The research team looked at focused recharge, which occurs when hillslopes funnel water into concentrated areas, such as streambeds. Streambeds often have sandy bottoms, which allow water to quickly infiltrate and prevent the deep-rooted shrubs from sucking it up. They then used data from a highly-monitored desert mountain slope in New Mexico to simulate the effects of woody plant encroachment and climate change on water resources.

They discovered that not only did the shrubs increase focused groundwater recharge but that they did so even under conditions where climate change reduced the amount of rainfall.

They also modeled a more extensive form of shrub encroachment called “thicketization,” in which plants grow in dense stands with no bare patches, and found that, as in prior flat landscape research, the shrubs reduced the amount of groundwater recharge on slopes, as well.

On hillslopes, bare soil in between patches of shrubs are necessary to drive water into streambeds. Increased runoff increases focused groundwater recharge.

Climate change will most likely increase groundwater recharge by making rainstorms larger, but less frequent. Larger storms increase the amount of runoff that reaches sandy-bottom channels and increases groundwater recharge. Findings from this study suggest that vegetation will also play an important part in groundwater recharge in the future.

“It is striking that ecosystem composition is what controls projected future changes to groundwater recharge,” Schreiner-McGraw said. “This does not mean that climate change is not important, but that vegetation change is potentially more important and something that scientists and land managers should focus more effort on understanding.”

Though the study took place in New Mexico, the research team indicates that it can be applied to similar environments. For example, large parts of California are also desert savannahs and Southern California and the Central Valley have landforms similar to those found in the New Mexico study site. These areas could experience similar hydrological processes, though atmospheric rivers create storms that are very different than monsoon storms, so more research is required.

This work is also an example of advancing science for the public good through interdisciplinary research.

“Without a close collaboration across different disciplines, in this case hydrology and ecology, it would be difficult to use scientific methods to support land management decisions,” Vivoni said. “Approaches like this often require teams from multiple institutions.”

The paper, “Woody Plant Encroachment has a Larger Impact than Climate Change on Dryland Water Budgets,” is published in Scientific Reports. Additional co-authors include Osvaldo Sala of ASU’s School of Life Sciences, the Global Drylands Center and the Julie Ann Wrigley Global Institute of SustainabilityHeather Throop of the School of Earth and Space Exploration and the School of Life Sciences, and Debra Peters with the USDA Agricultural Research Service.

This article was written by Holly Ober of the University of California, Riverside with contributions from Karin Valentine.