Have you ever wondered what goes on beneath a river? Rivers, lakes and streams – all different types of “surface water” are often interconnected to the underlying groundwater aquifer. This system is known as Interconnected Surface Water (ISW) – and it supports a special type of habitat known as Groundwater Dependent Ecosystems (GDEs). What a mouthful, right? These ecosystems support plant and animal species that live in and along waterways and wetlands where groundwater provides critical water, especially where surface water can be seasonally scarce. Many rare and endangered species in California rely on these types of ecosystems. The Sonoma Valley GSA has identified data gaps in our understanding of how surface water conditions and water use activities affect GDE’s.
Using grant funds from the Department of Water Resources (DWR), in 2024 and 2025 the GSA conducted ISW and GDE studies to strategically fill these data gaps and better understand how we can manage our groundwater resources to avoid significant depletion of surface water and maintain the vitality of groundwater dependent ecosystems.
To evaluate ISW, the GSA conducted seepage runs, compared stream flow measurements with groundwater levels, and piloted temperature sensors as a proxy measurement where groundwater data is unavailable. The ISW analysis indicates varying degrees of surface water interconnection throughout the basin. This study’s findings will help the GSA further refine the ISW sustainable management criteria and help comply with DWR guidance.
The GDE analysis focused on reassessing GDE acreage and health in Sonoma Valley. The GSA had initially estimated GDE acreage when developing the Groundwater Sustainability Plan (GSP), by comparing the Sonoma Vegetation Digital Elevation Model (DEM) with 2015 groundwater levels. This analysis was updated using groundwater levels from 2023. The updated GDE analysis estimated that Sonoma Valley has 50 less acres of GDE than previously understood, with a total of 3,328 acres of freshwater marsh, oak woodland, riparian forest, wetlands, and vegetation dependent on groundwater. To estimate the health of these areas, the study compared their normalized difference vegetation indexes (NDVI). NDVI is essentially how green vegetation is, which is an indicator of health or stress. Results showed that 75% of GDEs Sonoma Valley are moderately healthy or very healthy, but 40% are showing declining trends from 2015-2024.
These studies greatly improved the GSA’s understanding of ISW and GDEs in Sonoma Valley. These results alone do not provide enough information to make policy decisions or establish protective metrics, but give further direction to fill remaining data gaps. Through these collaborative efforts and focused scientific studies, we are working towards long-term sustainability and health of our groundwater ecosystem.
To learn more, read the final report here.