THE USE OF A COUPLED SIMULATION-OPTIMIZATION MODEL TO GUIDE GROUNDWATER DEVELOPMENT IN THE UPPER KLAMATH BASIN, OREGON AND CALIFORNIA

Over the past decade, water management priorities in the upper Klamath Basin of Oregon and California have shifted to protect habitat for endangered and threatened fishes. The result has been substantial reductions in the amount of surface water diverted for agriculture along with increased demand for groundwater, particularly in drought years. Consequently, water and environmental managers in the basin need methods to identify reliable groundwater management strategies that meet the twin goals of enhanced groundwater production and resource protection.

To address that need, we have developed a decision model that links groundwater simulation and optimization to identify groundwater management practices that provide additional water to irrigators and have acceptable impacts to groundwater and surface-water resources. The groundwater management model was developed working collaboratively with water users and resource managers and was designed to meet the complex set of goals and constraints associated with groundwater development in the basin. These include: (1) determining the spatial and temporal patterns of groundwater pumping that maximize groundwater withdrawal, (2) ensuring that the impacts of groundwater pumping on surface-water resources are within the limits allowed by the Klamath Basin Restoration Agreement, (3) ensuring that drawdowns due to managed pumping do not exceed the limits allowed by Oregon water law, and (4) determining the influence of climate variability on allowable groundwater pumping, including the necessity for pumping restrictions during extended drought cycles.

Initial work with the groundwater management model indicates that the groundwater system will be able to meet demand in many years; however, significant irrigation shortages can be expected in years of highest demand during extended drought cycles. Moreover, the groundwater management model has provided water users and regulators with information on the tradeoffs between groundwater-pumping decisions and the constraints that limit pumping impacts. The tradeoff analyses identified the constraints that exert the greatest control on pumping and determine how total pumping increases or decreases as these constraints are relaxed or tightened.