United States

History of Droughts

Droughts in the United States range from localized events and seasonal deficits to severe droughts that affect regional or continental areas over one or many years ( Cook et al. 2013; Diodato et al. 2007; Overpeck 2013). Drought is a defining feature of the intermountain western region of the United States, including the iconic events such as the Dust Bowl (1930s) and post-World War II (1950s) droughts that prompted a range of water infrastructure and management responses. In recent years, population growth in humid and semiarid areas alike has increased the exposure to drought events in California, Georgia and the Southeast, Hawaii, the Southwest, the Pacific Northwest, and Texas. In this context, some regions are vulnerable to substantial impacts even for drought events of moderate intensity, duration, and severity. Together with the continental scale droughts in 2012-2013, these events have made drought an issue of increasing national interest and concern, prompting coordination of monitoring, planning, and other drought management actions (Folger and Cody 2015).

River basins in the western United States have experienced sustained droughts and will be the primary focus here, highlighting the experience of the Colorado and Rio Grande/Bravo basins, two international rivers shared by the United States and Mexico. Here the primary focus will be the US portion of each basin.* The Colorado River Basin drains almost 700,000 km2

The interstate dynamics within Mexico and Mexico's delivery obligations to the United States in the Rio Bravo are beyond the scope of this analysis.

with its territory covering parts of nine states (seven in the United States, two in Mexico). The basin's hydroclimatology has been marked by sustained droughts, including the period from the late 1940s through the 1950s, which was the drought of reference for modeling and planning purposes until recently. Since 2000, the Colorado River Basin has experienced a dry period without precedent in the observed record, although reconstructions of the paleoclimate indicate droughts of longer duration and intensity (Udall and Overpeck 2017; Woodhouse et al. 2006).

The Rio Grande/Bravo Basin drains approximately 450,000 km2 (excluding endorheic zones), including almost equal territory in the United States and Mexico. Both the United States (upper Rio Grande) and Mexico (Rio Conchos) contain important tributaries shared by multiple states within each country. Like the Colorado River, the basin has experienced sustained droughts in the 1950s and since 2000. However, the history of drought in the Rio Conchos and Upper Rio Grande has not been correlated per the paleoclimate record, which means that droughts can affect one tributary without impacting the other (Woodhouse et al. 2012).

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