Kainji Lake lies between 9°30'N and 10°35'N latitudes and 4°25'E and 4°45'E longitudes (Fig. 1). This region is an area of Guinea savannah maintained by annual burning. The topography is of low relief lying at an altitude of 500 ft (150 m) to 1 000 ft (300 m) above sea level, with a highly dendritic pattern of drainage.
The human population density is low in the areas adjacent to the lake and generally low throughout the drainage basin. Intensive agriculture is carried out in a few places, principally in the drainage basin of the Sokoto River, tributary of the Niger and the lake. Except in a few places population density is of the order of 10–20 per square mile (Udo, 1970).
The soils are generally poor away from the river alluvium, being highly variable in physical composition, low in phosphate, very low in nitrate but fairly rich in potassium. The upland savannah soils are almost totally lacking in humus materials. The surface horizons are composed of clay, sand and stones, while silt fractions are confined to the lower layers. Data resulting from general surveys of these soils may be found in Pullen and deLeeuw, 1964.
Kainji Lake lies in a transition region, dominated by dry continental trades during the winter months and moist maritime equatorial air during the summer. In West Africa the boundary region, generally called the Intertropical Discontinuity or “Front” (ITD), moves north and south seasonally through only about 12° of latitude (Fig. 3), while in East Africa the seasonal change is as much as 30° (Cloudsley-Thompson, 1969). The movement of the ITD gives marked seasonal change to the region. This, in turn, exerts a strong influence on conditions in Kainji Lake.
3.2.1 Precipitation
The annual rainfall in the area of the lake is about 1 000 mm (40 inches). This rain is almost entirely confined to a period beginning about mid-April and extending to mid-October, and much of it is associated with line squalls occurring between late afternoon and early morning. Monsoon rains, associated with low dense cloud cover, are the next most important source of rainfall (Barry and Chorley, 1968). The average seasonal distribution of rainfall for the two nearest stations with extended records is shown in Fig. 4. Yelwa is near the upper end of the north arm (see Fig. 1). Mokwa is about 40 miles to the south-east of the dam. These records show a single peak in rainfall at Yelwa, while there is a substantial mid-summer drop in rainfall (little dry season) at Mokwa. There is considerable variation among years in the amount and distribution of rainfall. In many years a little dry season may be recognized at Yelwa as well. This period corresponds to the northernmost extension of the ITS.
The local rains are of high intensity and the runoff picks up clay into colloidal suspension imparting a blue-white colour to local runoff. This material is retained in the rivers and the lake and gives the name “white flood” to this water.
About half the annual inflow to the Kainji Lake originates in runoff from the basin south of Naimey as “white flood”, entering the lake beginning in mid-August and lasting until December. The balance of the inflow originates in rains at the headwaters of the Niger in the Guinea highlands. This water, having passed through the flats around Timbuktu, is much clearer and is known as the “black flood”. It is considerably delayed and rises in October continuing into May.
During the dry season, the vegetation is burned to encourage new growth for cattle and to aid in hunting. This burning is virtually complete and extends well into January. The fate of the nutrients released in burning is not well known. It may be assumed, however, that much of the nitrogen is carried into the atmosphere, while phosphates and other nutrients that mineralize in burning remain in the ash; the latter is left on the soil surface at or near the site of burning. Ash is also deposited on the lake from nearby burning sites. Burning is generally done during periods of low wind, and hence the ash is rarely carried very far. “Dust devils” are very frequent during the dry season. These break up the ash and carry it, along with other debris back into the air. As the nutrient concentrations in the lake are apt to be substantially affected by the fate of nutrients released in burning, it is unfortunate that so little appears to be known about the redistribution of this ash.
3.2.2 Temperature
Air temperature is the Kainji area range from 15°C to nearly 40°C (Fig. 6). The continental air of the dry season, with little cloudiness, results in extremes of daily fluctuation in temperature, and the lowest (night) temperatures occur during the periods of domination of this air mass. As the ITD moves northward, southerly winds increase humidity and temperature, and the highest daytime temperatures occur at this time when there is still little cloud cover. With the onset of the rains, air temperatures continue to fall until August, and there is little diurnal fluctuation. In September and October, the rains diminish and air temperatures rise until the return of the continental air.
3.2.3 Humidity
As with air temperature, the humidity in this area is closely associated with the movements of the ITD, and is quite variable. A maximum occurs in August and September at the peak of the rains, but a little later than the maximum northward extension of the ITD. The minimum occurs in late January or early February (Fig. 7). There is some evidence of a local effect of the lake in the records at Kainji, where the relative humidity is less variable than at Yelwa or at Mokwa. The effect is particularly evident in higher humidity during the dry season.
3.2.4 Solar Radiation and Cloudiness
Data on solar radiation are available at Yelwa and Bida but not at Kainji or Mokwa. Fig. 8 shows means of the Yelwa and Bida figures and the two are closely correlated. Radiation maxima occur in February–March and November and minima in August and January. There is a close correlation with mean monthly air temperatures. The radiation minimum in January is probably the result of dust haze of the “harmattan”, and lower solar altitude (6 percent), while the minimum in August is presumed to be the result of cloud cover which is maximum at the same time. Records of sunshine hours, converted to percent of maximum sunshine are similar, as shown in the same figure, but less regular. The “harmattan” minimum is hardly to be found in these data.
A rough conversion of Gunn-Bellani observations to absolute units can be made by assuming equivalence with comparisons with actionographs made by McComb and Iyamabo (1968) near Zaria, Nigeria. Using the Yelwa and Bida records an annual mean insolation of 502 g-cal/cm2/day was calculated. Using Budyko's (1956) tables of maximum insolation at a latitude of 10° and the records of sunshine hours at Mokwa and Yelwa, monthly means of insolation were calculated. The agreement with the data from the Gunn-Bellani measurements was good (annual mean 494 g-cal/cm2/day), but the latter showed less extreme values by month (see section on heat budget of Kainji Lake).
Outgoing long-wave radiation was also calculated by month from the meteorological data (based on records obtained at Faku, just below the dam). Its mean value for 1969–1970 was 94 g-cal/cm2/day, leaving a net incoming radiation of 400.
One may also estimate the photosynthetically active component of the incoming radiation following Tailing (1960). Assuming that 6 percent is reflected from the surface and that 46 percent of the remainder lies within the 400–700 mm band (visible light), approximately 213 g-cal/om2/day is available, or about 100 kergs/cm2/sec. Maximum values occur in February and minimum values in August. Insolation is rather lower than reported by Tailing for East African lakes.
3.2.5 Winds
The direction of the prevailing winds in the Northern Nigeria results from the major flows: the northerly trade winds which blow in the area from mid-November to mid-March, and the southerly maritime “westerlies” which dominate the flow the remainder of the year (Fig. 10). The boundary region is the Intertropical Convergence Zone or “front”. These winds are light averaging about 2.5 to 3 m.p.h. (Yelwa data). Squalls are frequent at the beginning of the rains (April through June) and again in September and October. The latter, however, are much less frequent. These squall winds generally reach peak velocities of about 45 m.p.h., though 70 m.p.h. winds have been recorded. These squalls raise average wind velocities to as much as 7 m.p.h. during May (Yelwa data), and generally blow from the east.
Unfortunately records of wind velocities are not available for the lake itself. Land and sea breezes are evident at the margins of the main section of the lake, particularly in the late afternoon (sea breeze) and early morning (land breeze). the prevailing winds are most in evidence from late morning to mid-afternoon, while storms tend to occur from late afternoon to early morning. Many of the latter appear to originate near the Jos Plateau.
3.2.6 Visibility
While fog is relatively rare in the area, visibility is often much reduced during the “harmattan” season owing to dust brought into the area from the Sahara. Throughout the “harmattan” season, visibility is rarely more than 6–8 km while it is often reduced to about 0.5 km. During this season a compass is necessary for navigation in the central basin, while during the remainder of the year landmarks are visible from all parts of the lake.
3.2.7 Climatic Summary
The climate of the Kainji area is strongly seasonal. This is of considerable significance to the limnological characteristics of the lake and should impress strong seasonal characteristics to both the growth and movement of the fish fauna and the operation of the fisheries on the lake.
