 250,000-300,000 hectares (ha) in the 1960s and 1970s to around 70,000-100,000 ha in recent times.
The largest scheme, at present, is the Kano River Irrigation Project ( krip ), upstream of Hadejia. Water supplies for the project are provided by the Tiga Dam, the biggest dam in the basin. Water is also released from this dam into the Kano River for use downstream.
The second major irrigation scheme within the river basin, the Hadejia Valley Project ( hvp ), is under construction. A barrage has been built across the river creating a storage pond capable of holding one week's irrigation water requirements. Linked to the hvp is the Challawa Gorge Dam on the Challawa River, upstream of Kano. This dam is designed to store water during the wet season. A number of small dams and irrigation schemes have also been constructed or are planned for minor tributaries of the Hadejia River.
In contrast to the Hadejia, there are relatively few projects disturbing the flow of the Jama'are River. There is only one small dam across one of its tributaries. However, there are plans for a major dam -- Kafin Zaki -- on the Jama'are. With a storage capacity twice as large as Tiga dam, Kafin Zaki will be the largest dam in the basin.
The researchers say that these irrigation projects have failed to consider the impact on the floodplain or any loss of benefits derived from the wetlands.
Water diverted by irrigation projects has an "opportunity cost", or alternative use, in the form of various wetland benefits provided by the floodplain. Barbier and Thompson analysed the net benefits forgone through floodplain losses in the basin and concluded that the benefits exceed the net agricultural production gain from the projects.
However, in the case of Tiga Dam, the researchers say that a regulated release of water to Kano in the wet season could reduce floodplain losses. However, there appears to be little justification for the construction of Challawa Gorge and other smaller dams.
The upstream projects are likely to irrigate more land upstream. However, once again, the analysis shows that the gains in terms of additional irrigation production values fall short of compensating for the loss in floodplain benefits.
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