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The current study is Part I of a two-part assessment of climate change impacts on hydrology and farm-level Greenhouse Gas (GHG) emissions. The effects of climate change on spring and summer water supply and spring and summer water demand related to the vegetation cover in the Keremeos Creek watershed in the Regional District of Okanagan-Similkameen (RDOS), southern British Columbia, Canada in the time-frame of 1961 to 2100 are investigated in Part I. The GENerate Earth SYstems Science input (GENESYS) spatial hydro-meteorological model is applied to predict the potential changes for the ensemble averages of 15 General Circulation Models (GCMs) for two Representative Concentration Pathways (RCP) scenarios (RCP 4.5 and RCP 8.5) in the 2020's (2011-2040), 2050's (2041-2070), and 2080's (2071-2100) relative to the 1961-1990 base period. Timing of snowmelt may be expected to occur one month earlier in all projected periods with RCP 4.5 and 8.5 except RCP 8.5 in the 2080's, which may be 2 months earlier relative to the 1961-1990 period. Based on predictions, there may be increases in total spring water supply from 35% to 39% and decreases in summer water supply from 36% to 79% relative to the 1961-1990, based on the three projected periods with RCP 4.5 and 8.5. Average spring vegetation water demand may increase from 20% to 47% but summer vegetation water demand may decrease from 10% to 29% relative to the 1961-1990 period based on the projected periods and emission scenarios. The spring and summer evapotranspiration estimates were controlled by the surface energy and soil moisture availability, respectively. These changes are expected to put stress on the future water resources management in the watershed. The results of Part I are then applied to Part II to estimate changes in farm-level GHG emissions and soil carbon storage.