Julie E. Laity
The core deserts of southwestern North America stretch from southeastern California to western Texas and from Nevada and Utah to the Mexican states of Sonora, Chihuahua, and Coahuila, and much of the peninsula of Baja California (fig. 19.1). Beyond these areas, semidesert conditions extend north to eastern Washington, south onto the central Mexican plateau, and east to link with the steppes of the High Plains (see chapter 12). In area, about 55% of the North American deserts are considered semiarid, 40% arid, and only 5% hyperarid. This chapter, however, focuses on the core deserts of the southwestern United States and northwest Mexico, namely, the Chihuahuan, Sonoran, Mojave, and Great Basin deserts.
North American deserts owe their aridity to a rainshadow effect caused by mountains blocking moisture of Pacific origin in the winter and from the Gulf of Mexico in the summer, the occurrence of a subtropical high-pressure cell, and cold currents along the western coast. Rainshadow effects are greatest in the Great Basin and Mojave deserts, whereas the effect of high pressure is important in the Chihuahuan and Sonoran deserts. Cold coastal currents play a role in the western deserts, particularly along the outer coast of Baja California.
Demarcation of the deserts has been based on multiple criteria, including climate, geology, vegetation physiognomy, and floristic composition, with the result that the defined boundaries of the desert may vary according to the criteria selected. From a climatic perspective, the deserts can be classified according to their temperature and the seasonality of their precipitation (fig. 19.2). The Great Basin, characterized by its northern position, high altitude, the receipt of 60% of its winter moisture as snow, mean monthly temperatures below 0°C from December through February, and mean annual temperature of 9°C, is considered a “cold” desert. The distinction between “cold” and “warm” deserts is based principally on winter temperatures, as all deserts are characterized by hot summer temperatures. The mean annual temperature of the more southern warm deserts is 20°C, and essentially all of their precipitation occurs as rain. The number of frost-free days averages 80 to 150 in the Great Basin and 210 to 365 in the Mojave, Sonoran, and Chihuahuan deserts (McMahon and Wagner, 1985). The Great Basin, Mojave, and western Sonoran deserts receive winter and spring precipitation derived from frontal systems that move in from the Pacific. Winter precipitation, falling from November to February, makes up 44% of the annual total in the Mojave Desert, 38% in the Great Basin Desert, 28% in the Sonoran Desert, and 18% in the Chihuahuan Desert. Moving from west to east from the Mojave to Chihuahuan Desert, the ratio of winter to summer rainfall decreases. The eastern Sonoran Desert mainly has a bimodal rainfall regime, with a strong primary maximum in July and August and a secondary maximum in February. The Chihuahuan Desert receives summer rain-