Location, location, location, says the Realtor and the Business Marketing Advisor. One is unlikely to encounter deserts in Delaware. Or France or Iowa or Malaysia. There is, Good Reader, a recipe for making them...
The first ingredient is location...
 |
| Desert landscapes are not always sandy. The Chihuahuan Desert is dotted with pockets of grassy shrublands from southeastern Arizona (shown here) to the western Concho Valley of southwest Texas |
Deserts, at least those we call Hot Deserts, generally form between 20 degrees and 30 degrees latitude. Atmospheric circulation patterns such as the Hadley Cell play key roles in creating conditions needed to bake land dry. Heated by the sun at the equator, air rises and expands and cools. Rain falls and lush tropical rainforests are created...
The air continues to rise, eventually coming down in the subtropics before it returns to the equator. As it descends, physics dictates air becomes compressed and warm. Its relative humidity plummets. Deserts form below these descending and dry air masses...
 |
| Chihuahuan Desert: Chisos Mountains in the Big Bend of Texas |
A person would (reasonably) expect all land under them should be desert country. But, obviously, this is not the case. We ask ourselves: why, at the same latitude, is wet and rainy Louisiana not an arid wasteland like West Texas? The answer: location, location, location...
The tendency to dryness is interrupted in certain places, especially on the eastern side of continents. Winds loaded and heavy with moisture from the ocean blows onshore. Rain falls. As the distance from the coast increases, the amount of moisture available to generate precipitation decreases. Mountain ranges create "rain shadows" blocking westward movement of clouds that already have very little moisture in them. In the United States, the hottest and driest desert country lies between the Rocky Mountains and the coastal ranges near the Pacific...
A tendency to dryness thanks to unpredictable rainfall is a second and, perhaps, most important ingredient in our recipe... The basic nature of desert precipitation is to be low, erratic, and often violent. It is also generally ineffective since the potential for water loss through evapotranspiration (pET) significantly exceeds actual rain totals in deserts. (Readers may recall, from a previous essay in this series, that studies by H C Trumble show an area is "water-deficient," i.e, a desert, when potential for water loss is three times or greater than precipitation) In a statistically average year, Tom Green County in Southwest Texas receives 18.2 inches of precipitation with a pET of nearly 72 inches. Few are statistically average years in deserts: a worldwide correlation exists between total rainfall and its predictability. The lower the annual rainfall for a location, the higher the year-to-year variation in amounts that actually occur...
 |
| Sonoran Desert: The giant saguaro cactus symbolizes the Arizona drylands |
An extremely arid place like Yuma, Arizona, might average 3.39 inches of precipitation yearly. The actual totals for a given climate cycle there, however, could be as low as 0.28 inches and as high as 11.42 inches...
Back in Texas, San Angelo fares better than Yuma. National Weather Service numbers give it annual average rains of 21.25 inches over the past three and a half decades. Nevertheless, precipitation is unpredictable: actual totals of under 10 inches and more than 30 inches have been recorded. Yearly rains of 20.25 inches to 22.25 inches, one inch plus or minus the average figure, fell only four times in the last thirty-five years...
The range of actual yearly rainfall totals in semi-arid deserts are not always as variable as those in very, very dry places. West Texas is dry largely due to distance from the warm waters of the Gulf of Mexico. Seasonal storms provide much of the annual rain in this dry country. Cold fronts and stationary hot air masses collide, triggering violent and sudden storms in spring and autumn...
But even these seasonal rains cannot be predicted accurately. For San Angelo, we see data showing September is a "wet" month that averages 2.95 inches of rain. True totals range from less than 0.01 inches to 11.0 inches during the ninth month of the year...
 |
| Mojave Desert: North America's smallest desert is known for Joshua Tree yuccas and for being one of the hottest and driest places on the continent |
North American deserts have "rainy seasons" during which most of a year's precipitation usually occurs. The Mojave and Great Basin see a "winter pattern" when Pacific Ocean storms move eastward and inland. The resulting precipitation is low intensity, may last for several days, and often cover large areas. From spring through autumn, storm cells from the Gulf of Mexico track to west and northwest to create a "summer pattern" in the Chihuahuan. These rains are likely to be convective thunderstorms-- intense, localized, and brief. A mixed pattern of summer and winter rain characterizes the Sonoran since it is centered between Pacific and Gulf storm systems...
Relative humidity tends to be below 50% in the North American deserts. This adds one more ingredient in our recipe for making a desert. Since water in air molecules acts to trap infrared radiation from both sun and earth, low relative humidity guarantees a large percentage of possible solar radiation reaches the ground during daylight hours and that much of the accumulated surface heat radiates back into the atmosphere after dark. Low relative humidity also aggravates aridity by limiting cloud formation, increasing the amount of sunlight that reaches the ground...
 |
| Desert Grasslands along US Highway 87 north of San Angelo, Tx |
Depending on relative humidity levels, maximum and minimum temperatures can vary from twenty-five degrees or less to fifty degrees or more during a twenty-four hour period. (This variation does not always affect desert cities due to the urban heat island effect.) Low relative humidity also translates into extremes between annual maximum high and low temperatures. Annual mean temperatures in deserts range from the mid-sixties to the mid-seventies degrees Fahrenheit...
Temperature, in conjunction with air movement and precipitation, plays a major role in determining evapotranspiration rates. High temperatures accelerate the process and low temperatures retard it. Summer heat, sustained for months at a time, rapidly bakes already dry land. We have yet another recipe ingredient...
 |
| Great Basin Desert: Mile after mile of sagebrush dots the dry country of Nevada and Utah |
Lack of plant life results in soils low in humus, those organic components formed from decomposition of plant or animal matter. Desert soils are, therefore, often light colored at the surface and low in organic matter. They have developed in areas where limited precipitation is insufficient to leach salts and other chemicals from the soil. These various minerals accumulate in the subsoil to form the hard layers often referred to as caliche in the southwest...
 |
| Arches National Park near Moab, Utah |
This combination of poor soil and bare ground can be added as two more ingredients for our recipe. Perhaps we can best describe them as flavoring: not all desert locales are characterized by aridisols and mile after mile without discernible vegetation...
Arid region soils are described by those who study their ability to sustain vegetation as immature, weakly developed in terms of soil profile, and alkaline. Desert soils do vary: they include sands, sandy or gravelly loams, shallow stony soils, alluvium, and scree-derived deposits...
Wind, too, is a major ingredient in creating a desert. Those of us who live in the dry country think of it as a constant... rarely stopping for more than a few minutes, always drying the land, eroding soil and rock to create ravines and mesas and fantastically shaped stone formations...
It may be that one or two readers recall my comments about the Irrigation Technology Center associated with Texas A&M University in an earlier installment in this series. A few days ago, I revisited the Center's website to pull up its data for San Angelo. The non-archived information covers the period from May 1, 1998 to June 26, 2012, some 3980 days...
 |
| Bison herd at San Angelo State Park in Texas |
During these many years, 183.55 inches of rain fell on the Concho Valley less than ten miles north of San Angelo. Potential water loss via evapotranspiration was a tad higher-- 745.24 inches. Daily numbers included: minimum relative humidity of 36 percent and winds of about 7mph. Solar radiation, on a statistician's day, averaged 18.01 megajoules per square meter...
Life in each of our four major North American deserts is more complicated and present than most people believe. Variations developed as regional responses to the common theme of aridity can be used to distinguish one desert from one another. At the same time, however, plants and animals in every desert share common patterns as universal responses to challenges posed by dryness, heat, wind, and poor soil quality common to arid and semi-arid deserts...
 |
| The Twin Buttes on the western side of San Angelo silently warn travelers they have entered the harsh and lovely drylands of Southwest Texas |
NEWS CORRAL:
The Toledo (Ohio) Blade offers some brief but well-considered thoughts about the reduction in forces which will accompany military drawdowns as US soldiers depart Afghanistan and Iraq and the nation moves toward a pre-9/11 strength.
Pat Magee of Corpus Christi believes oceans and surfing belong to the world. The former champion wave-rider opened a free-to-the-public museum where one can see the first surfboard made in Texas.
LRNARTS MARKETPLACE
Artwork by Louis R Nugent now available: For fine art prints and greeting cards, visit:
Fine Arts America now features West Texas painting, drawings, and photographs by Karen Boudreaux of Houston, Texas, Ken Brown Pioneer of Sand Springs, Oklahoma, Joe JAKE Pratt of Kerrville, Texas, David Pike of Lubbock, Texas, Judi Bagwell of Greenwell Springs, Louisiana, Suzanne Guirard Theis of Houston, Texas, Linda Cox of Graham, Texas, and Louis R Nugent of San Angelo, Texas at: http://fineartamerica.com/groups/west-texas.html?tab=overview
CREDITS
Note: All photographs for this essay were located through Google Images or Wikipedia, without authoritative source or ownership information except as noted: Chihuahuan Desert: Chisos Mountains at Big Bend from Brittanica.com; Joshua Trees in Mojave Desert from LonelyPlanet.com; Great Basin Desert Sagebrush from Chihuahuan Desert Nature Center; Arches National Park from USParks.com; Twin Buttes, oil on canvas, by Dwight Holmes from San Angelo Museum of Fine Arts; Desert Shrublands from cas.vanderbilt.edu; Bison Herd at San Angelo State Park from Texas Parks and Wildlife
No comments:
Post a Comment