The curiosity about the size of arid regions in the United States stems from the complex way scientists define a “desert” and the vast geography of the American West. Answering what percentage of the US is desert requires moving beyond the image of sand dunes to embrace a scientific definition of dry land. This approach reveals a significant portion of the country is constrained by permanent water scarcity.
Classifying Arid and Desert Environments
The scientific classification of dry regions relies on the Aridity Index (AI), which compares the long-term average of water supply to the atmospheric demand for water. This index is calculated as the ratio of annual precipitation (P) to potential evapotranspiration (PET)—the total amount of water that could evaporate and transpire from a surface. When potential water loss (PET) significantly exceeds the actual water received (P), the land is considered arid.
The United Nations Environment Programme (UNEP) uses this P/PET ratio to classify drylands, with an AI value less than 0.65 marking the boundary for all drylands. True deserts (hyper-arid and arid zones) have a very low AI, often corresponding to less than 10 inches (250 millimeters) of annual rainfall. Semi-arid regions, such as the North American steppe, receive slightly more moisture but still face a permanent water deficit and are often grouped with deserts when calculating the total dry area.
Calculating the US Desert Percentage
The percentage of the United States classified as desert depends entirely on the chosen definition. If the definition is limited to only the driest, classic desert ecosystems (hyper-arid and arid zones), the area is smaller, roughly between 13% and 20% of the contiguous US land area. However, including the vast semi-arid regions that struggle with a permanent water deficit causes the percentage to rise dramatically.
According to the U.S. Geological Survey (USGS), a comprehensive classification that includes all drylands—arid and semi-arid zones—reveals that approximately 40% of the land in the United States is considered dryland. This figure encompasses the four major deserts, the immense expanse of the Great Basin, and the western Great Plains. This large percentage is primarily a result of the rain shadow effect created by the Sierra Nevada and Cascade mountain ranges, which block moisture from the Pacific Ocean from reaching the interior West.
The Four Major North American Deserts
The four major desert regions anchor the 40% dryland figure and dominate the American Southwest and Intermountain West. They are distinguished by their geographical location, elevation, and seasonal precipitation patterns.
Great Basin Desert
The largest and most northern is the Great Basin Desert, a high-altitude, cold desert covering most of Nevada and parts of surrounding states. Most precipitation here falls as winter snow.
Mojave Desert
South of the Great Basin lies the Mojave Desert, characterized by extreme dryness and the presence of the iconic Joshua Tree. The Mojave is the smallest and driest of the four, featuring very hot summers and cold winters.
Sonoran Desert
The Sonoran Desert covers parts of Arizona, California, and Mexico, and is unique for its bimodal rainfall pattern. Receiving moisture in both winter and summer allows for a richer diversity of plant life.
Chihuahuan Desert
Extending across parts of New Mexico, Texas, and Mexico, the Chihuahuan Desert is a high-elevation desert that receives most of its moisture during the summer months. It is considered the largest in North America by some measures.
The Role of Arid Lands in US Ecology
Despite the scarcity of water, these arid lands are vital ecological systems, serving as biodiversity hotspots for specialized flora and fauna. Organisms in these regions have evolved adaptations to survive the moisture deficit and extreme temperatures. Plant life includes xerophytic species, which use waxy coatings and deep root systems to conserve water, and succulents, such as cacti, which store water in their tissues.
Animal populations exhibit similar traits, with many species being nocturnal to avoid the intense daytime heat. Some animals, like the kangaroo rat, obtain almost all the water they need from their food. However, these ecosystems are fragile and vulnerable to changes in climate and land use. The Southwest is currently facing “aridification,” a sustained state of dryness that goes beyond temporary drought. This long-term water deficit threatens the delicate balance of the ecosystems, increasing the risk of wildfires and placing strain on the limited water resources that support both human populations and natural habitats.