Assessing Riparian Woodland Response to Shallow Groundwater Availability

Across California, riparian woodlands serve as crucial refuges for a diverse array of rare and endangered species. Some of these woodlands are groundwater dependent ecosystems: communities that depend on groundwater near the ground surface. Many of these systems are vulnerable to drought and groundwater extraction, meaning resource agencies must consider possible impacts on these sensitive ecosystems in long-term groundwater management. However, there is a knowledge gap regarding the complex exchanges between surface water, shallow groundwater, and riparian vegetation in groundwater dependent ecosystems. To fill this knowledge gap, we analyze precipitation, hydrogeologic conditions, and vegetation health in one riparian woodland to characterize statistical relationships between depth-to-groundwater and ecosystem health. In studying the hydrogeology of the region, we find evidence of a pressure gradient that causes groundwater upwelling to the shallow subsurface. Groundwater upwelling, in addition to increased precipitation, may contribute to the shallowing of groundwater levels within the riparian woodland. Using remote sensing, we show ecosystem health improves when groundwater levels are shallower, especially during summer. These results reaffirm this ecosystem’s reliance on shallow subsurface flows during dry periods, meaning that resource agencies must prioritize the maintenance of shallow groundwater levels in this system. Our methodological framework can aid managers in assessing the baseline condition of other unique groundwater dependent ecosystems for future management. As interest in the protection of groundwater dependent ecosystems increases, the continued monitoring of these systems will be crucial to ensure the protection of these sensitive habitats from groundwater extraction and future droughts.

Acknowledgements

UC Santa Barbara Bren School: Derek Booth, Affiliated Researcher; Scott Jasechko, Associate Professor; Chris Jerde, Associate Researcher; Arturo Keller, Distinguished Professor; Ashley Larsen, Associate Professor; Trace Martin, PhD Student; Ruth Oliver, Associate Professor

Stillwater Sciences: Christian Braudrick, Fluvial Geomorphologist; Peter Downs, Senior Fluvial Geomorphologist; Bruce Orr, Co-Founder and Principal Ecologist

UC Santa Barbara Department of Geography: Christopher Kibler, Postdoctoral Scholar; Conor McMahon, PhD Student

UC Santa Barbara Marine Science Institute: Sean Carey, Ecological Restoration Technician; Tom Dudley, Principal Investigator; Evan Hobson, Project Staff; Li Kui, Project Scientist; Adam Lambert, Research Biologist

Ventura County Public Works Agency: Jeff Dorrington, Water Resources Specialist; Lauren Zaragoza, Resource Conservation Technician

Shawn Kelly, Executive Director, Santa Clara River Conservancy

Kathleen Kuepper, Hydrogeologist, United Water Conservation District

Bob McKane, Research Ecologist, United States Environmental Protection Agency

Candice Meneghin, Director, Fillmore and Piru Basins Groundwater Sustainability Agency (Santa Clara River Environmental Groundwater Committee Representative)

Tony Morgan, Vice President and Principal Hydrogeologist, Daniel B. Stephens and Associates

E.J. Remson, Senior Project Director, The Nature Conservancy