Patterns of Life

What do these different types of storms mean for life in the desert? How does wildlife cope with shifts from drought to flood within hours and then perhaps back to no rain again for months? How plants, animals, and people in the Sonoran Desert respond to the rainfall's variability is a particularly critical aspect of survival in the desert.

As we've seen, rain falls in the northeastern Sonoran Desert generally in a "bimodal pattern"—that is, twice a year: thunderstorms in the summer, and larger, gentler storms in the winter. These rainy times are not predictable, and a season's worth of rain can fall in a day or drizzle in over a month, or rain may not come at all. Given these conditions, survival in the desert has required adaptations—even, as we shall see, on the part of the modern urban dweller.

There are perhaps three things that characterize a good desert denizen: knowing how to wait, knowing how to hold onto what rain does fall, and knowing how to get down to business when opportunities arise. Consider the patience, resourcefulness, and speedy sex of spadefoot toads. Cued by the vibration of rain or thunder, spadefoot toads emerge from interments of ten to eleven months. Taking advantage of the temporary ponds from the rains, spadefoots pursue breeding with all the intensity of creatures denied for most of the year. After months of waiting for opportunity to knock, they begin the next generation of spadefoots within a day's time. "Patience" also characterizes the Gila monster, which does absolutely nothing for nearly nine months of a year. Mornings in April, May, and June will find the large lizard seeking bird eggs and baby quail, but most of the time it waits out the hot, dry days, living off the fat stored in its expandable plump tail.

The variability of rainfall is reflected in reproductive cycles. Many desert plants and animals do not automatically attempt to reproduce every year, but wait until sufficient rain has fallen to make the investment of energy worthwhile. Spadefoots will not emerge without a heavy enough rain to fill the temporary ponds, giving them time to mate and the tadpoles time to develop. Gambel's Quail and Rufous-winged Sparrows will not nest unless sufficient rain has fallen to support insect life and fruit development that will in turn supply baby birds with food. Brittlebush plants (Encelia farinosa) bloom generously most years, but hold back their yellow flowers in times of drought.

Many desert plants exemplify the ability to hold on to what one has. Barrel cactus (Cylindrocactus and Ferocactus spp.), saguaro cactus (Carnegiea gigantea), and succulents like agaves (Agave spp.) are well known for their ability to store gallons of life-sustaining moisture within. Saguaros grow visibly plump after a wet monsoon season. The extensive, shallow roots of these plants help them capture much of the ephemeral moisture before it evaporates. The intense competition for water when it is available is one reason desert plants are so well dispersed over the landscape. Other plants, such as mesquite (Prosopis spp.), push their roots many, many feet deep to tap into underground moisture.

Lifegiving Desert Oases

Riparian areas are the precious gems of the desert. These year-round streams or springs are the incubators of desert life. More than eighty-five percent of desert animals depend upon riparian areas for some phase of their life cycle.

Sonoran Desert riparian areas are sustained by the rain that falls in the mountains and foothills and migrates into the region's alluvial valleys and aquifers. As overpumping of groundwater occurs, the hydrologically-connected riparian areas are degraded and eventually lost. Cottonwood-willow forests that once lined many rivers in the Sonoran Desert, including the Salt, Gila, Santa Cruz, and Rillito rivers, have disappeared as groundwater pumping and surface water diversions disrupted flows and lowered the water table below the plants' root zone.

It is estimated that more than ninety percent of Arizona's riparian areas have been lost in the past century, and many of the remaining areas are imperiled by surrounding urban, agricultural and mining development. The spectacular San Pedro riparian corridor in southeastern Arizona, for example, home to an estimated 400 species of birds, 83 mammal species, and 47 reptile and amphibian species, faces a bleak future because of groundwater pumping in nearby Sierra Vista, and Fort Huachuca, and by burgeoning housing developments near the river.

Efforts to protect the San Pedro and the other few remaining riparian treasures will require concerted and dedicated action on a number of fronts to prevent their demise.  

People and Rain

Desert survival has required that human life also adapt to scarce and variable rainfall. Traditional Tohono O'odham farmers plant tepary beans, squash, corn, melons, and other crops bi-annually, in order to take advantage of the rhythm of summer and winter rains. Their fields are designed to catch water washing across the land after storms, often channeling it to areas that have been prepared for planting. A single summer or winter rain can make or break a harvest, and some years, the fields are not planted at all. The Tohono O'odham also scatter their desert plots widely among several washes, in order to maximize the chances that even scattered thunderstorms will soak at least one field.

Perhaps as a reflection of their sensitivity to the vagaries of Sonoran Desert rainfall, the Tohono O'odham seem to dislike jumping to any conclusions about the weather. Linguist William Pilcher noted that the Tohono O'odham avoid any assumption that rain will fall for sure: ". . . it is my impression that (they) abhor the idea of making definite statements. I am still in doubt as to how close a rain storm must be before one may properly say t'o tju (it is going to rain on us), rather than tki' o tju:ks (it looks like it may be going to rain on us)." Life-giving rain, upon which Tohono O'odham have traditionally been utterly dependent, is not taken for granted, and when it falls, is considered good fortune.

Many modern farmers of large-scale commercial crops, in comparison, have so divorced themselves from the natural rhythm of the desert that they actually dislike rain. Cotton, for example, is grown solely on irrigation water. To increase their yields, some farmers "stress"their plants slightly by withholding water, and the plants put out bigger blooms as a result. Any natural rainfall during this period is seen, therefore, as interfering with the growers' management plans.

Contemporary city dwellers, likewise, are mostly buffered from the vagaries of desert precipitation by modern technologies such as irrigation, cooling systems, and bridges. Pumped water, swimming pools, lawns, and air conditioning allow city people to live comfortably through the desert's hottest times. Is modern urban society therefore immune to the unpredictability of desert rain?

The changeable nature of desert rainfall has required adaptation, even on the part of the city dweller. This is particularly evident with regard to the paradoxical hazard of arid lands—flooding. The natural desert tendency to flood is exacerbated in cities. Over paved surfaces like roads, rain water will move more than eight times as fast as it could in a wooded area. And in urban areas, the proportion of runoff, that is, the water which flows over the surface rather than sinking into it, is about four times that of undeveloped desert. So, not only is more runoff moving much faster in a city, it has less opportunity to soak into the ground. This problem is worsened as more earth disappears under asphalt and pavement, and as more people live closer to flood plains and arroyos.

While Tohono O'odham farmers and desert spadefoots welcome floodwaters, a typical urban response has been irritation and, especially in the face of violent storms, fear for human life and safety. For decades, as cities in the region grew at tremendous speed, the urban response to the threat of flooding was to pour concrete. Natural drainages were widened, straightened, channeled, and lined in concrete. Water that fell over a large urban area quickly flowed into concrete ditches and rushed away. Millions of dollars went toward engineering projects including huge storm drains to accommodate the hundred-year floods that inexplicably seemed to occur every seven years.

Yet, in spite of the investments in flood control, when the big, bad storms hit, such as that causing the flood of 1983, human engineering has repeatedly met its match. In fact, the "structural" approach often serves to worsen flooding and other problems. Concrete ditches move floodwaters away fast—so fast that unlined channels downstream suffer worse erosion as they are hit with more and faster-moving water. The water quality of this runoff also presents a problem, since rain collected from streets, parking lots, and buildings carries sediments, pollutants from cars, and nutrients from fertilizers used in landscaping. Additionally, a cement-lined wash cannot serve as a recharge route to the underground aquifer, an important natural function of desert washes. And a wide cement ditch detracts from the aesthetic appeal of a neighborhood.  

The Value of an Urban Wash

Over the past several decades, city dwellers have gradually realized that engineering cannot completely remove their vulnerability to the threat of floods. In addition, some people have also come to view runoff from desert rains not as a nuisance to be guided away, but rather as a resource that can support desert wildlife and recharge underground aquifers. Engineering projects are still a major aspect of flood control plans, but Tucson, Phoenix, Scottsdale, and other cities, have begun to look to the desert's natural drainage system as part of the solution.

A newcomer may not even notice the network of arroyos around a city, or think much of the dry beds natives call "rivers."But for desert city dwellers the arroyos are assets. Contemporary city planning has prohibited construction in flood plains and encouraged the development of extensive river park systems. These parks provide attractive recreation areas for city dwellers, and also buffers during a flood. Small, unlined washes through urban neighborhoods offer pathways and cool, green spots sheltering native plants and urban wildlife such as quail, roadrunners, javelina and coyotes. The unlined urban washes also allow rainwater to soak into and recharge underground aquifers.

City people in the desert may never respond to the arrival of the first summer monsoon with the enthusiasm of the spadefoot toad, but there is a sense of joy as the first drops fall. The changes in city flood-control decisions indicate an ethic of urban desert living that welcomes desert wildlife along arroyos, acknowledges vulnerability to desert storms, and seeks answers, at least in part, in the desert's natural flow. These plans are expensive, yet they enjoy real political and economic support. So when the rains come and the washes begin to run, perhaps more people are pausing to watch the arroyo vegetation turn green, breathe in the smell of wet earth, and wonder at the marvelous event of a desert storm.  

Desert Storms: Additional Readings

Carr, Jerry E. National Water Summary 1988-89—Floods and Droughts: Hydrology, Water-Supply Paper 2375. Washington, D.C.: U.S. Geological Survey, 1991.

Olin, George. House in the Sun: A Natural History of the Sonoran Desert. Tucson: Southwest Parks and Monuments Association, 1994.

Sellers, William D., Richard H. Hill and Margaret Sanderson-Rae. Arizona Climate: The First Hundred Years. Tucson: University of Arizona, 1985.

Smallwood, J. B., Jr., ed. Water in the West. Manhattan, KS: Sunflower University Press, 1983..                                                                            
  

The website http://geochange.er.usgs.gov/sw/changes/natural/ has information on a number of topics related to climate change and land use in the Southwest