The hydrology of Lake Victoria

Abstract

Evidence is presented to show that Lake Victoria is triply stratified the whole year round. The conclusions of FISH (1957) that a uninodal internal seiche operates in the lake is shown to be invalid, and the oscillations of isotherms at E.A.F.R.O. Open Lake Station observed over some years are explained by the day to day wind changes. It is calculated that an internal seiche could operate between the middle and bottom layers, but not between the middle and surface layers, since any such seiche would break down the stratification. Surface waves of the type which normally induce internal seiches are commonly observed on the lake, so it is concluded that the extreme shallowness and great length of Lake Victoria cause a rapid frictional damping of any incipient seiche. Lake Victoria has an annual heat budget of about 9,000 gram/cals/cm2. This amount of heat is lost between March and August and regained between August and March. The loss of heat is occasioned mainly by the increase in evaporation from the lake in the period May to October, and partly by an increase in conduction and backradiation. The increase in evaporation rate between May and October assists in disposing of the surplus water derived from the rains in April. The lowest layer in the lake is believed to derive from the inflow of rivers. This layer mixes with the middle layer at the south end of the lake. An apparent area of mixing is suggested north of the Sesse Islands, also. Current measurements indicate a general northward flow of surface water in response to the wind. This is accompanied by a compensating flow of the two lower layers southwards. This latter movement is mainly phasic but probably has a small residual component. Lake level readings from gauges round the lake indicate that the west, north and north-east portions of the lake rise more rapidly in level than the south portion, from April to July.