Groundwater in the Upper Deschutes Basin, Oregon

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Groundwater monitoring in the Deschutes Basin shows water-level declines are larger than might be expected from climate variations alone, raising questions regarding the influence of groundwater pumping, canal lining, and other human influences.

The upper Deschutes Basin is among the fastest growing regions in Oregon. The rapid population growth has been accompanied by increased demand for water. Surface streams, however, have been administratively closed to additional appropriation for many years, and surface water is not generally available to support new development. Consequently, ground water is being relied upon to satisfy the growth in water demand.

The growing reliance on ground water, and the increased development of the resource, has resulted in concerns among basin residents, water users, and natural resource agencies. The most common questions pertain to the capacity of the resource to accommodate increased use and the potential for ground-water pumping to diminish streamflow. A secondary concern is the possible consequences of lining irrigation canals to reduce the substantial leakage. Reducing irrigation canal leakage could result in the lowering of the water table in certain areas.

The U.S. Geological Survey conducted a study of the ground-water system of the upper Deschutes Basin to develop a quantitative understanding of the regional hydrology, and to provide tools to allow resource managers and basin residents to evaluate the possible effects of various development scenarios. The study was conducted in cooperation with the Oregon Water Resources Department, the cities of Bend, Redmond, and Sisters, Deschutes and Jefferson Counties, and the Confederated Tribes of the Warm Springs Reservation of Oregon.

OBJECTIVE

The objective of this study was to provide a quantitative understanding of the ground-water hydrology in the Middle Deschutes Basin in order to provide resource managers, planners, and the general public the best information available with which to make water maanagement decisions. Specifically, this information includes: a compilation of basic ground-water data, a description of the geologic framework of the regional flow system, a quantitative description of the flow system including estimation of the hydrologic budget, an evaluation of ground-water/surface water relationships, an analysis of the effects of present canal leakage, and development of the capability to estimate of the effects of present and future development on ground-water levels and streamflow.

APPROACH                                     

The study was conducted in two phases. Phase I included the characterization of the hydrogeologic framework using surface geologic maps and lithologic data from wells. This phase also included estimation of the rates and distribution of recharge to the aquifer system from precipitation (using a mass-balance approach), canal leakage, and deep percolation of irrigation water applied to fields. Discharge from the aquifer system to streams was estimated using streamflow records and gain-loss measurements. Discharge to wells was estimated using pumping records, analysis of satellite imagery (for identification of crop areas and types), and water-right information. Water-level elevation maps were created using water levels measured in wells precisely located in the field. The last part of phase I consisted of construction and calibration of a steady-state numerical flow model. Phase II of the study involved construction and calibration of a transient ground-water flow model. The models can be used to test various future ground-water development scenarios and canal lining scenarios, particularly with regard to their effects on regional ground-water levels and streamflow.

Groundwater flow model simulations indicate that climate variations have the largest influence on groundwater levels throughout the upper Deschutes Basin, and that impacts from pumping and canal lining also contribute but are largely restricted to the central part of the basin that extends north from near Benham Falls to Lower Bridge, and east from Sisters to the community of Powell Butte. Outside of this central area, the water-level response from changes in pumping and irrigation canal leakage cannot be discerned from the larger response to climate-driven changes in recharge. Within this central area, where measured water-level declines have generally ranged from about 5 to 14 feet since the mid- 1990s, climate variations are still the dominant factor influencing groundwater levels, accounting for approximately 60–70 percent of the measured declines. Post-1994 increases in groundwater pumping account for about 20–30 percent of the measured declines in the central part of the basin, depending on location, and decreases in recharge due to canal lining account for about 10 percent of the measured declines. Decreases in recharge from on-farm losses were simulated, but the effects were negligible compared to climate influences, groundwater pumping, and the effects of canal lining and piping.

Observation well data and model simulation results indicate that water levels in the Cascade Range rose and declined tens of feet in response to wet and dry climate cycles over the past two decades. Water levels in the central part of the basin, in contrast, steadily declined during the same period, with the rate of decline lessening during wet periods. This difference is because the water-level response from recharge is damped as water moves (diffuses) from the principal recharge area in the Cascade Range to discharge points along the main stems of the Deschutes, Crooked, and Metolius Rivers in the central part of the basin. Water levels in the central part of the basin respond more to multi-decadal climate trends than shorter term changes.

Groundwater-flow simulations show that the effects from increased pumping and decreased irrigation canal leakage extend south into the Bend area. However, the only wells presently monitored in the Bend area are heavily influenced by the Deschutes River, which dampens any response of water levels to external stresses such as groundwater pumping, changes in canal leakage, or climate variations.