Simulated changes in shallow groundwater and vegetation distributions under different reservoir operations scenarios

The objectives of this study were to develop and use a linked groundwater and vegetation model to simulate groundwater and vegetation distributions in a riverine and reservoir-fringe system under different reservoir operations scenarios. This study was conducted where Little Stony Creek flows into East Park Reservoir on the east front of the Coast Range, northern California. A numerical groundwater model was used to model mean depth to groundwater during the growing season for water years 1980-1999 for each of five community types identified on the study site. Multiple vegetation models were developed, each of which described the probability that a given community type would occur primarily as a function of modeled mean depth to groundwater during the growing season and secondarily as a function of flooding. Four scenarios representing four different reservoir operations were simulated: existing condition, existing condition with late drawdown, full drawdown, and full pool. A groundwater backwater effect caused by the imposed reservoir stage extends to portions of the terrace, but the most pronounced effects occur on the delta. Consequently, the most pronounced changes in vegetation distributions also occur on the delta. Compared to the existing-condition scenario, modeled vegetation distributions do not change under the existing condition with late-drawdown scenario, a xeric herbaceous community type is greatly expanded under the full-drawdown scenario, and mesic herbaceous, scrub-shrub, and forested community types are greatly expanded under the full-pool scenario. The results of this study are twofold. First, the linked groundwater and vegetation model is relatively simple to construct and can be used to efficiently simulate multiple surface-water and groundwater management scenarios. Second, changes in reservoir operations can have pronounced effects on shallow groundwater and associated vegetation distributions in riverine and reservoir-fringe systems. Thus, the effects of changing reservoir operations must be considered if the management of shallow groundwater and associated plant and wildlife habitat resources is to be successful.