Tuesday, December 13, 2011

More Desert Than Not

Texas west of the 100th meridian and south of the 32nd parallel tends to be more desert than not.  Exactly how much is desert can be (and has been) debated.  But when all is said and done, the fact remains that Southwest Texas is hot and dry.

Historians and natural scientists tell us the pre-settlement Concho Valley was basically prairie country.  Accounts by Spanish explorers such as Juan Dominguez de Mendoza and pre- Civil War U S Army surveyors do much to support this theory.  Early descriptions rarely mention mesquite and say the creeks were filled with rushing water. 

But they also contain hints about the true character of the Concho Valley.  As LT Francis T Bryan traveled past Brady Creek in the spring of 1849, he described the grass as "abundant and good for grazing."  Not too many days later, he saw the grazing conditions near Dove Creek as only "tolerable...the grass being old and dry."

Bryan was one of the first people to notice there are actually two Concho Valleys.  One lies east of San Angelo and the other is west of town.  These two valleys exist because rainfall in Texas gradually decreases as one travels from the border with Louisiana to the one with New Mexico.  Annual rainfall averages culled from the Texas Handbook Online paint a clear picture of the land's increasing aridity:  Mason County: 25"; Concho County: 23"; Tom Green County: 18.2"; Reagan County: 16".

An 1876 report on Texas agricultural and mineral resources by geologist S B Buckley comments on precipitation in various parts of the state.  His totals roughly correspond to current figures.  Average regional rain totals were likely not much different during LT Bryan's day.

When naturalist Vernon Bailey passed though the Concho Valley in May 1899 he described the San Angelo area as "an open mesquite plain in the genuinely arid region" boasting "great stretches of smooth surfaces with only short grass and little desert plants."  Bailey's list of plant life on the road to Sterling City could have been written today: juniper, lotebush, wolfberry, catclaw, cactus, mimosa, agarita, yucca, and javelina bush.  Two years later, Harry Oberholser traveled from San Angelo to Ozona and saw the same species, adding joint fir and Mexican persimmon to the list.

A Bureau of Soils survey of West Central Texas conducted in the 1920s summarized the area as mainly "grasses and a few small trees."  The surveyor's catalog of important regional species echoes Bailey and Oberholser and also includes creosote bush, tarbush, and allthorn.

This emphasis on vegetation is meant to illustrate an important point:  the basic character of a place is reflected in its plant life.  Grasses, shrubs, and trees have no choice but to adapt to their environment.  Humans and animals can move from one place to another.  A plant can't.

These reports show that Chihuahuan Desert scrub has been part of the western Concho Valley landscape for over a century.  Archaeological evidence tells us mesquite grew here hundred years before the first rancher of European descent arrived.  

West Texas shrubs and grasses deal with erratic and irregular rains.  Focusing on Tom Green County, we have fifty-four years of Irrigation Technology Center data that puts San Angelo's average annual rain and snowfalls at 19.20".  But this is offset by a potential evapotranspiration rate of 71.34".  In short, the city enjoys semi-arid desert conditions. 

National Weather Service records generate a somewhat higher annual average of 20.91" at Mathis Field.  Data from 1977 to the present show our actual totals were as low as 12.95" in 1977 and as high as 32.05" in 2008.  The variation between average and actual is even more pronounced when we look at individual months during those same thirty plus years.   A hodgepodge of numbers ranging from 0.12" to 11.24" results in average monthly rains of 3.09" for May.

Our rains are not only infrequent, they are generally ineffective.  A statistically average year sees almost a month's worth of falling moisture but an inch or more of rain will come only four times.  To gauge the usefulness of our precipitation, we also have to factor in the drying power of the area's steady wind and high percentage of solar radiation received by the ground here.

Let us not forget water loss through plant transpiration.  The quality of our soils plays a role too but we can be thankful they are a bit more suited to farming and ranching than the dirt west of the Pecos.

Mother Nature made earning a living from the land hard enough but we didn't help her.  Land bordering on being desert country was pushed into that status by overgrazing newly fenced rangelands, controlling wildfire, and the introduction of prolific non-native species like salt cedar.

An 1898 Agriculture Department study by H L Bentley estimated that, in the decade prior to his report, 64 head of cattle were being grazed per square kilometer on ranges in this part of Texas.  Such overgrazing increased soil compaction and disrupted soil structure which, in turn, led to higher runoff rates and more water loss.  Animal hooves trampled grasses that spread via surface runners and further increased the amount of bare soil.  

Damage caused by overgrazing was not immediately obvious.  But, over the course of several decades, the number and size of woody plants increased across the western Concho Valley and Trans-Pecos grasslands.  Relatively low value tobosa grass filled a void left by the buffalo grass killed off by too many hungry animals in too small a space.  Degraded grasslands transformed into a collection of mesquite, prickly pear, and yucca separated by bare soil and patchy weeds.

How do we deal with the future water needs of the Concho Valley?

We can start by understanding that a number of long range climate forecasts for West Texas call for gradually increasing temperatures.  A few optimists see slightly more rainfall in our future.  This best case scenario means our aridity level will remain roughly the same.  But, should we see the same amount (or less) in the way of precipitation, there will be little debate that we are a bona fide desert.

We can realize that West Texas rainfall averages are just that-- averaged numbers.  These numbers do not always correspond to a consistent pattern.  It is true that we get rain in the spring and autumn months when warm and cold air masses collide.  But we don't know when or if this will happen in a given year.  In short, the sooner we stop telling ourselves that we "should be" at X inches of rain in May or Y inches in November, the better.

We can carefully think through any future plans to "restore" the land to its "natural" state.  Don't forget that salt cedar was introduced to help protect our rivers and streams.  Razing every pencil cactus and desert sumac in Tom Green or Irion County might create a nice dull landscape but it won't make the rain any less sporadic-- or the heat less intense.

None of these "we can dos" will solve our water problem.  But they can help change the way we think about the issue.  And that might help us all in the long run.   Turning a blind to eye to the degraded rangelands and brush-clogged streams is not a wise option.  Fixing them properly means we take the reality of our environment into account.

Some say we should pray for rain.  I am not convinced we should ask God to change a perfectly good design for the universe because we are unwilling to use the resources that we have been given more wisely than we have.  There is a reason giant yuccas and century plants can grow here.  It is because our part of the Concho Valley is a hot and dry land, more desert than not, a transition zone where short steppe grasses become the outer edges of the Chihuahuan Desert.  And it has been that way for a long time.


  1. Louis, as a botanist and an editor for a scientific journal....I think I am qualified to say, this is an excellent article. I very much enjoyed reading it!!

  2. Thank you, Pat. At some point, I hope to put together a longer piece (book length) about the Western Concho Valley. The human history of the area is pretty interesting and would be part of it but the larger focus would be on the land and our impact on it.