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Horticulture in the semi-arid, Okanagan valley is dependent on irrigation. Our objectives were to determine crop water requirements under climate change scenarios and to compare potential demand with current water use and supply. Methods were developed to integrate crop water demand data with spatial climate and land use data. Equations for seasonal crop coefficients were developed. Equations to predict daily solar radiation and daily maximum and minimum temperatures from monthly data were also derived as a basis for estimating PET. Future climate data (Canadian Global Coupled Model -CGMC1) were compared with 1961-1990 normals. Climate data were spatially downscaled from a 3.75degreeslatitude x 3.75degrees longitude grid output through the PRISM (Parameter-elevation Regressions on Independent Slopes Model) to a 4km x 4km grid. Land use data were acquired from a variety of sources and incorporated into a GIS and overlain with the PRISM grid cells to create unique polygons. Calculations of crop water demand were performed for each polygon. Crop water demand was totaled on a region and Irrigation District basis. Overall average predicted water use data for present day conditions were compared with values of expected water use to test the crop water demand model. Predicted values were slightly lower than expected values (745 mm/year vs 820-1000 mm/year). This was attributed to the coarseness of the PRISM grid, which resulted in large elevation changes within cells and underestimation of temperatures. Total annual water consumption for the period 1996-1999 reported by the major Irrigation Districts was reasonably similar to that predicted by the model (46.9 m(3) x 10(6) vs 51.8 m(3) x 10(6)). Thus the model was considered adequate for prediction of effects of climate change. For the region as a whole, estimated crop water demand increased by 37%, from 745 to 1021 mm/year (80 to 110 m(3) x 106) between the present day and the 2070-2099 scenario. Some Irrigation Districts may not be able to meet the increased demand.