The Sonoran Desert is clearly distinct from nearby deserts (e.g., theGreat Basin,Mojave, andChihuahuan deserts) because it provides subtropical warmth in winter and two seasons of rainfall (in contrast, for example, to the Mojave's dry summers and cold winters). This creates an extreme contrast between aridity and moisture.[4]
The Sonoran desert wraps around the northern end of theGulf of California, fromBaja California Sur (El Vizcaíno Biosphere Reserve in central and Pacific west coast, Central Gulf Coast subregion on east to southern tip), north through much ofBaja California, excluding the central northwest mountains and Pacific west coast, through southeastern California and southwestern and southern Arizona to western and central parts ofSonora.[5]
To the north in California and northwest Arizona, the Sonoran Desert transitions to the colder-winter, higher-elevationMojave,Great Basin, andColorado Plateau deserts.
The desert's sub-regions include theColorado Desert of southeastern California; and theYuma Desert east of the north-to-south section of theColorado River in southwest Arizona. In the 1957 publicationVegetation of the Sonoran Desert, Forrest Shreve divided the Sonoran Desert into seven regions according to characteristic vegetation: Lower Colorado Valley, Arizona Upland, Plains of Sonora, Foothills of Sonora, Central Gulf Coast, Vizcaíno Region, and Magdalena Region.[6] Many ecologists consider Shreve's Vizcaíno and Magdalena regions, which lie on the western side of theBaja California Peninsula, to be a separateecoregion, theBaja California desert.
Within the southern Sonoran Desert in Mexico is found theGran Desierto de Altar, with theEl Pinacate y Gran Desierto de Altar Biosphere Reserve, encompassing 2,000 square kilometres (770 sq mi) of desert and mountainous regions.[7] The biosphere reserve includes the only activeerg dune region in North America. The nearest city to the biosphere reserve isPuerto Peñasco ('Rocky Point') in the state ofSonora.
The Sonoran desert has anarid subtropicalclimate and is considered to be the mosttropical desert inNorth America.[8] In the lower-elevation portions of the desert, temperatures are warm year-round, andrainfall is infrequent and irregular, often less than 90 mm (approx. 3.5") annually. TheArizona uplands are also warm year-round, but they receive 100–300 mm (approx. 4–12") of average annual rainfall, which falls in a more regular bi-seasonal pattern.[2]
Many plants not only survive, but thrive in the harsh conditions of the Sonoran Desert. Many have evolved specialized adaptations to the desert climate. The Sonoran Desert's bi-seasonal rainfall pattern results in more plant species than any other desert in the world.[2] The Sonoran Desert includes plant genera and species from theagave family,palm family,cactus family,legume family, and numerous others. Many of these adaptations occur in food crops.Mission Garden is a living agricultural museum that showcases foods that have been grown in the Sonoran Desert for over 4000 years.
The Sonoran is the only place in the world where the famoussaguaro cactus (Carnegiea gigantea) grows in the wild.[10]Cholla (Cylindropuntia spp.),beavertail (Opuntia basilaris),hedgehog (Echinocereus spp.),fishhook (Ferocactus wislizeni),prickly pear (Opuntia spp.),nightblooming cereus (Peniocereus spp.), andorgan pipe (Stenocereus thurberi) are other taxa of cacti found here. Cacti provide food and homes to many desert mammals and birds, with showy flowers in reds, pinks, yellows, and whites, blooming most commonly from late March through June, depending on the species and seasonal temperatures.
Ascending from the valley upbajadas, various subtrees such asvelvet mesquite (Prosopis velutina),palo verde (Parkinsonia florida),desert ironwood (Olneya tesota),desert willow (Chilopsis linearis ssp.arcuata), andcrucifixion thorn (Canotia holacantha) are common, as well as multi-stemmedocotillo (Fouquieria splendens). Shrubs found at higher elevations includewhitethorn acacia (Acacia constricta), fairy duster, andjojoba. In the desert subdivisions found on Baja California, cardon cactus, elephant tree, andboojum tree occur.[11]
The Sonoran Desert is home to a wide variety of fauna that have adapted and thrive in the hot, arid desert environment, such as theGila monster,bobcat,mule deer,pronghorn antelope,jackrabbit,burrowing owl,greater roadrunner,western diamondback rattlesnake, andelf owl. There are 350 bird species, 20 amphibian species, over 100 reptile species, 30 native fish species, and over 1000 native bee species found in the Sonoran.[12] The Sonoran Desert area southeast ofTucson and near the Mexican border is vital habitat for the only population ofjaguars living within the United States.[13] TheColorado River Delta was once an ecological hotspot within the Sonoran desert due to the Colorado river in this otherwise dry area, but the delta has been greatly reduced in extent due to damming and use of the river upstream. Species that have higher heat tolerance are able to thrive in the conditions of the Sonoran Desert. One such insect species that has evolved a means to thrive in this environment isDrosophila mettleri, a Sonoran Desert fly. This fly contains a specializedP450 detoxification system that enables it to nest in the cool region of exudate moistened soil. Thus, the fly is one of few that can tolerate the high desert temperatures and successfully reproduce.
The largest city in the Sonoran Desert isPhoenix, Arizona, with a 2017 metropolitan population of about 4.7 million.[14] Located on theSalt River in central Arizona, it is one of the fastest-growingmetropolitan areas in the United States. In 2007 in the Phoenix area, desert was losing ground tourban sprawl at a rate of approximately 4,000 square meters (1 acre) per hour.[15]
The next largest cities areTucson, in southern Arizona, with a metro area population of just over 1 million,[16] andMexicali, Baja California, with a similarly sized metropolitan population of around 1,000,000. The metropolitan area ofHermosillo, Sonora, has a population close to 900,000.Ciudad Obregón, Sonora, in the southern part of the desert, has a population of 375,800.[17]
During the winter months, from November to April, the daytime temperatures in the Coachella Valley range from 70 °F (21 °C) to 90 °F (32 °C) and corresponding nighttime lows range from 46 °F (8 °C) to 68 °F (20 °C) making it a popular winter resort destination. Due to its warm year-round climatecitrus and subtropical fruits such as mangoes, figs, and dates are grown in the Coachella Valley and adjacentImperial Valley. The Imperial Valley has a total population of over 180,000 and has a similar climate to that of the Coachella Valley. Other cities includeBorrego Springs,Indio,Coachella,Calexico,El Centro,Imperial, andBlythe.
Straddling theMexico–United States border, the Sonoran desert is an important migration corridor for humans and animals. The harsh climate conditions and border militarism mean that the journey can be perilous, usually moving at night to minimize exposure to the heat.[18]
^TheSonoran Desert National Monument was created in 2001 in Arizona, to enhance protection of the unique resources of the Sonoran Desert, with 2,008 square kilometers (496,000 acres). :Reference:Sonoran Desert National MonumentArchived 2009-01-26 at theWayback Machine, Bureau of Land Management, U. S. Department of the Interior. Accessed on line June 17, 2009.
