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4.3 Tidal Irrigation, The Gambia

Introduction

The Gambia is one of the smallest countries in the Sahel region of West Africa, surrounded on all sides by Senegal, except on the western side of the country which borders the Atlantic Ocean. The Gambia is bisected by Gambia River and lies in the east-west direction between the longitudes 160 50' and 130 45' W and latitudes 130 00' and 13o 50' N. The country is approximately 480 km long and nowhere is it more than 50 km wide. Its total surface area is about 11 000 km2, with about one third of its surface area covered by the Gambia River and the marsh lands along its banks. The main topographic feature is the low lying basin of the Gambia River. The river runs through the whole length of the country with only a few points above 50 m in elevation. Its capital is Banjul.

The Gambia has savanna vegetation and lies in the Sahel of West Africa. The climate is uniform across the country due to its small size and relatively flat features. The country has a single rainfall season annually, which starts in June and ends in October. The rainfall in the country varies evenly from 1 100 mm in the south-west to 700 mm in the north and east. The rainfall is highly seasonal with all but 1% or 2% falling in the raining season.

The Gambia River rises in Guinea and passes through Senegal before finally entering The Gambia for an approximately 500 km journey to the sea. The flow in the river is highly seasonal. The maximum flow occurs at the end of the rainy season in late September or October with a flow of about 1500 m3/s. The minimum dry season flow is less than 4.5 m3/s, both measurements taken at Gouloumbo in Senegal. Due to the large variation in river flow and the flat nature of the country's terrain, the Gambia River is tidal, and thus saline, for much of its length.

The position of the interface between the freshwater and saltwater varies with river flow. During the low flow period, the freshwater-saltwater interface, defined as the point at which the salinity is 10 ppt, is 250 km from the sea. Under high flow conditions, this interface is located 150 km from the sea. Due to the perpetually saline conditions which exist in the Gambia River and its tributaries for 150 km from its mouth, where the population centres and tourism facilities are located, surface water is rarely used as a source of potable water in The Gambia. The potable water demand for urban areas, tourism, industry, and irrigation and livestock watering comes from groundwater sources.

Groundwater is available in all parts of The Gambia. The country is located on one of the major sedimentary basins in Africa often referred to as the Mauritania/Senegal Basin. It is characterised by two main aquifer systems with water table depths varying from 10 m to 450 m.

Technical Description

This technology is intended to supplement rain fed agriculture. The availability of tidal water at high tide was used as source of irrigation water supply. Due to the use of this technology, a double cropping of rice is achieved annually in a country with seven months of dry season.

The land along the Gambia River is relatively flat, and, since the river is tidal all through its length in The Gambia, tidal irrigation schemes become feasible. Tide heights vary from 3.5 m at the mouth of the river to 0.9 m at Basang, 310 km upstream. Special intake structures were constructed with gates which, when opened at high tide, allowed tidal waters to enter irrigation channels leading to the farms. During high tide, the gates were opened from 3 to 24 hours, depending on the size of the area to be irrigated. In two rice growing areas, at Jahally and Pacharr, tidal and pump irrigation are coordinated. Tidal heights of 1.3 and 1.0 exist in the Gambia River at Jahally and Pucharr, respectively (Figure 45). Tidal water is utilized to irrigate low lands nearer the banks of the river while water is pumped from the river to irrigate large areas of land at higher elevations. The project began operations in 1983 and 1984 at Jahally and Pacharr, respectively.

This technology is described in Part B, Chapter 1, "Agriculture."

Figure 45. Jahally irrigation pumping units.

Effectiveness of the Technology

The technology has been successful in paddy rice cultivation as a rural development project. Using tidal irrigation, double cropping of 167 ha and 850 ha was achieved annually at Jahally and Pacharr. Similarly irrigation of 440 ha and 125 ha is achieved annually at Jahally and Pacharr, respectively using pump irrigation (Figure 46).

Figure 46. Jahally irrigation field.

Costs

The projects were financed by the Gambian Government, the International Fund for Agricultural Development (IFAD), the African Development Bank (ADB), and the German Government. During the design and construction stages, the project management was in the hands of the financier and exact capital cost figures were not available from the current local project management. Nevertheless, the estimated cost of the project, in 1983/84 dollars, was approximately $7.5 million. Assuming a 7% inflation rate, a 25-year life, and 7% discount rate, the annual cost of project may be estimated at $643 583, or $40/ha. Current operation and maintenance costs are $220/ha/yr. The resultant yield per hectare is 9 tons/yr, incurring an annual cost per unit of output of $70/ton.

Suitability

The technology is appropriate in areas where there is a river with a relatively flat basin and high tide intrusion. Arable land must be available near the banks of the river. The rainfall in the area must be sufficient to encourage constant and high river flows. The technology is also good for use in areas with fairly large rivers and sufficient rainfall to keep the water level high. The rivers must also be tidal.

Operation and Maintenance

Trained local staff must be available to perform the farming operations and management. Additional manpower needed to implement this technology include: (a) one power tiller operator for each 15 ha cultivated per month; (b) two tractor operators; and, (c) two experienced mechanics. There should be about 20% local community control or management.

Advantages

This technology is good because, once the intake structures and irrigation channels have been constructed, the operation is relatively cost free. Maintenance work on the irrigation channels and clearing of weeds and brush from the channels and irrigated area can be done by the local farmers.

Disadvantages

There is a difficulty being experienced in the availability of spare parts locally.

Environmental Benefits

The breeding of mosquitoes and snails is enhanced by water ponding on the farms, which could lead to public health concerns if control measures are not imposed.

Cultural Acceptability

No cultural inhibitions have been experienced. This technology provides for viable commercial farming in a poor rural area.

Further Development of the Technology

No further development appears to be required at this time.

Information Sources

Director, Department of Water Resources, 7 Marina Parade, Banjul, The Gambia.

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