Colorado River extremes

Overview

Colorado River Extremes table

Events of Concern Noted by Water Managers	Climate-Change Trend Defined	Impact of Change	Linkage to Climate Change Drivers	Relevant Research*	A Recent Example (details in text)
Intensifying heat waves	Hotter periods that last longer	- Air quality declines - Water quality declines - Increased fire danger - Ecosystem mortality events - Health and safety impacts, especially for outdoor workers and vulnerable populations	greenhouse gases trap more heat, lead to warmer temperatures	US: Meehl et al., 2009; Cutting et al., 2011; Wuebbles et al., 2017; Fahey et al., 2017; Vose et al., 2017 Global: Guo et al., 2018; Masson-Delmotte et al., 2021	In the summer of 2020 and 2021, the Colorado River Basin experienced heat wave conditions that exceeded multiple records for consecutive number of days, with temperatures surpassing historical conditions.
Diminished snowpack	Less precipitation falls as snow vs. rain, snowmelt occurs earlier	- Reduced streamflow - Earlier peak flows - Water shortages - Recreation limited	more precipitation falls as rain than as snow, warmer temperatures cause early snowmelt	Basin specific: Reynolds et al., 2020 Western US: Barnett et al., 2005; McCabe et al., 2007; Barnhart et al., 2016; Harpold and Molotch, 2015; Li et al., 2017; Harpold and Kohler, 2017; Painter et al., 2018; Yan et al., 2018; Musselman et al., 2017, 2018, 2021; Davenport et al. 2020 US: Berghuijs et al., 2014	Snowpack monitoring stations throughout the West reveal that snowmelt is already occurring earlier in the year.
Long-duration drying (Sustained declines in runoff efficiency)	Streamflow declines even when the same amount of water falls as precipitation	- Reduced streamflow - Water shortages - Agriculture declines	warmer, thirstier atmosphere leads to more evaporation/ transpiration/ sublimation of water, drier soils, and less water reaching streams before returning to atmosphere	Basin specific: Vano et al. 2012, 2014; Woodhouse et al., 2016, 2021; McCabe et al. 2017; Udall and Overpeck, 2017; Hoerling et al., 2019 Western US: Das et al., 2011; Gonzalez et al., 2018 US: McCabe and Wolock, 2016	The Upper Colorado River Basin in 2020 and 2021 had unexpectly low run-off volumes.
Extensive wildfires	More areas burned	- Public safety threatened - Air quality declines - Water quality declines - Increased erosion	increased temperatures, increase drying and reduce snowpack while lengthening the fire season	Western US: Abatzoglou and Kolden, 2011; Jenkins et al., 2014; Abatzoglou and Williams, 2016; Holden et al., 2018; Kean et al., 2019; Zhang et al., 2020; Brey et al., 2021 US: Wehner et al., 2017	In 2020, Colorado experienced the three largest wildfires in recorded history.
Short-duration intense wet and dry system shocks	Precipitation intensity increases; each storm brings more water	- Flooding - Increased erosion - Dam safety concerns + Increased groundwater recharge in some locations + Drought relief	a warmer atmosphere holds more water, so when it rains or snows there a great chance that more precipitation will fall in any given event; additionally, storm are now developing in warmer and more humid environments, ocean surface temperatures drive an increase worldwide in size and amount of storms	Basin specific: Gutmann et al., 2016; Shamir et al., 2019; Corringham et al., 2019 Western US: Zhu and Newell, 1994; Corbosiero et al., 2009; Dettinger, 2013; Rutz et al., 2015; Alexander et al., 2015; Swales et al., 2016; Demaria et al., 2019; Gershunov et al., 2019; Davenport et al., 2020; Payne et al., 2020; Rhoades et al., 2020; US: Easterling et al., 2017; Kossin et al., 2017 Global: Trenberth, 2011; Pendergrass and Hartmann, 2014; Swann et al., 2016; Fischer and Knutti, 2016; Simpson et al., 2016; Kossin et al., 2017; Pendergrass et al., 2017; Sippel et al., 2019; Heinze-Deml et al., 2020;	In 2013, the “Front Range Flood” in Colorado resulted from record rainfall and urban runoff volumes.
Short-duration intense wet and dry system shocks	Rapid onset dry year (flash drought)	- Water shortages - Agricultural losses - Recreation limited - Increased fire danger	large-scale atmospheric circulation changes increase temperatures, modify precipitation, reduce cloud cover, and increase wind speeds	Great Plains and Eastern US: Hoell et al., 2020; Ford and Labosier, 2017 US: Otkin et al., 2018; Christian et al., 2019; Pendergrass et al., 2020 Global: Hoffmann et al., 2021	Recent example not within the Colorado River Basin.
Amplified wet and dry swings (climate whiplash)	Multi-year dry spells broken by a couple of very wet years	- Dam safety concerns - Increased erosion - Management challenge (expecting drought and get too much rain instead)	a combination of more moisture and weakening of overall atmospheric circulation results in a drought-pluvial see-saw	California/Western US: Swain et al., 2018; Wahl et al. 2020; Dettinger 2013 Global: He and Sheffield, 2020; Pendergrass et al., 2017; Pendergrass and Gerber 2016; Konapala et al. 2017; Schwarzwald et al., 2021	July 2021 was the wettest month on record in Tucson, which came after the "nonsoon" of 2020.
Dramatic decline in monsoons	Timing of monsoon is later or no monsoon Average rainfall from monsoons lessens Change in frequency and intensity of extreme rainfall events	- Water shortages - Agriculture loses - Less relief from summer heat	warmer temperatures change atmospheric circulation patterns in ways that inhibit convection and cloud formation	Southwest US: Cook and Seager, 2013; Pascale et al., 2017; Luong et al., 2017; Demaria et al., 2019; Carroll et al., 2020 Global: Emori and Brown et al. 2005; Wang et al., 2021	In 2020, Arizona’s monsoon was characterized as a “nonsoon” for delivering near record low precipitation.