SIMULATION OF REGIONAL GROUNDWATER FLOW IN THE OREGON CASCADES – SUCCESSES, LIMITATIONS, AND NEXT STEPS

Regional groundwater flow models have been developed for basins draining most of the east side of the Cascade Range in Oregon. The complexity of these models varies spatially according to the degree of knowledge of the subsurface, availability of calibration data, and driving issues. Models tend to be detailed and complex in developed areas adjacent to the Cascades, and simple and generalized in the mountains. Although generalized, these models simulate large-scale hydrologic behavior of the Cascades reasonably well, particularly the spatial distribution of and climate-driven temporal variations in groundwater discharge to major streams and springs. Smaller-scale features in the Cascades, such as small groundwater-fed streams and springs, however, are not simulated consistently well because model calibration is driven by larger, more volumetrically important features. Present models have proven useful for addressing many groundwater management questions, and have been helpful in developing regional water-management strategies. There is increasing interest, however, in the hydrologic response of specific small streams and springs to water- and land-management decisions and climate change, a task to which the present regional models are not well suited.There is a considerable body of knowledge of the hydrology of the Cascade Range at the individual spring and stream-reach scale that includes field observations, descriptive geologic and hydrologic studies, geochemical studies, and analytical, statistical, and physically based models. While much of this work provides important insights into the hydrology of the Cascades, there is as yet no tool with which to evaluate the hydrologic response of small-scale features to the full variety of forcing variables occurring at a range of scales. The next challenge, therefore, is to combine our knowledge of local-scale hydrology with the larger regional-scale models to provide quantitative capability that spans and properly couples processes across a range of spatial and temporal scales. Two possible approaches are refined-grid models nested within larger, regional models, and coupled groundwater/surface-water models (such as GSFLOW) discretized at appropriately fine scales. Both of these approaches will require additional data of diverse types.