WATCH / Time2WATCH projects towards the implementation of a permanent observatory of groundwater in Kenya – A first hydrogeological model of the Chyulu Hills

In semi-arid southern Kenya, the Chyulu Hills consist of an alignment of Quaternary scoria cones and basaltic lava flows. This ~80-km-long, NW–SE volcanic fissure vent hosts underground water resources of importance to the local rural population and the savanna ecosystems. The subvolcanic topography allows groundwater to flow south and east, resulting in a line of springs along the base of the hills. Several springs are partially tapped to supply water for drinking water and farming activities. Mzima spring, in the south, yields 70% of the total outflow of the Chyulu Hills watershed, and 10% of Mzima water is diverted from its local use to supply the city of Mombasa, 200 km to the southeast. This generates conflict between local residents and regional water resource authorities. It is therefore crucial to quantify the water resources of the Chyulu Hills and establish to what extent these are suitable for sustainably supplying the local and wider regional population in the future, in a context of global change. The WATCH and Time2WATCH projects (2024–2026), funded by the Centre National de la Recherche Scientifique (France), aim to assess and monitor Chyulu-wide water budgets by setting up a multidisciplinary observatory combining meteorological, geophysical, geological, hydrogeological, and land use/land cover evaluations. This observatory was elaborated in close collaboration between Kenya (University of Nairobi, Technical University of Kenya, Regional Centre on Groundwater Resources Education, Training & Research) and France (Université Lumière Lyon 2, Université Claude Bernard Lyon 1, Université Marie and Louis Pasteur, Université de Montpellier, Sorbonne Université). The present contribution mainly focuses on preliminary hydrochemistry results. Their integration across the entire observatory provides the first functional insights into the Chyulu Hills groundwater system.