Snowpack and streamflow amounts are projected to decline in parts of the Southwest, decreasing surface water supply reliability for cities, agriculture, and ecosystems.

This finding is from chapter 20 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: A central component of the assessment process was the Southwest Regional Climate assessment workshop that was held August 1-4, 2011, in Denver, CO with more than 80 participants in a series of scoping presentations and workshops. The workshop began the process leading to a foundational Technical Input Report (TIR) report.17ad4429-1321-4e7c-9cd5-3554eb0c3b38 The TIR consists of nearly 800 pages organized into 20 chapters that were assembled by 122 authors representing a wide range of inputs, including governmental agencies, non-governmental organizations, tribes, and other entities. The report findings were described in a town hall meeting at the American Geophysical Union’s annual fall meeting in 2011, and feedback was collected and incorporated into the draft. The chapter author team engaged in multiple technical discussions through more than 15 biweekly teleconferences that permitted a careful review of the foundational TIR17ad4429-1321-4e7c-9cd5-3554eb0c3b38 and of approximately 125 additional technical inputs provided by the public, as well as the other published literature and professional judgment. The chapter author team then met at the University of Southern California on March 27-28, 2012, for expert deliberation of draft key messages by the authors. Each key message was defended before the entire author team prior to the key message being selected for inclusion. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities, which include magnitude, timing, persistence and reversibility, likelihood and confidence, potential for adaptation, distribution, and importance of the vulnerable system.”3277e83c-e374-4ed5-b0a2-0adadfaf118d

Description of evidence base: The key message was chosen based on input from the extensive evidence documented in the Southwest Technical Input Report17ad4429-1321-4e7c-9cd5-3554eb0c3b38 and additional technical input reports received as part of the Federal Register Notice solicitation for public input, as well as stakeholder engagement leading up to drafting the chapter. Key Message 5 in Chapter 2, Our Changing Climate, also provides evidence for declining precipitation across the United States, and a regional study966bf116-8d6d-41f2-96be-4b66d3e729db discusses regional trends and scenarios for the Southwest. Over the past 50 years, there has been a reduction in the amount of snow measured on April 1 as a proportion of the precipitation falling in the corresponding water-year (October to September), which affects the timing of snowfed rivers. The implication of this finding is that the lower the proportion of April 1 snow water equivalent in the water-year-to-date precipitation, the more rapid the runoff, and the earlier the timing of center-of-mass of streamflow in snowfed rivers.40292b53-9f3a-42f4-9d48-5e614f3d7785 0d8b090e-e060-4f9d-a442-b7e050608a20 For the “recent decade” (2001 to 2010), snowpack evidence is from U.S. Department of Agriculture (USDA) Natural Resources Conservation Service snow course data, updated through 2010. One studyc9075dbc-f7c8-4d85-b534-e97282562b3e has analyzed streamflow amounts for the region’s four major river basins, the Colorado, Sacramento-San Joaquin, Great Basin (Humboldt River, NV), and the Rio Grande; data are from the U.S. Department of the Interior – Bureau of Reclamation, California Department of Water Resources, U.S. Geological Survey, and the International Boundary and Water Commission (U.S. Section), respectively. These data are backed by a rigorous detection and attribution study.87875dde-385b-4f57-b0ae-aa21648b2833 Projected trends59cf5a07-b64b-418c-acc6-73b7ff4d7d7e make use of downscaled climate parameters for 16 global climate models (GCMs), and hydrologic projections for the Colorado River, Rio Grande, and Sacramento-San Joaquin River System. Based on GCM projections, downscaled and run through the variable infiltration capacity (VIC) hydrological model,f312de7c-ebe1-447f-93c5-c2ddec528464 there are projected reductions in spring snow accumulation and total annual runoff, leading to reduced surface water supply reliability for much of the Southwest, with greater impacts occurring during the second half of this century.59cf5a07-b64b-418c-acc6-73b7ff4d7d7e f11e90fb-b100-4487-8bac-1a076166d623 5d7f76e8-7585-4fb9-89b0-98ef6eac31ee Future flows in the four major Southwest rivers are projected to decline as a result of a combination of increased temperatures, increased evaporation, less snow, and less persistent snowpack. These changes have been projected to result in decreased surface water supplies, which will have impacts for allocation of water resources to major uses, such as urban drinking water, agriculture, and ecosystem flows.

New information and remaining uncertainties: Different model simulations predict different levels of snow loss. These differences arise because of uncertainty in climate change warming and precipitation projections due to differences among GCMs, uncertainty in regional downscaling, uncertainty in hydrological modeling, differences in emissions, aerosols, and other forcings, and because differences in the hemispheric and regional-scale atmospheric circulation patterns produced by different GCMs produce different levels of snow loss in different model simulations. In addition to the aforementioned uncertainties in regional climate and hydrology projections, projection of future surface water supply reliability includes at least the following additional uncertainties: 1) changes in water management, which depend on agency resources and leadership and cooperation of review boards and the public;5030722b-8c1f-43f1-a4e3-5ee701be0a77 2) management responses to non-stationarity;c52f2539-9c5e-4ead-b8b7-f1884c5d662e 3) legal, economic, and institutional options for augmenting existing water supplies, adding underground water storage and recovery infrastructure, and fostering further water conservation (for example, Udall 2013e1a270c3-6eb4-4084-8267-b400584555ea); 4) adjudication of unresolved water rights; and 5) local, state, regional, and national policies related to the balance of agricultural, ecosystem, and urban water use (for example,67b69161-5101-418a-a6c9-1b6a80773305).

Assessment of confidence based on evidence: There is high confidence in the continued trend of declining snowpack and streamflow in parts of the Southwest given the evidence base and remaining uncertainties. For the impacts on water supply, there is high confidence that reduced surface water supply reliability will affect the region’s cities, agriculture, and ecosystems.

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