While much of Colorado is characterized by high-altitude alpine and semi-arid steppe environments, the state’s average precipitation is around 17 inches annually, which is too high for a desert designation. However, due to its varied topography, several large regions within Colorado meet the strict climatological and ecological criteria to be classified as a high desert, or cold desert. These areas function as true arid landscapes where precipitation deficits define the environment. This geographic complexity means that Colorado encompasses a range of arid zones, from semi-arid grasslands to genuine desert ecosystems.
Defining the Arid Landscape
A region is scientifically classified as a desert primarily by the balance between precipitation and potential evapotranspiration. The common threshold for a true desert is an average annual precipitation of less than 10 inches (250 millimeters). The more precise definition focuses on aridity, where the potential for water to evaporate from the land and transpire from plants far exceeds the actual moisture received.
In these arid and semi-arid environments, the rate of potential evapotranspiration is often two or more times greater than the annual rainfall, creating a perpetual moisture deficit. A semi-arid region receives slightly more than 10 inches, supporting short grasses and scrubland, often referred to as a steppe. Colorado contains vast stretches of this semi-arid land, but its driest parts fall below the 10-inch threshold, fitting the definition of a desert.
The Geographic Divide: Climate Zones and the Rain Shadow Effect
Colorado’s diverse climate is a direct result of its extreme elevation changes and the presence of the Continental Divide. This mountain barrier dictates where moisture falls across the state. Prevailing weather patterns bring moisture-laden air from the Pacific Ocean eastward.
As this air encounters the high peaks of the Rocky Mountains, it is forced upward, cooling rapidly and causing moisture to condense and fall as heavy precipitation, predominantly on the western slopes. This process is known as the orographic effect, or the rain shadow effect. By the time the air descends on the leeward side of the mountains, it is significantly drier.
This mechanism results in a stark contrast: high-elevation areas can receive more than 60 inches of moisture annually, while lower-lying areas or mountain-ringed valleys are left parched. The Eastern Plains are drier than the Western Slope’s windward side, but the driest areas are often deep interior valleys that are in a double rain shadow, shielded by mountains on all sides. This geographic isolation creates the conditions for desertification.
Regions That Qualify as High Desert
The most definitive desert environment in Colorado is the San Luis Valley in the south-central part of the state, which is often called the highest-elevation true desert in North America. This vast, flat valley sits at an average elevation of about 7,700 feet and is ringed by the Sangre de Cristo and San Juan Mountains. The valley floor receives a meager 7 to 9 inches of precipitation annually, easily meeting the classification criteria for an arid climate.
The unique ecology of the San Luis Valley is characterized by plants adapted to cold, dry conditions, such as greasewood, saltgrass, and various sagebrush species. The aridity, combined with the high altitude, results in a cold desert climate with a short growing season and extreme temperature fluctuations. The Great Sand Dunes National Park and Preserve, located within the valley, is a stark visual testament to the powerful combination of high winds, aridity, and sediment deposition that defines this desert landscape.
Grand Valley
Another area frequently classified as a high desert environment is the Grand Valley around Grand Junction and Fruita on the Western Slope. Situated at a lower elevation of about 4,800 feet, this region receives an average annual precipitation of approximately 9.4 inches, placing it just under the 10-inch desert threshold. The surrounding mesas and canyons feature the classic desert-shrubland ecology of the Great Basin, dominated by drought-resistant plants.
The landscape is defined by extensive tracts of sagebrush steppe and pinyon-juniper woodlands that thrive on the arid plateaus and slopes. These trees, including the single-leaf pinyon pine and various junipers, are markers of the Western Slope’s high desert environment, surviving in conditions of low moisture and highly alkaline soils. Therefore, while Colorado is not a desert state in its entirety, several significant areas function ecologically and climatologically as genuine high deserts.