The project involved hydrological analysis of the Sagana river catchment, a catchment located
northwest of Nairobi. The catchment is a part of the central basin, one of the five main basins
of Kenya.
Using WHO maps, the catchment area was estimated to be 1938 km2 which translates to about
10% of the upper Tana River catchment. Tana River is the most important river in the country in
terms of water and hydro-electric power provision. There was a need, therefore, to study the
area and get its hydrological information.
The data used for the study was obtained from the Kenya Meteorological Department and the
Ministry of Environment, Water and Natural Resources. This data is from the good network of
stations established in the area and collected by experts over a long period. Rainfall data, river
flow data and sedimentation data was used.
Rainfall data was analysed for volumes, monthly patterns and yearly trends. River flow data was
processed for trends and fluctuations; averages and high flows (flood flows).
By the use of Thiessen polygon method, average rainfall in the area was found to be 995.03 mm.
Two rainfall seasons were obtained, March to June and October to December. It was also found
that the amount of rainfall received in the area remained almost constant with high rainfall
years coming after a period of every ten years.
The area experienced high flow with frequent flood peaks. In the two-case-study tributaries of
Sagana River, using the flow duration curves, flows with a 95% probability of occurrence was found to
be 2.2m3/s in station 4AC03 and 3m3/s in station 4AD01.
Water balance assessment indicated adequate quantities of water for use and potential
conservation measures for the excess water. Therefore the area has a huge potential for water
conservation which could be done by reservoir development.