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ur rr';uiiilll''riiit;autaii�auuiu auuiriid1 uruuuiriioff airv� liiil1ccdII to iii ourulc rr(:M� (:� iii uurul t;llh�r (:� III oirt;llh�r (m t; �uunuld C kl (� t; iii uurul iii ourult;(Nrr a irul d IIp uri uruig, airuld d1k:�curvm� (� iiiuurul t;llhmr c% t�", (''r llp(''rcivaullllll t;llmr „ o ut;llhr (r t;, iuuurui IIp uri uruig a iruld tru ur. ur. rNir. Runoff, which accumulates as streamflow, is the amount of precipitation that is not evaporated, stored as snowpack or soil moisture, or filtered down to groundwater. The proportion of precipitation that runs off is determined by a variety of factors includ- ing temperature, wind speed, humidity, solar intensity at the ground, vegetation, and soil moisture. While runoff generally tracks precipitation, increases and decreases in precipitation do not necessarily lead to equal increases and decreases in runoff. For ex- Water II'' tesouir °ceem Projected Changes in Annual Runoff -20, ..rub - 2 5 11D �0 40 Percent Milly et 01 . Projected changes in median runoff for 2041 -2060, relative to a 1901 -1970 baseline, are mapped by water - resource region. Colors indicate percentage changes in runoff. Hatched areas indicate greater confidence due to strong agreement among model projections. White areas indicate divergence among model projections. Results are based on emissions in between the lower and higher emissions scenarios 91 ample, droughts cause soil moisture reduc- tions that can reduce expected runoff until soil moisture is replenished. Conversely, water -sat- urated soils can generate floods with only moderate additional precipitation. During the last century, consistent increases in precipitation have been found in the Midwest and Northeast along with increased runoff. 149,110 Climate models consistently project that the East will experience increased run- off, while there will be substantial declines in the interior West, especially the Southwest. Projections for runoff in California and other parts of the West also show reductions, although less than in the interior West. In short, wet areas are projected to get wetter and dry areas drier. Climate models also consistently project heat - related summer soil moisture reductions in the middle of the continent' 11,142,146,149 lurid au�ur(� as wli(:Nir(:�r sirio pacic (91110 ur. iii ounulau�t;(� s, t; Ilhm� t;iii ur. iii uurull ' ��at' ur� ru uu�ul��aft' III coirultaii uurul uu(� to slflft to (M�urlll iiie ur iii uurul t;llh�mr IIp uri uruig airi(Jflows will Ill rr Ilya rr ir iii uunui Ilautrr truue°rlmiin(rr. Large portions of the West and some ar- eas in the Northeast rely on snowpack as a natural reservoir to hold winter precipita- tion until it later runs off as streamflow in spring, summer, and fall. Over the last 50 �ry a rie years, there have been widespread temperature - related reductions in snowpack in the West, with the largest reductions occurring in lower elevation mountains in the Northwest and California where snowfall occurs at temperatures close to the freez- ing point .142,"' The Northeast has also experienced snowpack reductions during a similar period. Observations indicate a transition to more rain and less snow in both the West and Northeast in the last 50 years. 143,154-156 Runoff in snowmelt- dominated areas is occurring up to 20 days earlier in the West, and up to 14 days earlier in the Northeast 151,151 Fu- ture projections for most snowmelt- dominated ba- sins in the West consistently indicate earlier spring Simulated Changes in Annual Runoff Pattern Jaur't Feb kiair Afar 1 ,,i.yi Jl,uir JLA Aug Sep Oct dery Dec 5 �w'ft';am'tl1; % "ti C::Fiu ist�aris�tri et aI,. General schematic of changes in the annual pattern of runoff for snowmelt - dominated streams. Compared to the historical pattern, runoff peak is projected to shift to earlier in the spring and late summer flows are expected to be lower. The above example is forthe Green River, which is part of the Colorado River watershed. 45