Case study of Ewaso Nyiro catchment in Kenya

Rainwater moves most very fast on the steep slopes of the Aberdare Ranges, and the Nyandarua Highlands. These areas receive relatively high rainfall and have steep gradients, causing water to flow quickly into tributaries such as the Nanyuki, Naromoru, Burguret, Timau, Ewaso Narok and Isiolo rivers before joining the main Ewaso Nyiro River. During heavy rainfall, surface runoff is accelerated where vegetation has been cleared or cultivated intensively.

Rainwater is stored temporarily in several natural reservoirs within the catchment:

• Forests on Mount Kenya and the Aberdares intercept rainfall and slowly release water.
• Soils in the upper catchment absorb rainfall, storing soil moisture for crops and vegetation.
• Groundwater aquifers, including those around Lake Ol Bolossat, store infiltrated water and maintain river flow during dry periods.
• Wetlands and swamps, especially the Lorian Swamp, store floodwaters and release them gradually downstream.
• Small reservoirs, pans and dams constructed for irrigation and livestock also provide temporary storage.

How do these fast-flowing and storage areas influence water availability in the catchment?

The interaction between rapid runoff and water storage determines the availability of water throughout the year.

• In the upper catchment, forests, wetlands and groundwater recharge help regulate river flow by releasing water slowly after rainfall.
• In the middle and lower catchment, particularly in Isiolo, Samburu, Wajir and Garissa, communities depend on this delayed release because rainfall is low and highly variable.
• If forests are degraded or wetlands drained, rainfall flows away rapidly as runoff instead of infiltrating into the ground. This results in: higher flood peaks during rainy seasons; lower river flows during dry seasons; reduced groundwater recharge; more frequent drought impacts on pastoralists, irrigated agriculture and wildlife.
• Increasing upstream water abstraction for irrigation has also reduced downstream flows, sometimes causing sections of the river below Archer's Post to become intermittent during dry seasons.

Can you identify a local river, lake, or wetland whose condition cannot be understood without considering upstream land and groundwater processes?

A good example is the Lorian Swamp in Isiolo County.

The health of the Lorian Swamp depends almost entirely on processes occurring hundreds of kilometres upstream. Rainfall captured in the forests of Mount Kenya and the Aberdare Ranges infiltrates into soils and groundwater before feeding tributaries that eventually reach the swamp. Consequently:

• Deforestation in the upper catchment increases erosion and sediment transport.
• Expansion of irrigation reduces river discharge reaching downstream areas.
• Over-abstraction of groundwater lowers baseflow that normally sustains the river during dry periods.
• Soil erosion increases sediment deposition within the swamp.
• Climate variability further reduces inflows.

As a result, the ecological condition of the Lorian Swamp—including its wetlands, grazing resources, biodiversity and seasonal flooding—cannot be understood without considering upstream land use, water abstraction, forest conservation and groundwater recharge. The same applies to river reaches through Samburu and Buffalo Springs, where river flows support wildlife, pastoral livelihoods and domestic water supplies.