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Warm Deserts

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Name	†	Color on Map	CEC Code‡
Mojave Basin and Range	✓		10.2.1
Chihuahuan Deserts	✓		10.2.4
Sonoran Basin and Range	✓		10.2.2
Baja Californian Desert	✓		10.2.3

† Status: ✓ = Complete ○ = Needs Image … = Incomplete ∅ = Stub Only

‡ This code refers to the CEC's Level 3 ecoregion codes for North America, see here.

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↑About the Warm Deserts

The Warm Deserts are a region encompassing the warmer deserts in North America, consisting of two discontinuous pieces. The western portion extends from Nevada and southwest Utah through California and Arizona into most of Baja California and Sonora. The eastern portion is mostly located in Mexico, east of the Sierra Madre Occidental mountain range, extending north into west Texas and southern New Mexico.

The climate here is arid and subtropical. There is significant seasonality to temperature but little frost: low elevations are mostly either frost-free or experience only occasional light frost, but temperatures can get colder at higher elevations. Unlike the Cold Deserts to the north which are mostly produced by rain shadow, this region, which opens to the ocean along its westernmost border, is heavily influenced by a persistent high-pressure system, the North Pacific High. This system diverts moisture-laden weather systems to the north and south as they come inland, which leads the deserts to extend the whole way to the Pacific coast in the central portion of this region. This region thus has a diversity of inland areas with drier air and more continental climate influence, to coastal areas with more moderate temperatures and higher relative humidity but very low rainfall. The Eastern portion of this region is influenced by a mix of of rain shadow, much like the cold deserts, and the effects of this high. This high pressure zone, which becomes stronger and moves north in the summer, and becomes weaker and moves south in the winter, leads there to be a seasonality of precipitation: the far northwest of this region receives more rainfall in winter, but the rest of the region receiving more rainfall in summer, with the effect becoming more pronounced moving farther south and east.

Vegetation cover varies considerably throughout this region. Some portions are dominated by large cacti, whereas in others, grasses are more dominant. Much of the vegetation is adapted to utilize short bursts of high water availability, going dormant for months at a time, and then becoming active in response to the presence of water. Plant diversity is often higher than in the cold deserts, owing to the greater portion of plants able to survive in the warmer temperatures, but the higher temperatures can also produce additional stress from heat and drought. Much of this region features low mountain ranges interspersed with the broad bottomlands; these areas of higher elevation also have higher rainfall and support denser vegetation cover, usually scrubland but reaching to forests in a few small regions along the highest peaks. These areas, owing to their isolation from other forested regions, have a high portion of endemic species.

Land use varies considerably throughout this region, with parts of it left largely untouched and ecologically preserved, and others of it intensely utilized and severely degraded. Major land uses here include ranching and grazing, irrigated agriculture, and mining. The irrigation for agriculture is both taken from major rivers, especially the Colorado river, as well as groundwater aquifers; both uses have large environment impacts. Historically, this region was sparsely populated, but more recently, the population has exploded in the major cities. The largest metro areas in this region include Phoenix, AZ, Las Vegas, NV, the cross-border metro of El Paso-Juaréz, and Torreón, Coahuila, Mexico. There are also numerous smaller cities and metro areas. Water diversion both for urban use and agriculture has severely reduced the flow of water in the major rivers of this region, especially the Colorado river, devastating the ecosystem in the Gulf of California. The Chihuahuan desert in the southeast is also facing increased human-induced desertification.

Different parts of this region border radically different ecoregions. To the west, this region transitions gradually into Mediterranean California, as the rainfall increases slightly and the desert becomes Chapparral. To the north, as temperatures cool, the western portion transitions smoothly to Cold Desert, but east of the rockies, the desert transitions to the relatively-moister South Central Semi-Arid Prairies. Southeast of the Sonoran desert, the region transitions to tropical dry forests of the Western Pacific Coastal Plain, Hills, and Canyons, and in Baja California, to the Sierra and Plains of El Cabo. Mountain ranges and changes in altitude produce more abrupt borders with the Western Sierra Madre Piedmont, and in the north, the Upper Gila Mountains.

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