Geographically, our American Southwest includes the southwestern one-quarter of the United States. It incorporates the western one-third of Texas, all of New Mexico, Arizona, Nevada and Utah, and the southern one-quarter of Colorado. Topographically, the “Staked Plains” of Texas form the eastern boundary; the Sierra Nevada Mountains its western edge. The San Juan and Sangre de Cristo mountains delineate its northern rim. The demarcation line established in the Gadsden Purchase (1853) serves as the southern boundary. Included here are parts of the Sonoran and Chihuahuan deserts, the Colorado, Rio Grande and Gila Rivers, and the principal population centers of Santa Fe, Albuquerque, Tucson, Phoenix, El Paso and Las Vegas (NV). While mountains, plains and deserts exemplify diversity, aridity and sunlight provide the two principal characteristics that unify the Southwest.
Water is scarce in the Southwest. There are few rivers. The largest, the Colorado and the Rio Grande, pale in comparison with the Ohio, the Missouri and the Mississippi. Precipitation in the form of winter snowfall in the San Juan, Sangre de Cristo and Sierra Nevada mountains provides the only reliable water for the entire region. Ironically, these same mountain ranges ring moisture from the air before it reaches the region, subjecting the area to long periods of drought. Further, an unrelenting sun bakes the region daily, compounding the perennial scarcity of water.
Throughout the Southwest, eighty percent of all available sunlight reaches the earth annually. This means that, on average, the unfiltered rays of the sun hammer the land and its people nearly 300 days each year. Furthermore, the ring of mountain ranges surrounding the Southwest isolates it from the moderating effects of any ocean, making it subject to large discrepancies between daytime and nighttime temperatures, which may vary thirty to fifty degrees each day. The harshness of the climate is reflected in the appearance of the land. It has a hard and brittle look about it. Plants are spaced far apart, separated by dry and sun-baked soil. Vegetation, which would normally add moisture to the air through transpiration, has evolved mechanisms to retain moisture for long periods in order to survive extended periods of drought. As a result, most of the Southwest experiences a “moisture deficiency” greater than twenty inches per year. In other words, irrigation equal to twenty or more inches of precipitation per year is required to grow crops, regardless of how rich the soil might be. Sunlight and aridity create a harsh environment here where survival depends, and has depended for centuries, on access to sufficient supplies of water.