Lake levels and glaciers: Indicators of the changing rainfall in the mountains of East Africa

There is glacial evidence from the mountains of East Africa indicating that the climate of this region was wetter in the past. The slow response of glaciers to short period precipitation changes make it difficult to correlate glacial changes over the last few decades with instrumental rainfall measurements. The smaller Rift Valley lakes respond much more rapidly yet also show evidence of higher level stands which correspond to periods of increased precipitation at similar times to the mountain glaciers.

Such changes in the level of Lake Naivasha have been well documented for more than 50 years and some measurements are available back to the 1880’s. A simple water balance model has been constructed for Lake Naivasha. The model has been calibrated against local precipitation, evaporation and river inflow data for the period 1960–75 and transformed into a predictive equation for annual precipitation using lake level change and the lake surface area as predictors. The model was tested against the measured precipitation data for the period 1935–59. Correlation between predicted rainfall and Naivasha’s rainfall for this 25 year period was 0.76 while between predicted rainfall and the average rainfall for two stations in the foothills of the Aberdare mountains, from where 80% of the river inflow to Naivasha originates, it was 0.83 (the 99% significance level was 0.51).

The water balance model of Lake Naivasha indicates that the average precipitation in this region of Kenya (a) during the last decade of the last century was 0.15 m y⁻¹ greater than the average for the 1935–75 period and (b) during the pluvial period 10–12000 B.P. was around 0.35m y⁻¹ greater than the average for 1935–75, and that these increases were similar both around Naivasha and in the mountains of western Kenya. This leads us to suggest that the snout elevation of the Lewis glacier on Mount Kenya at 4270m during the 1890’s and the snout elevations inferred from the early Holocene moraines at 3500m on Mount Kenya and Kilimanjaro were in equilibrium with precipitation increases of a similar magnitude.