Juniper infestations intercept much of an area's natural rainfall

Edwards Plateau municipalities in heavily infested juniper country lose much of their annual rainfall before it ever hits the ground, according to two Texas A&M University scientists.

“Thick stands of mature juniper (commonly called cedar trees) can actually intercept 40 percent of an area’s natural rainfall,” said Keith Owens, Texas Agricultural Experiment Station range researcher at Uvalde. “The percentage lost increased dramatically if the rainfall is light.”

Owens and colleague Robert Lyons, Texas Cooperative Extension range specialist at Uvalde, spent three years studying the evaporation and interception water loss from juniper trees across Edwards Aquifer Recharge area.

“This project proves what many have long suspected – too much juniper exacts a heavy toll on the Texas water supply. “Look closely at a juniper tree sometime. They are ideally made to catch rainfall and redirect it to the base of the tree. The water actually hits the tree and runs back toward the trunk. Often no moisture gets to the ground until the tree is hit by a hard rain of an inch or more.”

The two scientists conducted the study in areas averaging 24 to 34 inches of annual rainfall. The project was conducted at 10 locations in eight counties (Bexar, Blanco, Comal, Hays, Kendall, Kerr, Medina and Uvalde). The test area stretches across the Edwards Aquifer recharge and drainage area. Extension agents in these counties assisted in locating and establishing the test plots.

“During one year of observation, the Hays County site received over 36 inches while the Kendall County site got just 10.12 inches,” Owens said. “ But no matter how much or how little fell, the amount of rain falling and the percent intercepted by the trees were about the same across the board. “At the end of the three-year study, we averaged all of the 2,700 total rain events, both heavy and light, that fell over the 10 sites. We found that 35 percent of all the precipitation that falls on juniper trees hits the canopy and evaporates, the litter beneath the trees intercepts 5 percent, and 60 percent actually reaches the ground surface. Of that 60 percent, much is taken up by the tree for growth that leaves little or no water left for aquifer recharge in the heavier juniper-infested areas.

“This research shows that in an area which receives 30 inches of rain in a year, only 18 inches of that total actually reaches the ground surface under a juniper tree. That means 12 inches of rain a year does not reach the ground for either plant growth or potential aquifer recharge.”

Individual storm data generated from this study is available at:

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