Water in the Desert

Duration:

16.033 seconds

Alamo Wash is one of many seasonally flooded streambeds at Organ Pipe Cactus National Monument. During the dry seasons, this empty arroyo provides shelter and a cooling network for the plants and animals that live in and around it. NPS Video/Ami Pate

 

The Sonoran Desert is sometimes nicknamed the "Green Desert". This label comes from the lush, verdant spring of vegetation during the seasonal rains. In the dry seasons, you may have to look a little more intently, but water’s work on the environment is evident everywhere. From the deepest arroyos carved away by seasonal floods to the shallow, basin-like tinajas that may hold water on the driest of days, water shapes the landscape. Animals and plants here are well adapted to survive in dry conditions, but rain is a welcome and necessary event in order to truly thrive. Learn more about water’s effects in the desert below or read up of the 2012 floods at the monument.

 

 

NPS photo

Waiting in Anticipation for Rain

The Sonoran Desert is hot in the summer, with scorching temperatures reaching up and over 110°F (43 °C). During Dry Summer (May and June), the ground bakes, many plants wither and fall dormant, and animals retreat to shelter during peak daylight hours. For time immemorial, the Tohono O’odham people dipped into food stocks built up from winter to compensate for scarcity of fresh food. Every organism in the desert waits for the life-giving monsoon rains that are expected to fall during Wet Summer (July through August).

The shift in seasons begins as a shift in the wind. The dry wind that blows from the north gives way to winds that blow moisture in from the Gulf of Mexico. In the southern distance, clouds that sparsely dot the sky transform into walls of thunderheads. Closer and closer the storm cells creep, building up south of Sonoyta, Mexico, and the Tohono O’odham Nation to the east. With a crack of thunder, the clouds burst. The monsoon is here.

 

NPS photo

From the Highest Peak to the Deepest Arroyo

Large storms can move mountains, but even the effects of a small amount of rain can be felt throughout the desert network.

Down the Mountain

Water flows in the direction of least resistance and can wear away at stone surfaces. When a storm happens over the Ajo mountains, the water runs down the mountainsides, collecting together, gaining speed, and picking up loose gravel and stones. As this torrent continues its journey down, canyons and slides are carved in the water’s path, and small pools called tinajas will form, where some water comes to rest.

The water that does not stop at a tinaja will continue down the mountainside, gathering momentum and loosening boulders. Towards the base of the mountain, and the beginning of a wash, you may notice a collection of large boulders resting along a slope. This area is where the water lost some of its momentum and deposited heavier boulders as it began to spread out from the canyon to fill the wash.

Down the Wash

Although the water that reaches the wash has lost some momentum, it can still easily fill and overflow a small streambed. As water works its way to lower elevation, plants and animals in and near the wash become subject to the raw power of the torrent. During this time, plants may be uprooted, and sheltering caves may be inundated. Debris is carried swiftly down stream and may create damming effects if it encounters a barrier. The water will roar forward until it dissipates.

 

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With Water Comes Life

As the rain and floods ebb away, the evaporating water cools the landscape by up to 50°F (10°C) since before the storm. Animals quickly come out of their shelter and seek out what water has puddled on the landscape. During the calm after a storm, the desert air is thick with the earthy, charred scent of creosote. Mirror-like pools of water, over ankle-deep may form on playas, and toads and insects demand attention through their serenades. The saguaro fruit ripens, and the Tohono O'odham begin their new year.

The water slowly softens the hard desert crust, or "caliche", seeping into the ground to be used by plants. Within one day, dull creosote bushes have brightened, the barren ocotillo begin to leaf out, and the yellowed grass greens with new growth. Perhaps some hardy wildflower species will send up shoots and buds, attracting butterflies and other pollinators. The Green Desert has sprung to life.

 

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Two Seasons of Rain

The monsoon can be unpredictable, whereas winter rains are considered more dependable, like clockwork. The hot, pounding, explosive storms of the monsoon contrast against the cool, calm, steady rains of the winter. Both transform the desert into a lush landscape full of plant and animal activity.

Seasons Change Again

As the monsoon ends, winds shift again to blow in cool air from the Pacific. The mountains to the west catch most of the moisture, and the winds that reach the Sonoran Desert are mostly dry. Warm and dry fall settles in and life slows back down, but thanks to cooler temperatures, there is still activity abound. Soon the winds shift again slightly, blowing wet weather in from the north Pacific, down through the Great Basin in the north, and down through the Sonoran Desert.

A Different Kind of Rain

Winter rains are truly soaking, with some showers lasting throughout the day, softening the soil and allowing plant seeds to take hold. During this time, the desert cools to a balmy 40-60 °F (4.6-15.5°C), and water sources are reliable throughout the environment. Tinajas are regularly filled, and washes distribute water throughout the landscape. Many mammals and birds enjoy the abundance of food and water, while reptiles take shelter, waiting for the warmer weather of spring and summer.