Biome: Desert

Categorization of Deserts

In addition to being broadly categorized into tropical/subtropical and temperate, deserts are categorized according to their level of aridity, traditionally by assessing annual precipitation. However, moisture input should not be considered without factoring in moisture loss, particularly in the limited moisture systems of deserts. Furthermore, precipitation in deserts is too variable to be the sole basis of categorization. Most geographers measure the aridity of a desert according to the ratio between annual precipitation and evapotranspiration (the combined loss of moisture through evaporation and transpiration by plants). These criteria were developed by UNESCO in the 1970s, but several similar formulas have been developed over the past century.

The extent to which mean annual precipitation (P) falls below mean annual evapotranspiration values (ET), reflects a level of aridity that falls into one of three categories: (1) semiarid, (2) arid, and (3) hyperarid. This measurement is sometimes altered to include other factors. For example, calculations of this ratio are often made for set periods of time to account for climate variability, and some formulas factor in the water storage capacity of plants and soil. Nevertheless, categorizations are most commonly based on the following:

• Semiarid deserts: P/ET value <0.5 and mean annual precipitation 200 to 500 mm.
• Arid deserts: P/ET value <0.20 and mean annual precipitation between 80 to 200 mm.
• Hyperarid deserts: P/ET value <0.03 and mean annual precipitation <80 mm.

The delineation of different deserts is challenging at best, and satellite images now show us how the boundaries of deserts can shift in just 1 year—the sands of the Sahara Desert in North Africa continually shift and China's desert land expands every year due to desertification processes. In addition, a desert that is considered arid one year could be semiarid in another due to a rare spike in rainfall. To overcome these variations, there is an important temporal element to the categorization of deserts, usually requiring decadal measurements at minimum. Other measurements have also been used to help identify where a desert area begins and ends, such as vegetation distribution, geomorphology, or landscape type.

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