Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation in Africa>
4.3 Tidal Irrigation, The Gambia
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.
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.
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.
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.
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
There is a difficulty being experienced in the availability of spare
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.
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.
Director, Department of Water Resources, 7 Marina
Parade, Banjul, The Gambia.