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Rapid population growth in the intermontane, semi-arid, Okanagan Valley of Western Canada has led to water shortages with increasing debate over competing water interests. Nevertheless, the relationships between the various water resources of the Okanagan remain poorly defined. Stable isotopes of hydrogen (delta H-2) and oxygen (O-18) were used to assess rainfall importance and the sources and flux of water to and from rivers and lakes in watersheds, and to evaluate the origin of ground water resources. Precipitation isotopes resulted in a meteoric water line of delta H-2 = 6.6 (delta O-18) - 22.7 for the Okanagan Valley. Isotopic seasonality in precipitation was evident, with summer precipitation clearly affected by local recycling of water vapor. The H-2 and O-18 of surface waters were more positive than mean annual precipitation, indicative of basin-scale evaporation of surface waters; however, Okanagan Lake and its downstream river and lake system were isotopically synchronous, indicating that they behaved as a single well-mixed hydrologic unit. Isotopic mass-balance modeling revealed similar to 35% of inflow to the Lake Okanagan watershed was lost to evaporation, validating a meteorological water balance model for the region. Highland bedrock was recharged with snow-melt and early spring rains, with the isotopic composition dependent on elevation. Ground waters in the Valley bottom aquifers west of Osoyoos were recharged by irrigation water obtained from the Okanagan River system, with no evidence of recharge connections from the highland bedrock. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.