Cover of the hydrology report from the Sasumua site.

Nairobi city, like other major cities in Africa, relies on distant forested catchments for its water supplies. One of such catchments is the Sasumua watershed, which supplies 15% of Nairobi’s water demand.

Land use changes in upstream areas of the Sasumua watershed have led to increased sedimentation and water contamination. Ensuring proper hydrological functioning of the Sasumua watershed is therefore of critical importance to the future water supply of Nairobi.

Given the importance of the Sasumua watershed and the urgency of the problems affecting it, the World Bank with the World Agroforestry Centre (ICRAF) and partners undertook a study to identify sources of sediments and pollutants in order to facilitate decision making on targeting interventions.

The study was aimed at laying the foundation for a reward scheme for hydrological services provided by farmers in the watershed. The project’s objective was to assess the nature, extent, severity and impacts of land degradation in the Sasumua watershed, thus providing technical information indispensable to the development of a payment for environmental services (PES) mechanism in the watershed.

The activities undertaken under this study mainly focused on:

1. carefully delineating the watershed from which service users obtain their water;
2. identifying critical areas within the watershed that are (or potentially may) affect the provision of the desired services (for example, areas prone to erosion that may be contributing to sedimentation of waterways); and
3. calibrating and validating hydrological models so as to be able to estimate the changes in watershed services that would result from land use changes.

The results of the retrospective land use land cover analysis showed that the main drivers of land use change in the catchment are population increase, intensified agriculture, land tenure issues (as the land is becoming increasingly freehold) and policy changes. The location of the Sasumua reservoir downstream of Njabini township, intensive farmlands and an agro-processing factory all represent a major challenge to catchment conservation.

Physical and chemical characteristics and also nutrient levels of surface water were sampled from various points along rivers Mungutio, Little Sasumua, Major Sasumua and Kiburu. Sedimentation was high for most sampled points.

The result team obtained feedback and review from stakeholders, especially the Water Resources Management Authority (WRMA), Nairobi City Water and Sewerage Company (NCWSC) and the Farmers’ Forum. The stakeholders expressed appreciation for the value of the research studies. Most participants endorsed the results as an accurate reflection of the situation in the catchment.

The results of the study provide us with both opportunities and challenges for the design and implementation of an incentive scheme for environmental service providers. We are however in the formative stages because we are in the process of undertaking further studies (including socio-economic analysis, analysis of the institutional architecture, carbon stock measurements, production economic analysis) to inform mechanism design.

From the findings, it can be concluded that there is a need for a more detailed environmental audit and physical planning in and around Njabini town. Open grazing and tree harvesting in riverine areas and the forest reserve need to be more strictly controlled. Water abstraction for multiple uses – including horticulture and planting of fast-growing Eucalyptus trees – is a major concern in the catchment with many stakeholders concerned that rivers are drying as a result of over use.

For the detailed report, please click here >>.