Nigeria has been divided into eight hydrological areas or provinces. Table 3.4.1 shows the total and percentage arears covered by each hydrological area. The Chad basin area with more than 21 per cent has the largest, while the Niger south has the smallest area. The decrease in the density of the drainage network from south to north is easily noticed; it is due to the combined effect of hydroclimatic and geological factors.
RIVER FLOW REGIMES AND NEED FOR REGULATION
The pattern of seasonal variation which tends to be repeated from year to year is called river or flow regime. Parde (1947) classified river regimes into four major types, namely: simple, complex 1, complex 11 and torrential regimes. The simple regimes have two hydrological seasons only, a flood season during and immediately after the rainy season, and a low water period corresponding to the dry season.
The Niger River, the principal river system in Nigeria, belongs to a regime that is typical of tropical rivers which have the complex regime of the second degree, and characteristic of most of the world's large rivers, such as the Nile and the Zambezi. The torrential regime is exemplified by some of the headwaters of the Hadejia and Rima rivers. The flows are intermittent, occurring mainly during the short rainy season. The regime characteristics discussed above are important determinants of the usefulness of the rivers for navigation, hydro-electric power generation, or any other purpose which requires a certain minimum stage or flow over a given period of time.
They also influence the cost and efficiency of developing water resources for human survival. Where the volume of flow is adequate and sufficiently stable under natural conditions, it becomes cheaper to harness the river for development, e.g. for hydro-electric power production, without having to incur large expenses on complex regulating structures. In general, the more favourable the river regime is, the less storage capacity is needed to be provided in order to maximise available yield or run- off.
Table 3.4.2 shows the annual run-off and specific yeild of the major river systems in the country. It is a good summary of the surface water resources potential.
The specific yield is the discharge per unit area of the basin at the stations shown in the table. The specific yield gives clear indication of the humidity or aridity of the basin in question: a basin which receives high rainfall over all or much of its area normally bestows high specific yield to rivers draining it. On the other hand, rivers draining dry or semi-arid basins have low specific yields. For example, the Benue drains much more humid area than the Niger and contributes some 60 per cent of the Niger-Benue system flow, even though it accounts for less than 35 per cent of the basin area.
The rivers carry their sediment load as bedload (5-6 per cent), suspended and siltation solution load. The latter load is responsible for the building up of the alluvial valley, and an important factor of reservoir sedimentation. The implications of excessive sediment trans- port include aspects of impaired water quality, basin degradation, soil loss and deterioration, valley aggravation and reservoir sedimentation.
Hydrological Areas/Surface Water Provinces in Nigeria