This article was orginally published in the May 4, 1997 edition of the San Antonio Express News. It was written by Joe Foln, who is the paper's writer for agricultural issues.
This probably isn't going to shock anyone much: Juniper, alias mountain cedar, takes a lot of water out of the ground.
However, researchers working west of San Antonio now know how much water, and they also know that junipers use almost twice as much of it as do oak trees.
Some rather involved measurements, done in northern Uvalde County, indicate that a 15-foot-tall juniper uses about 35 gallons of water per day in a typical Hill Country ranching environment.
According to Keith Owens, range ecology professor at the A&M Research and Extension Center at Uvalde, the research also indicates that a live oak tree of the same size might consume only 19 gallons per day.
In a telephone interview last week, Owens said, "We're developing a water use simulation mode.
The computer model could calculate increased water yields as the plant density were reduced for a particular site, he said.
Owens said follow-up research will examine water use by three grass species: Texas wintergrass, sideoats grama and curlymesquite. It also will look at water-usage changes during regrowth by live oak, persimmon and mountain laurel.
A news release describing the project indicates scientists went to considerable trouble to come up with their water-use numbers. Researchers measured carbon dioxide intake and water loss from individual leaves on six trees-three oaks and three junipers - in each of nine watersheds. After measurements were taken the leaves were clipped and fed into a meter that determined their exact surface area.
Meanwhile , the nine watershed areas were divided into three groups. All of the juniper and oak canopy was removed from three sites and 70 percent was taken from three more, while the final three were left uncleared.
Finally, runoff was measured for each area, before and after clearing.
The combined data allowed Owens and his colleagues to determine how much water the trees used, how much stayed in the soil and how much ran off into streams, the release stated.
With the information that's been developed, a landowner might manage juniper trees with water use in mind.
However, Owens added that esthetic concerns, as well as federal regulations on endangered species habitat destruction, could affect juniper removal decisions.
As for total water recovery, Owens said it may vary with location, soil depth and slope.
Research by others near Sonora indicated a gain of 100,000 gallons of water per acre where all vegetation except grasses was removed. Other research in Medina County involved removal only of juniper, leaving oak and other shrubs. That test indicated 60,000 gallons per acre of water. His own results fell between the two, Owens said.
However, he said he's unaware of a means to determine exactly how much water gained through juniper removal could be recharged into the Edwards aquifer. New variables would enter that equation, such as how far water has to travel before it reaches fractured limestone that allows recharge, Owens said.
In the meantime, however, economic and environmental considerations for preserving or removing juniper in the Hill Country now can be tied to a pricetag, expressed in gallons of water.