The Desert Environment

Deserts are land areas that receive less than 25.4 cm (10 inches) of rain, unevenly distributed throughout the year. A number of climatic processes produce desert areas. The most influential are the six cells of cold air currents that descend at the poles and near the Tropic of Cancer and Tropic of Capricorn. These air currents, driven by the sun and rotation of the earth, create areas of relatively warm, dry conditions. Many of the world’s deserts live in a “rain shadow,” an area to the leeward side of a mountain range that prevents the small amount of moisture that is present in the air to move over the mountains. As the air rises, the moisture cools and precipitates in the higher elevations. Therefore the area in the “shadow” of the mountain range receives little moisture. The air that does descend is quite dry and adds to the evaporative effect. The Atlas Mountains shadow the Sahara, the Andes the Patagonian, the Great Dividing Range the Australian, and the Sierra Nevada and Cascades the Great Basin deserts. The amount of rainfall is not an absolute indicator of “dryness” because the rate of evaporation and timing of the rainfall must also be taken into consideration. The amount and type of vegetation, soil composition, altitude, average temperature, wind speed, and solar radiation all contribute to “dryness” and desert formation. Antarctica would be the world’s largest desert by the definition of less than 25.4 cm (10 inches) of rainfall annually, some areas of that continent having had no recorded rain in 30 years. There is a large amount of water present in the form of ice, but it is not available for use by plants. Antarctica has its own special survival problems not associated with precipitation and, for the purposes of desert survival, will not be considered here. In contrast to Antarctica stands the northern coast of Alaska, which receives less than 10.2 cm (4 inches) of rain annually yet is quite wet because evaporation is so low.

Deserts are one type of environment on Earth that is increasing in total area, likely because of human as well as geologic factors. Overgrazing, destruction of forests, global warming, and other aspects of increased human population contribute to desertification. Currently about 15% of the land area of the earth is desert (30% if Antarctica is included) (Figure 44-1; Table 44-1). Most of the earth’s deserts can be found between 30 degrees south and 30 degrees north latitude, making them hot as well as dry. These deserts include the Sahara, Arabian, Kalahari, Australian, Atacama, Thar, Namib, and southwest United States. About 50% of Africa is desert; the Sahara by itself is almost as large as the United States. About 8% of the United States, or 776,996 square kilometers (300,000 square miles), is desert. Most of the U.S. desert areas are adjacent to national parks and forests and are frequently visited, for example, the Grand Canyon, Big Bend, Arches, Zion, Organ Pipe, Joshua Tree, Great Basin, Saguaro, and Capital Reef. Beyond 40 degrees south and north latitude and at elevations over 3048 m (10,000 feet) are the “cold” deserts, which have wide swings in temperature, for example, the Patagonian, Turkestan, Gobi, and Taklamakan. The large temperature variations in desert regions are greater at higher elevations and latitudes but are present in all deserts. Lack of vegetation, cloud cover, and ground-water surface allows 90% of solar radiation to reach the desert surface. By contrast, a forest may reflect 50% to 60% of the solar radiation, and its vegetation disperses the rest. At night, lack of cloud cover and vegetation allows almost 100% of the accumulated heat to escape, as opposed to only 50% from a humid climate. This explains why the desert temperature may reach 48.9° C (120° F) during the day and drop to 4.4° C (40° F) at night. Tropical rain forests may only reach 35° C (95° F) during the day, but at night the temperature only drops to 29.4° C (85° F).

FIGURE 44-1 Desert areas of the world. Many of these areas are growing.

TABLE 44-1 Deserts of the World

It might seem that the extreme desert climate would only allow for sparse life, but that is not the case (Figure 44-2). Death Valley, one of the harshest environments in North America, where air temperatures have been recorded at 56.7° C (134° F), has 600 species of plants, 30 species of mammals, 25 species of reptiles, and 2 species of fishes. Oases are found in most deserts. They are isolated depressions usually fed by a constant source of water. Underground springs and wells supply moisture for plants and animals. Often one must dig to find water at the lowest point of the depression. Many named oases have supported camel caravans, allowing them to move from oasis to oasis and thus cross an otherwise impenetrable desert. Desert way stations and ancient cities have sprung up along these routes. Many ancient oases have wells hundreds of feet deep and because of overuse are gradually drying up. When the water is used up, the oasis disappears, along with its desert life.

FIGURE 44-2 Desert landscape. Note the physical features and abundant flora.

All desert flora and fauna have one guiding principle for survival, which is to conserve water. The ground surface of the desert has the highest temperature because of the direct effect of solar heat and wind. Therefore, during the hottest times of the day, most animals are either below the surface, in underground burrows, or above the surface in available vegetation, cacti, trees, or shrubs. Most animals forage from dusk until dawn because temperatures are cooler. Some mammals, such as kangaroo rats, never drink but obtain necessary water through plant seeds. Plants have evolved a number of survival skills to maintain water, including stomata that are closed during the day and crassulacean acid metabolism (CAM) photosynthesis. The latter allows for accumulation of carbon compounds at night through the dark reaction. These compounds are converted to carbon dioxide during the day when the stomata are closed. Other adaptations include stem photosynthesis in plants without leaves, thick cuticles, water storage tissues, and widespread shallow root systems. Desert plants also have evolved a variety of defense mechanisms, such as production of toxic compounds that act as herbicides to other plants, and the formation of needles, spines, and thorns that dissuade browsing animals. Obviously humans are not able to evolve these physiologic changes but must rely on behavior, technology, and other adaptations to mimic the methods used by indigenous desert dwellers.

Preparation

All things being equal, preparation improves the likelihood of survival. However, things are never equal, so luck is probably the most important, albeit the most uncontrollable, factor. The controllable factors are mental and physical conditioning, clothing, survival kit adequacy, and survival skills. These may allow one to survive even in the most extreme conditions.

Mental preparation is key to any survival situation. The “will to survive” has been shown to be the most important factor in the outcome of a number of situations. Knowledge of the terrain features, weather, animal and plant life, and potential hazards should all be studied before travel to a desert area. Not only does this increase one’s chances for survival, but it also enhances enjoyment of the desert environment. Practical experience in finding water and food, navigation, and constructing shelters is more valuable than reading about it. Time spent in attending a course on survival in general or desert survival in particular may be invaluable if one is later in a true survival situation.

Physical conditioning and acclimatization are as important for desert travel as for mountaineering. Desert travel is difficult under most circumstances. The terrain is rough and may include sand dunes, sharp loose rock, flash floods, steep grades, and hot surfaces. Lower-body conditioning helps prevent the ankle and knee injuries that can force a survival situation in a harsh climate. Acclimatization may take 10 to 14 days and involves three well-described physiologic adaptations. These are increase in sweat volume and number of active sweat glands, decrease in concentration of electrolytes in sweat, and sweating at lower body temperature. These can be induced before arriving in a hot climate by the use of a sauna or vigorous exercise to raise body temperature (see Chapters 10 and 11).

Clothing

Clothing selection for desert travel is somewhat different than for most other wilderness activities. The less exposed skin, the better. Although cotton is not appropriate for most cold, wet climates, it is useful in the desert. Light-colored clothing reflects sunlight and lowers skin temperature. Ripstop cotton (cotton material with nylon threads latticed within it) is best because it resists rips that are common in the desert. It is light enough to allow heat to escape, does not have a clammy feeling in low humidity, and protects against some ultraviolet rays and blowing sand. If ripstop cotton is not available, any tight-weave cotton is adequate. Long sleeves and long pants are a “must” to protect against spines, thorns, splinters, and insects. More important, they offer some protection against solar radiation that causes sunburn and increased body temperature, and they trap more cool air next to the skin. Sweat that is trapped decreases water loss through evaporation. Trousers can be tucked into the tops of socks to protect from insects; sleeves should be not be rolled up in order to minimize risk for sunburn and heat gain. Gaiters can be worn to protect lower legs and the inside of footwear from sand, rocks, and dust. In a survival situation, puttees (wraps that extend from the tops of the shoes to the knees, either over trousers or bare legs) can be made of strips of cloth, elastic bandages, or stockings to protect the lower legs. They can be incorporated into the socks and wrapped to above the knees in a fashion similar to gaiters. Because of the wide temperature swings, a pile jacket or sweater is necessary at night. Layering, just as in cold climates, is the best means of preserving body warmth. In a survival situation, any insulating material, such as seat cushions, newspapers, or dry grass, can be used to insulate whatever clothing is available. Although wind is more of a problem than is rain, a Gore-Tex jacket is also recommended, especially for “cold” deserts. A wide-brim hat or kepi (a cap with a cloth extending from the back protecting the neck) is necessary to protect the head, face, neck, and ears. In a survival situation, an expedient head covering can be made from whatever material is at hand (Figure 44-3). A cotton cravat, bandanna, or handkerchief can be used to keep the head and neck cool by soaking the material in water (if plenty of water is available; do not use precious drinking water) and then placing it on the head, followed by a hat. Alternatively, it can be wrapped around the neck and shoulders underneath a shirt. Commercially produced (Cooldanna, Climatech, StaCool) neck wraps and vests, which contain crystals that can be soaked in water and then become cool through a chemical reaction, are also available but probably do not add much in the way of total-body cooling in extreme environments. The cravat can also be used during dust storms to protect the nose and mouth, as a towel, or to absorb moisture from plants when obtaining water.

FIGURE 44-3 Expedient head coverings.

High-top (15.2 cm [6 inches]) boots composed of leather or synthetic materials are necessary to prevent sand, rocks, and burrs from entering the boots, support the ankles on rough terrain, insulate the feet from hot surfaces, and prevent the boots being pulled off in soft ground. Boots should be well broken in before hiking. Military-issue boots with metal spike protection can become extremely hot in desert conditions and should be avoided. Running shoes do not insulate the feet well and may become extremely hot. Socks should not be cotton, because of the risk for blisters and lack of wicking and insulation. Polypropylene or a combination of polypropylene and wool is best for socks because of less friction and thicker material. Foot care is extremely important, especially in a survival situation when walking is the only means of transportation. Feet should be inspected for blisters, foreign bodies, and abrasions on a regular basis while hiking. Socks should be changed frequently, at least twice a day, to allow them to dry out and to remove accumulated dust and sand. Leather gloves are desirable to protect hands from hot objects, plant spines, thorns and splinters, insects, and blisters. Abrasions and lacerations to the hands can quickly become infected in conditions where hand washing is difficult. Eye protection becomes very important in the desert, especially when traveling. Solar radiation, both direct and reflected, can cause keratitis similar to snow blindness. More commonly, blowing sand, dust, and insects may cause corneal abrasions and conjunctivitis. Contact lenses are difficult to manage in the dry, dusty environment. Tinted goggles are best, just as with mountaineering, but glacier glasses or standard sunglasses can be used. Duct tape, adhesive bandages, or other material can be used to fashion side shields for regular glasses to prevent sand and dust from entering through the sides. Insect head nets can be lifesaving, especially in African, Arabian, and Australian deserts, where insects and insect-borne diseases are particular problems.