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21: NORTHWEST
<br />Averaged over the region, the number of days with more
<br />than one inch of precipitation is projected to increase 13% in
<br />2041 to 2070 compared with 1971 to 2000 under a scenario
<br />that assumes a continuation of current rising emissions trends
<br />(A2),10 though these projections are not consistent across
<br />models.28 This increase in heavy downpours could increase
<br />flood risk in mixed rain -snow and rain -dominant basins, and
<br />could also increase stormwater management challenges in
<br />urban areas.
<br />Consequences and Likelihoods of Changes
<br />Reservoir systems have multiple objectives, including irrigation,
<br />municipal and industrial use, hydropower production, flood
<br />control, and preservation of habitat for aquatic species.
<br />Modeling studies indicate, with near 100% likelihood and for
<br />all emissions scenarios, that reductions in summer flow will
<br />occur by 2050 in basins with significant snowmelt (for example,
<br />Elsner et al. 201024). These reduced flows will require more
<br />tradeoffs among objectives of the whole system of reservoirs,30
<br />especially with the added challenges of summer increases
<br />in electric power demand for cooling31 and additional water
<br />consumption by crops and forests.'°'32 For example, reductions
<br />in hydropower production of as much as 20% by the 2080s
<br />could be required to preserve in -stream flow targets for fish
<br />in the Columbia River basin.33 Springtime irrigation diversions
<br />increased between 1970 and 2007 in the Snake River basin, as
<br />earlier snowmelt led to reduced spring soil moisture.34 In the
<br />absence of human adaptation, annual hydropower production
<br />is much more likely to decrease than to increase in the Columbia
<br />River basin; economic impacts of hydropower changes could
<br />be hundreds of millions of dollars per year.35
<br />Region -wide summer temperature increases and, in certain
<br />basins, increased river flooding and winter flows and
<br />decreased summer flows, will threaten many freshwater
<br />species, particularly salmon, steelhead, and trout.27 Rising
<br />temperatures will increase disease and/or mortality in several
<br />iconic salmon species, especially for spring/summer Chinook
<br />and sockeye in the interior Columbia and Snake River basins.36
<br />Some Northwest streams30 and lakes have already warmed
<br />over the past three decades, contributing to changes such as
<br />earlier Columbia River sockeye salmon migration37 and earlier
<br />blooms of algae in Lake Washington.38 Relative to the rest of
<br />the United States, Northwest streams dominated by snowmelt
<br />runoff appear to be less sensitive, in the short term, to warming
<br />due to the temperature buffering provided by snowmelt and
<br />groundwater contributions to those streams.39 However, as
<br />snowpack declines, the future sensitivity to warming is likely to
<br />increase in these areas.48 By the 2080s, suitable habitat for the
<br />four trout species of the interior western U.S. is projected to
<br />decline 47% on average, compared to the period 1978-1997.41
<br />As species respond to climate change in diverse ways, there is
<br />potential for ecological mismatches to occur — such as in the
<br />timing of the emergence of predators and their prey.38
<br />Adaptive Capacity and Implications for Vulnerability
<br />The ability to adapt to climate changes is strengthened
<br />by extensive water resources infrastructure, diversity of
<br />institutional arrangements,42 and management agencies that
<br />are responsive to scientific input. However, over -allocation
<br />of existing water supply, conflicting objectives, limited
<br />management flexibility caused by rigid water allocation and
<br />operating rules, and other institutional barriers to changing
<br />operations continue to limit progress towards adaptation in
<br />many parts of the Columbia River basin.43'44 Vulnerability to
<br />projected changes in snowmelt timing is probably highest in
<br />basins with the largest hydrologic response to warming and
<br />lowest management flexibility — that is, fully allocated, mid -
<br />elevation, temperature -sensitive, mixed rain -snow watersheds
<br />with existing conflicts among users of summer water. Regional
<br />power planners have expressed concerns over the existing
<br />hydroelectric system's potential inability to provide adequate
<br />summer electricity given the combination of climate change,
<br />demand growth, and operating constraints.' Vulnerability
<br />is probably lowest where hydrologic change is likely to be
<br />smallest (in rain -dominant basins) and where institutional
<br />arrangements are simple and current natural and human
<br />demands rarely exceed current water availability.43,45,46
<br />The adaptive capacity of freshwater ecosystems also varies
<br />and, in managed basins, will depend on the degree to which
<br />U.S. GLOBAL CHANGE RESEARCH PROGRAM
<br />491 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
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