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The positive relationship between water availability and carbon isotope discrimination (Delta) is well documented. There is also evidence that Delta is positively related to plant productivity. However, most of the data are concerned with cross species and genotypic variation in Delta relative to broad geographical differences in water availability or on general trends in local populations. We still lack quantified data on the response of localized populations to changing water availability, carbon isotope discrimination (Delta) and plant productivity, especially on a year to year basis. To provide data in this area, this study measured above-ground productivity in western wheatgrass (Agropyron smithii) and vegetative stem growth in big sagebrush (Artemisia tridentata), and carbon isotope discrimination in both species over several years of varying water availability. Estimates of Delta were regressed against spring soil moisture estimates, and plant productivity measurements against Delta estimates. For sagebrush, carbon isotope discrimination estimates were positively related to spring soil moisture levels (p = 0.027, r(2) = 0.11) and productivity estimates (p = 0.002, r(2) = 0.23). Carbon isotope discrimination for wheatgrass also regressed significantly with soil moisture levels (p = 0.039, r(2) = 0.10) and productivity estimates (p < 0.001, r(2) = 0.33). (C) 1999 Academic Press.