Rainfall in Organ Pipe Cactus National Monument

by Corey Reinig

Rain in the Sonoran Desert is defined by two wet seasons – one in the winter and one in the summer.  The desert has an average rainfall of between three and fifteen inches per year, depending on location.  Organ Pipe records around nine inches of rainfall. The two factors that cause local variations in rainfall are proximity to mountains and elevation.  About half of this rain falls in the winter wet season, while the other half is received in the summer wet season.  The winter wet season starts in early December and usually is over in late January.  The summer wet season is slightly longer, as it starts in July and lasts through early September.  Consequently, the wettest months are August, September, and December, while May and June are the driest.^[1]  Although it is predictable that most of the year’s rain will come during these wet seasons, rainfall during these seasons are quite unpredictable.  Most of the yearly rainfall usually comes in the form of couple of large storms.  For example, the town of Yuma, Arizona, one of the driest cities in the Sonoran Desert, can get a violent storm that can produce four inches of rain.  Of course, the opposite may happen, there are years which the wet season produces very little rain.^[2]

The desert avoids much rainfall largely in part to the Cascades and the Sierra Nevada Mountain ranges.  Rain clouds, that form over the Pacific and move east, cannot not rise over these mountains without dumping their precipitation first.  As the clouds move over the mountain ranges they rise and therefore cool.  This cooler air cannot hold as much water vapor as warmer air and therefore has to drop its precipitation.  By the time these storm clouds make it to the desert, they usually have little rain to offer.  This process is called the rain shadow effect.  The summer monsoon system brings water caused by storms that have their origins in the Pacific.  Air in the desert, which is warmed by the hot summer sun, rises and creates a vacuum, and air moving in the Pacific fills this vacuum. In these months not even the mountains can prevent all the storms from reaching the desert.  These storms also start further south and circle north to the Sonoran Desert.^[3]  The summer rains are often violent as they drop a lot of rain in a short period of time.  For example, Lukeville has recorded a two inches of rain falling in an hour.[4]  These storms can often create flash floods.  The desert soil does have the ability to absorb a substantial amount of water; however, the rain drops much too quickly.  Rain water runs quickly off the hot floor as it rushes towards streams and rivers.  As it travels it picks up sand, rocks, and plants.  The water erodes streams and makes channels.  The quick moving water can reach riverbeds in a manner of minutes.^[5]

The winter rains are caused by low pressure systems, which bring storms from the Pacific down to the desert region instead of bringing them to the northern part of the United States.  These winter storms, although also erratic, are much easier for forecasters to predict due to the presence of a low pressure system.  These rains are also longer and gentler, as they may last for a couple of days.  These soaking rains are not nearly as capable of producing large violent storms that their summer counterparts offer.^[6]

The dry, hot Sonoran desert can also create a phenomenon which is unique to desert storms, called “frustrated thunderstorms”.  These storms drop rain, but the precipitation is absorbed before hitting the ground.  An observer on the ground would not get wet from these storms but would notice the vigra, which are the trailing vapors steams of rain.^[7]

The heaviest and the fiercest storms which the Sonoran Desert experiences are from tropical storms that start from the Pacific, these storms cause severe desert floods.  When these tropical storms move east over land they lose momentum, but they can increase their power from the warm waters of the Gulf of California.  Even if these storms never make it to land, they can still produce rain in the desert.  These storms are rare as they only occur a couple of times every decade.  However, these floods are often very memorable.^[8]  For example, tropical storm Octave which hit the Pacific coast in 1983 produced flood waters that destroyed 1300 homes and displaced 10,000 people.  Communication and travel was also interrupted as power lines and roads were washed away, including a section of Interstate 10 between the cities of Phoenix and Tucson, Arizona.^[9]

The occurrence of El Nino affects the amount of precipitation that the Sonoran desert receives.  In years that El Nino is in effect, the chance of tropical storms increases. El Nino occurs when trade winds in the Pacific weaken, causing warm water to pool.  As this warm water moves eastward, it affects atmospheric pressure, which causes trade winds to weaken more, which causes more warm water to pool.  As a result of El Nino, tropical storms get their start further east and have a greater chance to create tropical storms which will make it to the American Coast.  Most desert floods have happened in years which El Nino occurs.  The opposite can also occur, La Nina, when the Pacific has unusually cold waters which results in exceptionally dry years over the Sonoran.^[10]

Rainfall in the Organ Pipe helps to provide essential surface water.  The most important surface water in the monument exists in the form of tinajas, or rock tanks, which are depressions in solid bedrock caused by flowing water or wind.  They are important because they are the most frequent form of surface water and they are fairly evenly distributed throughout the park.[11]  In all there are sixty, but only one of them is perennial, meaning it never goes dry.  There are several ways in which tinajas are formed; the most common in the park are plunge-pool tinajas, which are created by waterfalls carving into the bedrock.[12]  All tinajas rely on rainfall to supply the water.  At the end of each rainy season, these tinajas can fill to be several feet deep. During the course of the dry seasons they lose most of their water through evaporation.  Spring Arroyo Tinajas, the only perennial tinjas, supplies water to a small local herd of white-tailed deer.  Throughout the park, tinajas provide water to wildlife. People have also used them as a water source in the desert for thousands of years.  Today, back country users still take advantage of this water source.[13]

Natural springs are another form of surface water that can be found in the park.   There are eleven springs found in the park, most of which are found in the mountains.  Quitobaquito Springs is the biggest and the most well-known of these springs.  This natural spring seeps thirty to thirty-five gallons of water per minute into the 50,000 square foot spring.  This spring has been used by native people for thousands of years, but was first recorded by a western explorer, Eusebio Kino, in 1698.  In the 1860’s, this water source was important for the travelers of the Camino del Diablo which traveled along the U.S-Mexico border.  Today the spring has been modified by running water along a ditch to fill the Quitobaquito pond, which is an important habitat for the endangered desert pupfish.[14]

Extreme rainfall and the lack thereof contribute to the uniqueness of Organ Pipe.  Plants and animals needed to evolve to survive in such dry environment by conserving water and by taking full advantage of rainfall during the desert’s wet seasons.  Likewise, humans who have lived in this environment have developed strategies to cope with the infrequent and unpredictable rainfall.  Natural surface water, such as springs and tinajas, has helped animals and humans to survive in such an arid environment.

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