Irrigation is the artificial application of water to the surface of the soil to create a conducive environment for the growth and survival of plants. This process is often carried out during the dry season, during periods of drought, or when water storage from rainfall is insufficient in the rainy season.
This article explores the irrigation resources available in Nigeria, which provide water for irrigation schemes in the country.
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Irrigation Resources in Nigeria
The Earth is estimated to have about 1,386,000,000 km³ of water, with 97.22% of it found in oceans, 2.1% in glaciers and icebergs, and approximately 0.03% circulating annually through precipitation, transpiration, and evaporation.
This 0.03% of water includes snow, rain, flowing surface water, underground recharged water (percolation water), and atmospheric evaporative water vapor. This natural water circulation is referred to as the hydrologic cycle or water cycle.
Nigeria’s total annual renewable water resources are estimated at 286.2 km³. Annual internally produced resources amount to 221 km³, consisting of 214 km³ surface water and 87 km³ groundwater, with 80 km³ of the latter overlapping between surface water and groundwater.
External water resources (from neighboring countries) such as the Niger, Cameroon, and Benin rivers also contribute to the country’s water supply. Exploitable surface water resources are estimated to represent 80% of the natural flow, about 97 km³ per year.
Annual extractable groundwater resources are about 59.51 km³, with 10.27 km³ found in the south. The total dam capacity is estimated at 44.2 km³. In the year 2000, total annual water withdrawal was estimated at 8 km³.
Agriculture was the largest water user, accounting for 5.5 km³ (69%) of the total water withdrawal, followed by the domestic sector with about 1.7 km³ (10%).
Surface Water for Irrigation
Surface water is cheaper and easier to use compared to groundwater. The composition of surface water varies depending on the terrain through which it flows, as the minerals dissolved in the water change accordingly.
The kinds and amounts of material dissolved in surface water can vary widely, which may affect its suitability for irrigation. There are four principal surface water basins in Nigeria:
- The Niger Basin: This basin covers an area of 584,193 km² within Nigeria, which is 63% of the country’s total area, and spans a large area in central and northwestern Nigeria. The major rivers in the basin are the Niger River and its tributaries, including the Benue, Sokoto, and Kaduna rivers.
- The Lake Chad Basin: Located in the northeast, this basin covers 179,282 km², or 20% of Nigeria’s total area. It is the only internal drainage basin in the country. Important rivers in this basin include the Komadougou Yobe and its tributaries, such as Hadejia, Jama’are, and Komadougou Gena.
- The Southwestern Littoral Basins: Covering 101,802 km², or 11% of Nigeria’s total area, these basins originate in the hilly areas to the south and west of the Niger River.
- The Southeastern Littoral Basins: With major rivers such as the Cross and Imo Rivers, this basin spans 58,493 km², or 6% of Nigeria’s total area, receiving much of its runoff from the plateau and mountain areas along the Cameroon border.
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Groundwater for Irrigation
Recent increases in irrigated land areas have been made possible by the greater use of groundwater. Groundwater is stored in underground reservoirs, typically found in porous rock formations known as aquifers.
These aquifers can extend several kilometers or even up to 100 kilometers. Groundwater generally maintains a more constant temperature, contains fewer sediments, and has fewer dissolved materials compared to surface water.
However, drilling wells and pumping water from underground reservoirs are more expensive than utilizing surface water.
Nigeria has extensive groundwater resources, located in eight recognized hydrological areas, along with local groundwater found in shallow alluvial (fadama) aquifers near major rivers. The following are the key groundwater zones in Nigeria:
- The Sokoto Basin Zone: Comprising sedimentary rocks in northwest Nigeria, with yields ranging from below 1.0 to 5.0 liters per second (L/s).
- The Chad Basin Zone: Comprising sedimentary rocks with three distinct aquifer zones—Upper, Middle, and Lower. Borehole yields range from 1.2 to 1.6 L/s in the Upper unconfined aquifer and from 1.5 to 2.1 L/s in the Middle aquifer.
- The Middle Niger Basin Zone: Comprising sandstone aquifers with yields between 0.7 and 5.0 L/s and the Alluvium in the Niger Valley, yielding between 7.5 and 37.0 L/s.
- The Benue Basin Zone: This is the least exploited basin in Nigeria, extending from the Cameroon border to the Niger-Benue confluence. Sandstone aquifers in this area yield between 1.0 and 8.0 L/s.
- The Southwestern Zone: Comprising sedimentary rocks bounded by coastal Alluvium in the south and the Basement Complex in the north.
- The South-Central Zone: Made up of Cretaceous and Tertiary sediments centered around the Niger Delta, with yields ranging from 3.0 to 7.0 L/s.
- The Southeastern Zone: Comprising Cretaceous sediments in the Anambra and Cross River basins, with relatively low borehole numbers due to abundant surface water resources.
- The Basement Complex Zone: This area, covering over 60% of Nigeria, consists of low permeability rocks. Groundwater occurs in the weathered mantle and fracture zones, yielding between 1.0 and 2.0 L/s.
Nigeria is endowed with a well-drained network of rivers, streams, and extensive groundwater resources that support irrigation activities. These irrigation resources span the entire country and, if properly utilized, can significantly enhance food production, contributing to food security and poverty reduction among the rural population.
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