What Is a Desert?

Although many people visualize deserts as dry, desolate wastelands, the term actually defines a wide spectrum of landscapes and plant and animal population densities. The Sonoran Desert does have seas of sand and expanses of desert pavement that are nearly devoid of visible life, but most of it is more reminiscent of a sparse woodland savanna.

The common denominator of all deserts is extreme aridity— water is freely available only for short periods following rains. Desert is often defined as a place that receives less than ten inches (250 mm) of annual average rainfall, but this definition is inadequate. For example, the Pacific coast of northern Baja California and the north slope of Alaska both receive less, but those places are vegetated with chaparral and tundra, respectively. An accurate measure of aridity must compare rainfall (abbreviated P for precipitation) with potential water loss through evaporation and transpiration (the loss of water from leaves). Potential evapo-transpiration (abbreviated pet, the water that would be lost from evaporation and transpiration if water were present to evaporate) is difficult to measure accurately, but is crudely estimated to be sixty percent of pan evaporation (the water that evaporates from a wide pan of water exposed to the weather). Pan evaporation varies severalfold within a local area depending on slope and exposure to wind, so it is applicable only to the specific site where it is measured. Tucson receives an average of twelve inches (305 mm) of rain a year, while the pan evaporation is about 100 inches (254 cm). In other words, the climate of Tucson could evaporate eight times more water per year than is supplied by rain, a pan evaporation to precipitation ratio of 8:1. Using the sixty percent estimate for PET, Tucson's PET/p ratio is 4.3; climatologists classify areas with ratios higher than 3.0 as semiarid. This moisture deficit presents a significant challenge to the biota, but is not large compared to that of hyperarid deserts such as that around Yuma, Arizona which has a PET/p ratio of 30, or the interior Sahara Desert's 600.

A concise nontechnical definition of a desert is "a place where water is severely limiting to life most of the time." (Without the word "severely" the phrase defines semiarid habitats such as grassland, chaparral, and tropical deciduous forest.) Though desert plants and animals must cope with scarce water, the common perception that they are struggling to survive is grossly inaccurate. The native biota are adapted to and usually thrive under these conditions and, in fact, most of them require an arid environment for survival. Look at it this way: if a desert received much more rain, it wouldn't be a desert. A different, wetter, biome would replace it. Thus an alternative and more positive definition might be: "A desert is a biological community in which most of the indigenous plants and animals are adapted to chronic aridity and periodic, extreme droughts, and in which these conditions are necessary to maintain the community's structure." (The desert biome requires chronic aridity, but not all of its component species do.)