With less than a foot of rain annually and summer temperatures routinely exceeding 104 degrees, you might wonder how any plant or animal gets enough moisture to survive. And yet any visitor to the McDowell Sonoran Preserve can readily find evidence of a thriving ecosystem (over 2,000 flowering plants alone). So how does the Preserve get enough water to support it? Here’s a quick look at this amazing phenomena.
The Wet Facts
The McDowell Sonoran Preserve gets water from two naturally-occurring sources—rainfall and groundwater. No human-directed irrigation is used.
Each year, nearby Scottsdale receives approximately 11 inches of rain. The majority of that rainfall comes during two rainy seasons.
The summer monsoons can produce violent thunderstorms in July and August. Take a look at this video of one storm. Much of the rain from these large bursts runs off the soil or evaporates before it can soak into the ground. That can leave less water for plants and animals.
In December/January, the winter rains are less intense but can occur more frequently. They are usually the result of a low-pressure trough pushing the prevailing Pacific storm tracks over the Sonoran Desert.
To compound the scarcity of water, Arizona has experienced a drought for over 20 years now. While winter precipitation in early 2020 has led to some marginal improvements, long-term climate models indicate warmer and dryer than average conditions will persist.
The Great Adaptation
So how do Sonoran plants and animals adapt to less water? Look to their evolutionary structure to find the answer.
Desert cacti have an extensive system of shallow, wide-spreading roots so they can quickly soak up rain. In contrast, the desert Ironwood tree has deep roots that can tap down far enough to reach groundwater. Still others, like wildflowers, survive as seeds in the ground. They sprout only when there’s enough water to grow.
The leaves of plants can also play a part. Some have small “pores” in their leaves so moisture doesn’t evaporate as quickly. Succulent plants can absorb and store water in their leaves for long periods of time. Others, like the saguaro cactus, have folds that can expand to take in water, then close to store it.
Animals use other strategies to survive with less water. Some species nibble on juicy cacti to get their water. Others, like the kangaroo rat, have special digestive systems that get enough water from the seeds they eat. Larger animals, like coyotes, stay within a day’s traveling distance from water.
The Couch’s Spadefoot amphibian hides under the desert surface waiting for rain. The falling raindrops wake the spadefoot who then surfaces. Within 10 days, it must find a rainpool, mate and hatch tadpoles. The young ones then dig down and wait for the next rain to start the process over again.
Even human animals learn to adapt. Native American tribes living in the desert have adapted traditional growing methods for crops. In addition, they rely on desert plants for food and medicine.
Nature has an amazing way of creating complex ecosystems, even when water is scarce. Take one of the guided tours of the Preserve and learn more about the Sonoran Desert.