Drought in the American Southwest—Is it serious?

Andy May

The featured image is of the Big Bend, Texas, area in October 2015, when the drought index was moderately wet. Photos taken by the author.

Stories of some kind of catastrophic drought currently occurring in the American West are grossly exaggerated (with apologies to Mark Twain). While the western United States is dry due to its location in an area of ​​permanent high pressure and low rainfall, current drought conditions in the region are mild by historical standards. This article is primarily an update of two previous articles by Anthony Watts ( here and here ). Watts’ post has stood the test of time. It’s amazing how often “consensus” repeats old, debunked myths.

We should distinguish between meteorological droughts, which are long-term decreases in precipitation due to reduced cloud formation due to persistent high atmospheric pressure (IPCC AR6 WGI, pp. 1157 and 1760), and agricultural droughts, which are reduced soil moisture and reduced waterway flows (also known as for hydrological drought). Meteorological drought is a climatic phenomenon, while agricultural drought is only part of a meteorological phenomenon, as it can be mitigated through artificial changes in irrigation, dams, and other drainage systems designed to reduce the flow of precious fresh water into the ocean. This article is about meteorological droughts and long-term drought cycles.

Deserts are found in two zones around the Earth. In the northern hemisphere, they occur between approximately 20° and 40° north latitude. The Northern Hemisphere desert belt includes a small portion of southwestern North America, as shown in Figure 1, modified from (Cherlet et al., 2018). The curvature of this zone is due to the northern location and curvature of the ITCZ, or Intertropical Convergence Zone.

The ITCZ ​​is the climatic equator, which continuously moves from a southern position during winter in the Northern Hemisphere to a summer position in the far north and back again. Since the sun is always directly overhead in the ITCZ ​​at noon, evaporation is always greatest. The evaporated water vapor is denser than dry air and rises to form an updraft. When water vapor rises high enough, it condenses into rain and clouds, and the remaining dry air, which is denser than moist air, falls back. This creates areas of high pressure that lead to deserts.

The timing of drought in the southwestern United States is mainly controlled by ENSO (El Niño and La Niña), and is influenced by the approximately 22-year Hale solar cycle, the approximately 100-year Feynman solar cycle, the approximately 67-year AMO (Atlantic Decadal Oscillation), and the Pacific Ocean Decadal Oscillation (Mitchell, Stockton, and Meco). See also Jiang et al. 2019 (Jiang, Yu, & Acharya, 2019) here. Figure 2 shows the areas assessed, and Figure 3 is the assessment of climate proxies for the western continental United States.

The percentage of drought in the western United States is shown in Figure 3. PDSI values ​​less than -1 in the figure are considered drought conditions. Wayne Palmer designed the index that bears his name. It measures how abnormally dry or wet an area is based on its long-term average climate. An explanation and review of PDSI calculations can be found here (Heddinghaus & Sabol, 1991).

PDSI is objective and avoids drought arguments based on effects such as reduced stream flow (hydrological drought), reduced soil moisture affecting crops (agricultural drought), or economic impacts. The economic impact of drought can be caused by meteorological drought or by development (or overexploitation) where demand for water exceeds the amount of water available in the area.

The Palmer Index is based on precipitation and temperature records and a simple model of soil moisture supply and demand. It's not very effective over the short term, but it describes the situation well over the long term (many months or years). The index ranges from -10 to 10, with negative numbers indicating reduced soil moisture (drought) and positive numbers indicating wet conditions. Zero is neutral or “normal” as determined historically. One of the advantages of PDSI is that it is widely used and values ​​(from instrument readings or climate proxies) persist over many regions over long periods of time. Figure 3 depicts arid areas of the American West since AD ​​800. Drought severity is not only related to PDSI but also to the size of the affected area and the duration of the precipitation deficit. Prior to the instrumental era, the data used to create the index in Figure 3 came primarily from tree-ring proxies (Cook, Seager, Cane, & Stahle, 2007).

Compared with the beginning of the Little Ice Age around 1300 AD, current drought conditions in the western United States are relatively mild. It could be worse than what it is now. Matthew Bekker and colleagues (Bekker, DeRose, Buckley, Kjelgren, & Gill, 2014) noted in their study of tree rings around the Weber River in Utah:

“Although the instrumental period of the 20th century included a few extreme individual drought years, this was the fewest such years of the entire century. [576-year] reconstruction. Long-term droughts preceded instrumental records of greater duration, scale, and intensity, including the longest drought on record, which lasted 16 years, from 1703 to 1718.

Most of the dry years around the Weber River occurred in the 1400s and 1500s, well before significant human emissions of carbon dioxide and other human activities. There were several droughts in the 1920s^th The most severe period of drought in the past century was around 1934, as shown in Figure 4.

Using the same criteria and color scheme, Figure 5 shows the Palmer Drought Severity Index for the United States in July 2019.

In contrast to the dry years plotted in Figure 4, as shown in Figure 5, 2019 was relatively normal to wet across much of the country.

Figure 6 shows the situation in December 2024. The country is in much better shape today than it was in 1934.

Droughts in the Southwest always come and go in a semi-cyclical manner, generally with a cycle of about 22 years, and longer periods of about 100 years. When Pike's expedition traveled through the Southwest in 1806-1807, the weather was extremely dry, creating the myth of the “Great American Desert.” Later, in the 1870s, after the early transcontinental railroad was built, the area became even wetter and people called it the “Garden of the Great Plains.” (Cook, Seeger, Kane, & Stahler, 2007). Historical perspective is important.

Droughts can begin and end quickly, especially in drought-prone regions like the American Southwest. It is difficult to describe the drought characteristics of an area using static maps. For an animation of PDSI over the United States during a selected time period, see here. Especially when interpreting drought news coverage, this site can help keep some perspective.

Parts of the Southwest always seem to be in drought, but dry areas move around frequently. There were good years from 2009 to 2010, followed by drought from 2011 to 2014. 2015 to 2017 was pretty good, 2018 was drier, and 2019 to mid-2020 was better. Then in mid-2020, the weather started to get drier and now, with the exception of the Big Bend region of Texas and southern New Mexico, conditions are showing signs of improving. The conclusion is that although the American Southwest is a drought-prone region, droughts come and go and are much more severe than in the past, particularly between 900 and 1300 AD.

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