Impacts of Urban Areas on The Desert Southwest 
 Phoenix, Arizona

The Growth of Cities in the Southwest

Approximately 76% of the U.S. population lives in cities (Parlange 1998). The urban areas of the Southwest are growing at an enormous rate. The cities of Los Angeles (pop. 2.5 mil), Houston (1.7 mil), San Diego (1.1 mil), Dallas (1 mil), Phoenix (1 mil), San Antonio (1 mil), and San Jose (.8 mil) are among the 12 largest cities in the world (US Census Bureau 1992). Although, not all these cities are distinctly classified as being in the desert, they all have immense effects on the larger ecosystem of the Southwest.
 

The Negative Effects of Cities

Cities have several large effects on the desert ecosystem. First, they require enormous amounts of water which is piped in from rivers often hundreds of miles away. This can result in river beds drying up and the death of riparian zones. An example is the city of Phoenix, which in 1998, used 97 billion gallons of water. However city reservoirs only have a combined storage capacity of 450 million gallons and therefore the city is dependent on other water sources (Water Facts 1995). Dams have been built to divert the flow of both the Gila River, south of Phoenix and the Salt River, north of Phoenix. Over time, both rivers have become completely dry. The Los Angeles River, for similar reasons, is so dry that housing developers want to build within the river bed.  The largest river in the Southwest, the Colorado, has been diverted so much in order to send water to the dishwashers, showers and lawns of urban and suburban areas, that it no longer reaches the sea. Each day, 20 million people drink Colorado River water. Although not within the USA, the most drastic example of what can happen to natural water systems in the face of human expansionism is the drying up of the Aral Sea (pictured on left).  The Aral Sea was the fourth largest freshwater lake in the world, larger than Lake Huron and Lake Michigan.  However, as the input streams were increasingly diverted, the "Sea" began to shrink. Now there is only a little water left and the majority has become nothing but a dust bowl littered with the skeletons of decaying fishing boats (Luoma 1996).

In addition to water, cities require food. As a result agricultural lands spring up around cities. These lands require enormous amounts of water and completely alter the natural habitat. With agricultural industries comes the meat industry.  Cattle grazing in the Southwest has numerous negative effects for the desert, especially on riparian zones.

In addition to pulling in enormous quantities of water and food, cities give off many things. The cities use more oxygen than they produce. Therefore, CO2 is a major export of cities. Pollution can also be a problem. Finally, sewage and refuse are the main organic exports of cities.

The urban landscape itself is significantly altered from the natural landscape on which it sprung. Green lawns and concrete shopping areas spring up where before Creosote Bushes, Gila woodpeckers and Saguaro Cacti had flourished.  This is important when one realizes that the previously mentioned large cities cover nearly 2500 square miles of land (2.5 times the size of Yosemite NationalPark, not including the vast outlying suburban areas.) (US Census Bureau 1992). 

Cities have been so altered from their original state that they sometimes can have different daily temperatures than surrounding areas. Often, concrete city centers, via a "heat-island effect," become warmer than other areas because concrete absorbs heat. Vice versa is the "cool island effect" which occurs when sun radiation hits irrigated areas. Part of the heat energy evaporates water and is dissipated into the atmosphere. This occurs in many golf courses, cemeteries and parks within cities. In the city of Palm Springs, 75 golf courses have been created in the last 15 years. As a result, some climatologists see evidence that the city is becoming cooler than outlying areas. Microclimate studies in Phoenix have also verified the "cool island effect."  Temperature, on a calm day, has been shown to be 8 or 9 degrees cooler over a golf course than surrounding non-irrigated areas (Golf Course 1988).  The elderly golfer above left takes advantage of the "cool island effect" at a golf course in Sun City West, a retirement community in California. 

The flora has been altered as exotic trees, shrubs and flowers have been added to gardens, lawns and parks. Landscaping with these plants, in a desert environment that recieves less than 10 inches of rainfall a year simply does not make sense.  The fauna of the city environment has similarly been altered. No longer do Gila monsters roam freely where city asphalt stretches.  Despite all this, there are some signs that cities may have the potential to be a home for a wide diversity of native species.
 

Cities as a Place for Conservation

A study in the suburbs of Tempe, Arizona looked at the differences in bird diversity and abundance between a suburban transect and a nearby native riparian habitats which was dominated by cottonwood-willow and mesquite trees. Surprisingly, the suburban transects supported significantly higher bird densities in every month of 20 months. However, four non-riparian and non-native species (House Sparrow, Rock Dove, Inca Dove, and E. Starling) made up 52- 72% of the bird density each month. There were 10 native species that occurred at higher densities within the suburban area in at least one season. Overall, 53% of all riparian species and 85% of winter residents occurred in suburban Tempe. In Tucson, Arizona, a 26-fold increase in summer density from desert to urban habitats was noted. These results indicate that there may be some potential for developing suburbs and cities in such a way, that the desert ecosystem is not completely excluded. The cities and suburbs offer an abundance of food, water, shady areas, and micro habitats. In some ways, they can be viewed as desert oasis (Rosenberg 1987).

Xeriscaping

 

It has been estimated that half the water used by homeowners goes into landscape maintenance (watering the lawn). In order to save water and provide habitat and food for native birds, reptiles and mammals, the typical lawn should be replaced with more natural species. Xeriscaping is a term for landscaping gardens with desert plants. The designs can be incredibly beautiful and unique. In addition they save money by lowering water bills and require little effort to maintain (Water Conservation 1998).
 

Saguaro Cacti are becoming
more common in gardens and
public areas.
 

Native Species


 

In well irrigated urban areas, native riparian species such as cottonwood-willow and mesquite trees can be planted. These trees, and others, have been shown to harbor greater diversities of birds and other organisms than exotic and introduced plants.
 
 
 
 

Mesquite Trees, such as this one,
provide habitat for many native species.
 

Getting Rid of Invasive Species

Species such as cowbirds and other nest parasitizers need to be reduced in numbers if urban areas are to truly become a more hospitable home for native species. In addition, invasive shrubs and trees such as tamarisk must be eliminated.
 

Drip Irrigation

Drip irrigation in conjunction with xeriscaping has the potential to drastically lower the amount of water used in urban areas. Drip irrigation is the implementation of underground watering pipes that slowly saturates soils with small amounts of water. This greatly reduces the amount of water that evaporates into the atmosphere.
 

Restoration Projects

The Rio Salado (Salt River) Project is a plan that could restore the Salt River, north of Phoenix, to its natural state. The plan involves reintroducing a small, flowing stream and desert habitat to the currently dry riverbed. This project could renew a vital riparian corridor through the heart of one of the largest cities in the Southwest. Projects like this are crucial if we are to hope to incorporate cities with wildlife and "natural" things. Restoration projects have the ability to bring nature back to the areas from which it was formerly evicted (Rio Salado 1995).
 
 

A Paradigm Shift in Our View of the City

The city in the Southwest needs to be seen as a part of the desert, not as a separate region. Cities have enormous potential, especially in the Southwest, to provide an important habitat for many disappearing and endangered desert species. One positive sign of this change in how we view cities is the demarcation of the city of Phoenix as a Long Term Ecological Research Site. Traditionally, LTER sites have been situated in areas devoid of human influence (Parlange 1998). Viewing cities, and humans, as a part of the desert community will force us to be more responsible in how we design our cities. Then, maybe we will be able to conserve the wonderful diversity of the desert Southwest.

 

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Authored by Kevin Cox
May 2, 1999