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Predicting groundwater supply for an entire watershed in mountainous terrain required an approach that considered a wide range in data availability between valley bottom and headwater areas, large change in elevation, and steep topography. The methodology utilized the MIKE-SHE numerical code to simulate overland flow. actual evapotranspiration and recharge for data-rich areas, and a simpler, seasonal water budget for data-limited areas. Recharge estimates were combined to form spatially variable recharge boundary conditions for a larger-scale groundwater flow model of the entire mountainous watershed. Research focused on the BX Creek watershed. located in the north Okanagan Basin in British Columbia, one of Canada’s fastest growing and most water-limited regions. Groundwater recharge was found to vary from 0 to 20 mm/yr at lower elevations, and from 20 to 50 mm/yr at higher elevations. Simulation of the whole flow system illustrated that 58% of the groundwater flux from upland areas occurs through a relatively narrow alluvial fan aquifer that extends to the valley bottom, and the remaining recharge is nearly equally divided between groundwater flow through the mountain block (20%) and direct recharge (22%). Geochemical data from domestic water wells within the watershed suggest that water in the alluvial aquifer and bedrock are generally similar (i.e., common origin); however, stable isotope data indicate that groundwater in the alluvial aquifer may be derived from snowmelt recharge at a different time and elevation than snowmelt recharge to the bedrock. The combination of modelling results and complimentary geochemical and isotopic analyses of surface water and groundwater, provide an adequate first-order approximation of groundwater flow in the watershed. (c) 2008 Elsevier B.V. All rights reserved.