**2. Study area characteristics**

The Athi River flows from the southeast and north-eastward in the upstream reaches of the City County of Nairobi and then turning to the southeast in the north of Ol Doinyo Sabuk hill and flows along its boundary with Tana River drainage basin. The total drainage area is about 37,750 km<sup>2</sup> representing 64.4% of the Athi River Catchment Area. Some of its tributaries such as Lumi River, Lake Jipe, and Lake Chala flow into Tanzania and the Umba River flows from Tanzania to Kenya. Others such as the Rare, Mwachi, Pemba, and Ramisi rivers flow into the Indian Ocean making a total drainage area of 19,493 km<sup>2</sup> . Several springs exist in the basin such as Mzima, Kikuyu, Njoro Kubwa, Nol Turesh, etc., whose waters are used both for Municipal and domestic water supply.

The mean annual rainfall in the basin ranges between 600 mm in the central part of the area to 1,200 mm in the upstream area of the Athi River with an overall average mean annual rainfall of 810 mm. The JICA [1] report calculated the renewable water resource, which is defined as precipitation minus evapotranspiration, at 4.54 BCM/year in 2010 for the basin and per capita renewable water resources at 464 m<sup>3</sup> /year/capita. This is an indication that the basin is tilting towards a water scarcity scenario since the per capita value is less than the global accepted value of 1000 m<sup>3</sup> /year/capita (UN [3, 4]). According to the World Bank Survey [5], Kenya's renewable internal freshwater resources stood at 412cubic metres per capita.

The main land use activities in the basin include urban, residential, industrial, transport in the main urban centres (Nairobi and Mombasa) and agricultural and livestock keeping both in the middle and lower reaches of the river as shown in **Figure 2**. These various land use activities have significant impacts on the water resources in the basin. They cause significant water quality degradation in the basin, reduce the available water resources in the basin while increasing competition in water usage and conflicts. The overall implication is reduced water availability for various uses in the basin and hence affecting people's livelihoods potential and leading to increased vulnerability to climate change. This calls for better adaptive strategies to cope with reducing water availability and re-innovation of sustainable methods of water resources exploitation and use.

These various land use systems contribute significantly to pollutants, pollution and water quality degradation as well as to changes in river hydrology, which is worth investigation. The land use changes in a spatial manner from the rich agriculturally based system, through residential and urban to industrial, making it ideal for investigation of gradual water quality degradation downstream the river systems. The present study on land use changes and their effects on the water quality and livelihoods in the drainage basin is an eye opener to the problems related to the land-water nexus and development in the county and country in general [6].

#### **Figure 2.**

*Land use systems in the upper catchment area (Nairobi sub-basins). Source: Field data (2006–2017) and adapted from Kithiia [6].*
