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Water stress is intensifying across Kenya’s arid and semi-arid lands (ASALs), threatening livelihoods, ecosystems, and regional stability. This study employs the Water Evaluation and Planning (WEAP) system to quantify water stress across the Ewaso Ngiro North Basin, spanning five counties: Nyeri, Meru, Laikipia, Isiolo, and Samburu. The basin, originating from Mount Kenya and the Aberdare Ranges, supports approximately 3.4 million people across diverse livelihood systems ranging from intensive highland agriculture to extensive lowland pastoralism. The WEAP model was configured with 35 demand sites representing domestic, agricultural, livestock, industrial, and environmental water users across 12 sub-catchments. Model calibration against observed streamflow data (2010- 2018) achieved Nash-Sutcliffe Efficiency (NSE) values of 0.72-0.84 and percent bias (PBIAS) within ±12% across seven gauging stations. Climate change scenarios using IPCC’s AR6-CMIP6 projections under two scenariosSSP245 and SSP585, were incorporated to assess future water availability. Results indicate that annual renewable water resources are projected to decline by 18% by 2050, with dry season flows decreasing by 25-30%. The water stress index exceeded 70% in downstream counties (Isiolo and Samburu) during critical dry months. Unmet water demand under business-as-usual conditions is projected to reach 78 Mm³/year (24% of total demand) by 2045. Scenario analysis demonstrates that integrated interventions combining water use efficiency, ecosystem restoration, and coordinated governance can reduce unmet demand to 18 Mm³/year (6% of demand). These findings provide critical evidence for basin-wide water allocation planning and climate adaptation strategies in the Ewaso Ngiro North Catchment