Surface water origin and attenuation of meteoric water input signal in the transboundary Lake Malawi basin: Isotopic and hydrochemical evidence.

Understanding surface water origins and flow dynamics is essential for water quality protection and sustainable transboundary water governance. This study provides the first basin-wide isotopic (δ18O, δ2H) and hydrochemical characterisation of surface waters across the Lake Malawi basin, establishing a comprehensive baseline to support Integrated Water Resources Management (IWRM) amid growing climatic and developmental pressures. Seasonal hydrological regimes were identified: wet season surface flows dominated by tropical storm precipitation with minimal evaporation effect, and dry season flows sustained by isotopically distinct groundwater and spring discharges. Lakes exhibit strong evaporative enrichment (LEL slope: 5.5 ± 0.5; mean d-excess: -3.1 ‰), indicating kinetic fractionation. River and reservoir isotopic responses vary seasonally and spatially, with distinct transitions where mature plateau rivers meet escarpment headwaters. These rejuvenated rivers become progressively enriched across the rift valley lakeshore plains. While riverine δ18O and δ2H signatures fluctuate seasonally, lake and lagoon waters retain enriched signatures year-round due to longer residence times and internal mixing. Elevated total dissolved salts and corresponding isotopic enrichment suggest mixing with groundwater (influenced by water-rock interactions), and meteoric water inputs. The study underscores evaporation, groundwater, and meteoric water inputs as key drivers of surface water hydrochemistry, offering a robust framework for adaptive and evidence-based water resource management in tropical rift settings.