A Journey To The Desert's Edge, Part Two

 Note: this is the second in a series of occasionally appearing entries focusing on deserts in general and the drylands of West Texas in particular

"West of Fort Concho, it (mesquite) becomes dwarfed into a shrub of very large roots...." 

S B Buckley, State Geologist of Texas, 1876

Rangeland, Tom Green County, Texas

Wladimir Koeppen's great work, a massive analysis of the world's temperature and precipitation zones, failed to address the quality of mesquite in Southwest Texas but nevertheless still ranks as one of science's true leaps forward...

Ah, the history of climate classification.  A subject rivaled only by painting a mud fence brown with its potential for heart-pounding thrills and chills.  Let's ease our way into the excitement...

The New American Desk Encyclopedia serves me well.  A thick paperback, it's just the right size for a blogger who remembers there had been a Civil War battle at Pea Ridge but doesn't remember Pea Ridge is in northwestern Arkansas.  And the NADE has a fine little article about the dry country that touches on plant and animal life in the desert and the imbalance of rain and evapotranspiration.  It notes approximately one-third of the earth is desert, a total that must be accurate since it agrees with Wikipedia...

Another paperback that does right by me is The Deserts of the Southwest by Peggy Larson.  Ms Larson is highly regarded in her field and worked at the Arizona-Sonora Desert Museum near Tucson for many years.  Visitors to the city, by the way, won't waste their limited vacation time should they decide to spend a day exploring the Museum's 21 acres.  Strolling down two miles of pathway, one safely encounters more than 300 species of animals and 1300 varieties of desert plants-- mountain lions and gila monsters and eagles, creosote bushes and saguaros... 

Arizon-Sonora Desert Museum Brochure

Larson's book delightfully takes us on a tour across America's deserts.  Even a person who doesn't know isobars from temperature inversions can discuss dry climates intelligently after reading a few pages.   She says deserts cover about one-sixth of the planet...

This disagreement about how much of the world is desert... one-third or one-sixth... has a lot to do with a climate classification system officially unveiled in1900 by Wladimir Koeppen, originally a botanist, who spent his life making sense of the world's patchwork quilt patterns of rain and heat.  The dispute comes down to a simple question: how much rainfall does a desert receive in an average year?  Is it under 10" or less than 20"?  Are deserts simply empty tracts of sand interrupted with the occasional shrub or can they also be landscapes dotted with clumps of short grasses concealing yucca and cactus...

Koeppen's biographers say he was a small and courtly man who worked tirelessly to help underprivileged children.  An aristocrat, he ignored a tradition allowing him to place "von" before his last name.  His physician grandfather traveled to Russia at the behest of Catherine the Great who employed him to improve the country's public health system.  In the next generation, his father served Alexander II as a geographer and historian and received a country home on the Crimean coast from the Tsar of all the Russias for his services...

Wladimir Koeppen

Born in St Petersburg in 1845, Wladimir spent his childhood exploring the countryside adjoining the family estate.  He became fascinated by differences between plants that grew on the nearby plains and those he found by the shoreline and in the more distant mountains.  He made it his mission to unlock the mysteries behind the geography of the plant world and its relation to climate...

Wladimir Koeppen, PhD,  had given himself quite a challenge.  Few scientists paid attention to the subject after Aristotle divided Gaea into Torrid Zones, Temperate Zones, and Frigid Zones in the 4th Century BCE.  Edmund Halley, of comet fame, did study wind patterns affecting shipping routes in the 1700s.  George Hadley, a geographer who attempted to create a star map for the southern hemisphere, tried to account for some discrepancies in Halley's theories.  In 1817, the distinguished Baron von Humboldt created a world map showing regional temperature ranges.  No map of global rainfall patterns existed before 1882 when a chap named Loomis correlated the data to make one...

Sifting through the information available to him, Koeppen noticed five basic climate types corresponded to Earth's precipitation and thermal patterns.  It is almost an insult to his research to simply say he divided his "Dry Climate" into Arid (or desert) regions that saw an average of 10" rain or less per year and Semi-Arid (or steppe) locales receiving between 10" and 20" worth of precipitation...

Modified Koeppen Classification System

[Steppe, incidentally, is a troublesome word of Russian origin, that refers to grassland plains in semi-arid climates.  No exact English equivalent conveys the sense of the word although short-grass prairie comes very close.  An increasing number of scientists accept the term "desert grassland" to distinguish the grassy stretches of western Texas, southern New Mexico, and southern and central Arizona from cooler latitude shortgrass prairies.]   

The Koeppen system soon became (and remains) a standard way of describing Earth's climates.  It filled a major gap in the field.  Geography textbook writers appreciated it for its completeness and for providing school children with an easy to remember definition of a desert.  Cartographers liked the way it allowed them to draw fairly uncomplicated maps without having to use too many colors...

But, elsewhere in the world of publish or perish, it didn't take other scientists long to note minor flaws in the Koeppen system, especially its analysis of dry climates ...

After the rains

One flaw takes us back to southeastern Arizona and Tucson with an annual average of just under 12" of precipitation.  A strict reading of Koeppen unceremoniously dumps "The Old Pueblo" from lists of our nation's desert metropolises.  Yet, the town is clearly a dry place and likely to be described by the vast majority of residents and visitors as desert...

Half a mile down the road

To be fair to Koeppen, it should be noted this flaw is based on a factor that Koeppen recognized as existing but one he could not adequately analyze because of a lack of available data.  Desert aridity results not merely from lack of rain and high temperatures  and winds that seem to never quit blowing but also thanks to the amount of water lost to evapotranspiration...



[For the non-botanically inclined, evapotranspiration is the "loss of water from the soil both by evaporation and by transpiration from the plants growing thereon."  A simple way to define transpiration is to think of it as the water vapor escaping through plant leaves.]

One of Koeppen's critics, C W Thornthwaite, published "An Approach toward a Rational Classification of Climate" in the Geographical Review for January 1948.  Discussing "potential evapotranspiration," Thornthwaite cited H C Trumble's research in southern Australia which proved an area becomes moisture-deficient when demand for water is three times higher than the amount available through rain and snow...

Arid and Semi-arid Desert Regions (in Yellow)

Exactly what does this mean to someone who lives in a place like San Angelo, Texas?  Using 54 years of data collected by the Irrigation Technology Center of Texas A&M university, we learn the city has an annual average potential evapotranspiration rate of 71.34" with annual average rainfalls of 19.20".  The country around the western Concho Valley is hot and dry enough to burn away over seventy inches of water in a typical year.  Less than a third of that amount is normally available to sustain plant and animal life...

The aridity of the Concho Valley increases each westward mile across the Lone Star State.  By the time a traveler reaches El Paso, he has come to a place with average rains of 8" and average potential evapotranspiration rates of nearly 80"...

[Some writers, such as Michael Barbour and Norman Christensen who discuss the vegetation of North America in Flora of North America, would consider neither city as being located in a desert environment based on a definition that sets an average of 120mm (roughly 4 3/4") or less as the upper limit for "true desert" precipitation]

Peveril Meigs, offering an alternative to Koeppen and Thornthwaite, generated another approach to climate equally (if not more so) complex in its mathematics.  He suggested dividing dry areas into three categories: extremely arid, arid, and semi-arid.  Based on the way Meigs envisioned drylands, San Angelo might be described as a semi-arid and El Paso as arid ...

A land of yucca and cactus

Today, most scientists define deserts as places where potential water loss through evapotranspiration exceeds (or significantly exceeds) available water.  They also often employ Meigs' terminology and describe them as extremely arid, arid, and semi-arid.  But there is no universal agreement among academicians on when dry lands become deserts, their exact boundaries, or the amount of rain they receive.  Jeremy M B Smith, knowledgeable enough to discuss the subject for Encyclopedia Britannica,  for instance, contends certain areas may see as much as 24" of annual average precipitation and still be considered deserts...

Personally, I am inclined to agree with Smith when he says "Deserts are varied and variable environments, and it is impossible to arrive at a concise definition that satisfies every case . However, their most fundamental characteristic is a shortage of available moisture for plants, resulting from an imbalance between precipitation and evapotranspiration. This situation is exacerbated by considerable variability in the timing of rainfall, low atmospheric humidity, high daytime temperatures, and winds"...

Much can also be said for practical definitions.  Merriam-Webster's Collegiate Dictionary, Tenth Edition, says a desert is "arid barren land, especially:  a tract incapable of supporting any considerable population without an artificial water supply."  Other pragmatic sorts argue the dry country begins where non-irrigated farming is a fool's game and land is best used to graze livestock over wide areas...

And, for some, the desert begins where mesquites become shrubs with large roots...

West Texas Barrens

Note: Brochure illustration is from the Arizona-Sonora Desert Museum, circa 1996.  Photograph of Wladimir Koeppen, circa 1921, by Friedrich Becks, was located through Google Images and Wikipedia with no additional source information.  Map of arid and semi-arid deserts from Reader's Digest Great World Atlas, Reader's Digest Association, Pleasantville, 1968. Map of modified Koeppen System from Man's Domain: A Thematic Atlas of the World, McGraw-Hill, New York, 1968. All other photographs copyright by Louis R Nugent.  Publication data: Deserts of the Southwest, Peggy Larson, Sierra Club Books, San Francisco, 1977; New American Desk Encyclopedia, Signet Books, New York, 1982;  Merriam-Webster's Collegiate Dictionary, 10th Edition, Merriam-Webster, Springfield, 1993.  S B Buckley quote from Second Annual Report of the Geological and Agricultural Survey of Texas, Houston, 1876.