They have a saying in Texas: We don't have a water problem, we have a brush problem. The idea is that when shrubs and trees invade former grazing lands, they soak up so much groundwater that streams slow down and water supplies to cities and towns decrease. But a new study suggests that the opposite is true: Trees and shrubs on the prairies may actually help recharge the groundwater. The findings should force a rethink of land-management techniques for much of the U.S.'s former rangelands.
When the cattle ranches were founded on the southern Great Plains in the latter part of the 19th century—a development immortalized in the 1948 movie Red River—a lot of formerly productive and environmentally stable grassland shifted perilously close to becoming desert. That's because the cows ate away the vegetation and compacted the soil so much that its ability to absorb water declined. Those conditions persisted through much of the 20th century. Recently, mesquite and juniper trees have spread across the degraded landscape. This development, which is part of the land's recovery process, has raised new fears that the trees will leech water out of the ground, threatening water resources across areas such as the Edwards Plateau. That's the 140,000-square-kilometer area that feeds many of Texas's major rivers, such as the Nueces, Frio, Guadalupe, and Llano, as well as the Edwards Aquifer—possibly the most critically important water resource for the state.
But little research has tested this assumption. So hydrologists Bradford Wilcox and Yun Huang of Texas A&M University in College Station conducted field studies over 10 years and analyzed U.S. Geological Survey data going back to 1925 on stream flow in the Edwards Plateau area. As they report in an upcoming issue of Geophysical Research Letters, since 1950 the baseline stream flow—the amount coming from groundwater, not over the surface from rainfall—has doubled in areas where the trees have replaced the formerly degraded grasslands. Wilcox says the trees help aerate the soil, improving its ability to soak up rain.
The results were "very surprising," Wilcox says. They ran contrary to the prevailing idea, which was even supported by some research, that "when the trees move in, the water disappears." The remaining question, he says, is whether returning the area to lush grassland would improve the stream flows even more.
The research "contradicts the notion that trees and shrubs are drying up the prairie landscape," says soil scientist Mark Seyfried of the U.S. Department of Agriculture's Agricultural Research Service in Boise. Seyfried says the situation is similar on the plains of the northwestern United States, where juniper trees are invading and watersheds in the rangelands seem to be improving, relative to their conditions 60 to 80 years ago. In the case of the Edwards Aquifer in Texas, he says, "where you have the actual historical stream records, that kind of closes the door for me."