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Posted 4/27/2015

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By Ariane Pinson, Climate Science Specialist
USACE, Albuquerque District

During the next century, the Southwestern United States is anticipated to warm at a rate second only to Alaska, driving up evaporation rates, driving down soil moisture, and resulting in reduced stream flow, increased erosion/sedimentation, and increased wildfire severity and forest loss. These changes are likely to radically transform the region's watersheds, altering flood hydrology, further disrupting riparian ecosystems, and decreasing surface water supplies by 20 to 25 percent. With drought conditions anticipated to occur in 80 percent of the years between now and 2100, water is anticipated to be the defining issue of this century.

While drought is anticipated to increase, precipitation is unlikely to decrease. Instead, warmer conditions are likely to make small storms rare, and large storms less frequent and larger. As the time between rainfall events increases, soil moisture is likely to decrease, resulting in sparser vegetation cover, greater surface runoff, and increased erosion for a given-sized storm. Paradoxically then, both increased drought and increased flood magnitudes may occur under climate change.

A major concern for U.S. Army Corps of Engineers flood risk management projects -- whether they be a dam, levee, or lock -- is that these changes may increase the sediment load in streams of all sizes. This could potentially mean faster sediment accumulation behind dams resulting in a shortened reservoir lifetime, or increased sediment in navigation structures and channels, ramping up dredging-related costs. These concerns led the USACE Institute for Water Resources Responses to Climate Change Program, under the direction of Kathleen White, to sponsor a number of pilot studies to investigate these issues. The Cochiti Lake Sediment Yield project was one of these.

The Cochiti Lake project was designed to be a simple model of stream response to climate change. Data from an ensemble (or group) of climate models was used to understand how monthly precipitation might change in the future. This information was used to model potential future changes in runoff and streamflow on the Rio Grande, and to route these flows downstream to Cochiti Lake. The existing relationship between stream flow and sediment influx at the head of Cochiti Lake was then used to determine how much average sediment influx might change in response to streamflow changes in the future. The results were encouraging: average stream flows, and particularly spring runoff flows, are projected to decrease in the future, and these lower flows are likely to have reduced sediment loads. Under climate change, reduced rates of reservoir sedimentation were projected, resulting in longer reservoir lifetimes.

But spring runoff flows are not the entire story with respect to Rio Grande Basin climate change. Larger summer storms, while highly localized, are likely to increase sediment flux from tributaries to the Rio Grande, particularly should vegetation cover decrease or wildfires increase in extent and frequency (sediments on burned hillslopes are particularly susceptible to erosion). Thus, as the Rio Grande Basin becomes increasingly dry, more sediment is likely to be flushed into the Rio Grande and become available for deposition in Cochiti Lake during spring runoff floods. This kind of complex response was not captured in the initial modeling, and it is unclear at this point how the two trends will balance out in the future.

Since completing the pilot study, the Albuquerque District has continued to refine its understanding of climate change impacts to the district's area of responsibility through the consideration of climate change in the Civil Works planning process, and through outreach and coordination among Federal, State, Tribal, local and other partners.

With support from the Responses to Climate Change program, the Albuquerque District has collaborated with the U.S. Bureau of Reclamation on the Upper Rio Grande Impact Assessment and worked with the U.S. Department of Transportation on a climate change study for the Mid-Region Council of Governments (the Albuquerque area regional planning authority). These studies leverage analyses performed by each agency to reduce duplicative analysis and tailor complimentary work by multiple stakeholders. In collaboration with U.S. Forest Service staff and with support from the USACE Flood Risk Management Program, the Albuquerque District hosted a meeting of regional Federal staff to discuss projected impacts of climate change to regional watersheds, and available models and tools for understanding the problem. There is broad support for making this engagement a recurring event. The Middle Rio Grande Endangered Species Collaborative Program, a 16-signatory regional intergovernmental body, supported development of a literature synthesis on regional climate changes that has been the basis for public and professional outreach efforts.

All of these activities foster regional awareness of the scope of this complex problem and the range of strategies for adapting to, or mitigating, the impacts.