The Sonoran Desert

Deserts cover about 35% of the earth's surface. Most are located between 30° north and 30° south latitude. They are usually either in a "rain shadow" caused by high mountains, or located far from any large body of water. Although deserts vary widely in appearance, they share many characteristics: little precipitation (less than 10" annually), high average daytime temperatures, and high evaporation rates. Low humidity and the absence of clouds allow heat to escape at night. Temperatures may vary as much as 70° during a 24-hour period.

Some deserts have only one rainy season. Others, like the Sonoran Desert of southern Arizona and northwestern Mexico, have two. When rain comes both summer and winter, the desert has a greater variety of plants, especially trees. Winter rains are beneficial to wildflowers, but trees, being dormant in the winter, need summer rains to grow. Cacti bloom without much winter moisture, since they store water in their pads or stems.

Winter storms come in from the Pacific Ocean and generally last from November - March. Usually gentle, they may cover the entire state and last for several days.Snow is possible, but heavy accumulations are rare. Temperatures that remain below freezing for any length of time will kill many of the desert plants. Summer storms come up from the Gulf of California and generally last from July - September. They usually occur in the afternoon, are brief, widely scattered, and often extremely heavy with lots of lightning. There is a danger of flash flooding because the top layers of soil become saturated.

Water that collects in low-lying areas quickly becomes alkaline. Thick layers of calcium carbonate form as the lime-enriched groundwater evaporates. These deposits, known as caliche, can grow to be many feet thick and may eventually form an impassable barrier. This increases the likelihood of flash flooding, since rainwater is unable to soak into the ground. Many plants are not able to survive, since they cannot put down deep root systems.

There is an amazing diversity of life in the desert. There are snakes and cactus, but there are also frogs and wildflowers. All desert life has been forced to adapt to a frequent lack of water, as well as extreme temperature fluctuations. Many mammals spend the day underground, venturing out to search for food only at night. Others sleep through the hot, dry summer months. Large ears may help regulate body temperature. Light colored fur and feathers reflect heat and help the animal blend into the background. Some animals are able to survive on just the moisture they get from their food. Birds are able to cope with high temperatures since their feathers provide excellent insulation, and they can fly long distances to reach water. Reptiles cannot control their body temperature, and will die if not able to find shelter during the heat of the day.

Widely spaced plants reduce competition for moisture. Many desert plants store water in pulpy tissues or shed their leaves during the driest parts of the year. Others remain as seeds, possibly for years, waiting for the right conditions to allow them to germinate and grow. Hairy stems or thorns provide shade and break up air currents. Thick skins or waxy coatings reflect heat and reduce evaporation. Leaves close their pores during the day, or curl to avoid direct exposure to the sun. Root systems are either very shallow to catch light rains or very deep to reach the groundwater. The root system may actually weigh more than the aboveground portion of the plant.

Contrary to its rugged appearance, the desert is actually extremely fragile. Sprawling communities gnaw away at its fringes. Roads and utility lines cut through miles of prime habitat. Air and water pollution may be harming many species. Increased demands on scarce water supplies have lead to a drastic drop in the water table. Desert life has been able to cope with many changes, but it cannot adapt to the expansion of modern civilization. Only through an increased understanding of desert ecology will we be able to maintain this fragile environment for generations to come.

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