Box 2. Projected Changes in Southern Nevada’s Most Important Water Source: The Colorado River Basin
Since 2000, the Colorado River Basin has experienced an extended dry period in which the average annual water supply has been 18% lower than the historical average, contributing to depletion of water storage in the major reservoirs to less than half of capacity. This recent drought, along with the increasing recognition that rising temperatures impact the hydrology of the basin, has led to concerns about the long-term reliability of the basin’s water supplies. Research findings described in Colorado River Basin Climate and Hydrology: State of the Science (Lukas et al., 2020) demonstrate that the concerns are warranted. There is very high confidence regarding both future warming in the basin and in the role of emissions in leading to greater warming. Human-caused warming is already impacting droughts in the Colorado River Basin: an attribution study of the recent 2000–2018 drought indicates that it was made more severe by human-caused warming (Williams et al., 2020).
The future of precipitation in the basin is projected less confidently, so that it is not clear whether there will be more or less precipitation in the future overall. Studies have shown increasing variation from year to year, and on storm-to-storm scales, of basin precipitation. Consensus projections of overall shifts in hydroclimate driven by a warmer climate suggest a shift toward lower spring snowpacks, earlier melt and runoff, lower annual runoff volumes, and increasing water demand. Projected runoff changes are expected to lead to less streamflow overall (Udall & Overpeck, 2017), with the largest streamflow reductions projected for the Lower Basin downstream from Lees Ferry, Utah (i.e., the part of the basin from which Las Vegas extracts Nevada’s allotment of Colorado River water). A long-term perspective from tree rings and other paleoclimate data suggests that the Colorado River Basin has experienced droughts lasting many decades to many centuries, even in the absence of human-caused climate change (Lachniet et al., 2020; Routson et al., 2019; Williams et al., 2020; Woodhouse et al., 2010). These studies suggest that long-term drying (aridification) in the Colorado River Basin is a threat to water supply in southern Nevada and elsewhere, and the magnitude of future aridity in the Southwest will depend on the future trajectory of GHG emissions (Williams et al., 2020) and links with climate changes happening elsewhere (Lachniet et al., 2020).
For more information, please visit https://wwa.colorado.edu/publications/reports/CRBreport/