Building a data-driven conceptual model of the hydrological functioning of the Taita Hills tropical water tower system in Kenya

Tropical natural water towers regulate downstream freshwater resources, yet their hydro(geo)logical functioning remains poorly understood. This study integrates multi-year (2022-2025), multi-season hydroclimatic and tracer observations to build a data-driven conceptual model of a tropical metamorphic water tower at Taita Hills in Kenya. The collected dataset enabled rainfall partitioning, baseflow separation, multiple-method recharge estimations, hydrochemical and tritium analyses, and various spatial and multivariable analyses. Results show that the highland subcatchments, representing 43% of the catchment area, account for 87% of total discharge. Actual recharge is approximately twice as high in the highlands as in lower subcatchments. At the subcatchment scale, recharge ratios in more forested and elevated subcatchments (25% of rainfall) are about 1.5 times higher than in thicket-dominated subcatchments. The perennial behavior of most springs and streams, and the high baseflow contribution, indicate an important groundwater-buffering capacity. Evidence suggests that structural geology influences subsurface flow between subcatchments and baseflow in the streams. Spatial variations in lateral subsurface flow, higher specific discharge, and higher spring density in the south point towards cross-subcatchment transfers facilitated by lineaments. Hydrochemical and isotope analyses indicate increasing mineralization and (yearly to decadal) residence times from highlands to lowlands. These results indicate highland-lowland connectivity and show that the Taita Hills function as a groundwater-dominated water tower, where highlands are the main precipitation input zones, storage capacity is large, and preferential flow pathways sustain downstream and dry-season flows. More broadly, these findings reinforce the need to account for subsurface processes when assessing the hydrology of water towers.