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Model Analysis of the Hydrologic Response to Climate Change in the Upper Deschutes Basin, Central Oregon

Contact: Marshall Gannett

upper Deschutes Basin

The upper Deschutes Basin.(Click on map for a larger version.)

Background

Natural resource managers are facing considerable uncertainty regarding the hydrologic conditions that will result from climate change. Climate models predict a range of possible conditions in the Pacific Northwest over the next few decades, with temperatures warming between 0.9 and 2.9°C and precipitation changing from -9 to +12%. Analysis of about 20 climate models by Mote and others (2008) suggests that the most likely result of climate change in the Pacific Northwest will be warming, with little difference in precipitation amounts, but a shift in seasonality and form. The probable result will be a transition from snow to rain at intermediate and low elevations the Cascade Range, causing earlier runoff and reduction in the pulse of runoff and groundwater recharge associated with spring snowmelt. The ways in which basins will respond to these anticipated climate changes depends on the hydrogeologic setting.

The upper Deschutes River in central Oregon is a groundwater dominated stream with a mean discharge of approximately 4,000 cubic feet per second. The regional aquifer system that feeds the upper Deschutes River encompasses an area of roughly 4,500 mi2 that includes the eastern side of the Cascade Range. Because the Cascade Range is the dominant source of recharge to the regional aquifer system, resource managers are concerned about the possible ways in which the groundwater and surface-water systems in the basin might be affected by changes in the form and timing of precipitation and recharge resulting from climate change. Streamflow data from the basin suggest that the large storage capacity of the regional groundwater system will buffer the effects of changes in the seasonality of recharge in the Cascades, but watershed modeling by Tague and others (2007) shows that late-season groundwater-fed flows will be diminished, at least in smaller watersheds. To fully understand the possible responses of the basin to climate change will require analysis of a range of future climate scenarios using physically based spatially distributed models.

Because the Cascade Range intercepts much of the moisture from eastward moving Pacific weather systems, the hydrologic response of the Cascade Range has a strong influence on water resources in large areas of the high deserts of south central Washington, central Oregon, and northeastern California. The questions being addressed by the study, therefore, have wide applicability.

Development of water resource management strategies for the coming decades will require knowledge of the hydrologic conditions resulting from a range of future climate scenarios. Surface water in the upper Deschutes Basin is considered fully appropriated, and the Oregon Water Resources Department has been working to manage the highly coupled groundwater and surface-water systems in the basin using a system whereby the effects on streams resulting from groundwater pumping are mitigated by, for example, retiring surface water rights and leaving more water in the stream, with the goal of maintaining certain legally set instream flow rates. This strategy was designed, and is evaluated, on the basis of historic flows, so it is important to know the ways in which future flows are likely to differ from historic values. The need for better understanding of climate change effects also exists at the Federal level. The Bureau of Reclamation operates a number of storage reservoirs and a distribution system to provide primary or supplemental irrigation water to about 100,000 acres in the basin. Reclamation has concerns regarding how climate change affects their future ability to forecast supply, provide water for irrigation and aquatic ecosystems, and provide flood control in the basin.

This study will use the existing daily mass-balance recharge model and the regional groundwater flow model developed for the upper Deschutes Basin by USGS with downscaled climate-model output to identify probable future hydrologic conditions in the upper Deschutes Basin. The existing models were developed in the late 1990s and are documented in Gannett and others (2001) and Gannett and Lite (2004). The daily mass-balance model will calculate the form of precipitation, and the timing and amounts of runoff and groundwater recharge under future climate scenarios. This calculated recharge will be used to drive the regional groundwater model. Because streamflow and inflow to reservoirs are heavily groundwater dependent, the groundwater model will allow a better understanding of the coupling between climate, the stream system, and water supplies.

References

Gannett, M.W., and Lite, K.E., Jr., 2004, Simulation of regional groundwater flow in the upper Deschutes Basin, Oregon: U.S. Geological Survey Water-Resources Investigations Report 03-4195, 84 p.

Gannett, M.W., Lite, K.E., Jr., Morgan, D.S., and Collins, C.A., 2001, Groundwater hydrology of the upper Deschutes Basin, Oregon: U.S. Geological Survey Water Resources Investigations Report 00-4162, 77 p.

Mote, Philip, Salathe, Eric, Duliere, Valerie, and Jump, Emily, 2008, Scenarios for future climate for the Pacific Northwest: Seattle, University of Washington, Climate Impacts Group, Center for Science in the Earth System, Joint Institute for the Study of the Atmosphere and Oceans, http://cses.washington.edu/db/pubs/abstract579.shtml.

Tague, Christina, Grant, Gordon, Farrell, Michael, Choate, Janet, and Jefferson, Anne, 2008,  Deep groundwater mediates streamflow response to climate warming in the Oregon Cascades: Climatic Change, v. 86, p. 189-210.

Publications

SIR 2013-5092. Analysis of 1997–2008 Groundwater Level Changes in the Upper Deschutes Basin, Central Oregon, by Marshall W. Gannett and Kenneth E. Lite, Jr.

Banner-photo credits: 2,4,6, Oregon State University archives; 3, Portland General Electic; 5, Greg Burke; 7, John Hutmacher; rest, USGS

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