The rainfall in the catchment basin may be taken as 1250 millimetres per annum on the average. As the evaporation off the lake is probably the same, the area of the lake may be left out of the catchment altogether. The balance of the catchment basin amounts to 184,000 square kilometres, on which there is a mean annual rainfall of 230 cubic kilometres. The mean discharge of the Victoria Nile over the Ripon Falls appears to the approximately 580 cubic metres per second or 18 cubic kilometres per annum. This represents about ¹⁄₁₂ the mean rainfall. The greatest discharge of the lake seems to be about 850 cubic metres per second and the lowest 450. As the lake has risen in a single year 80 centimetres, which represents an increase of water of 48 cubic kilometres, and has fallen 60 centimetres which represents a decrease of water of 36 cubic kilometres, it will be seen that the discharges from the lake are factors of less importance in determining the level of the lake than the heavier rainfall and diminished evaporation in a year of good rain, and the lighter rainfall and increased evaporation in a year of poor rain. The great function of Lake Victoria in the economy of the Nile supply is the insuring of a nearly constant discharge of water into the Victoria Nile, and providing much of the evaporation which comes down in the catchment basin itself in the shape of rain. The principal feeders of Lake Victoria are the following streams:-- on the north (1) Lukos or Yala, (2) the Nzoia 250 kilometres long, (3) the Sio. on the east (1) the Nyando, (2) the Inyayo, (3) the Gori, (4) the Mara Dabash, (5) the Rawana. on the south (1) the Symiya, (2) the Moame. on the west (1) the Lohungati, (2) the Kagera, with its branches the Nyavarongo, the Akanyaru and the Ruvuvu, with a maximum length of 600 kilometres and a discharge varying between 140 and some 600 cubic metres per second. (3) the Ruizi with a length of 280 kilometres traversing much marshy ground in its course. and (4) the Katonga 250 kilometres long. The northern and western feeders are generally perennial streams, while many of the southern and eastern are torrents.