In December 2025, the Groundwater Resources Association of California hosted a two-day webinar to address the complex relationship between Interconnected Surface Water (ISW) and groundwater. The sessions focused on how groundwater extraction impacts surface water flows and the ecosystems that depend on them.
The webinar opened with Dr. Vivek Bedekar, a consultant with SS Papadopoulos and Associates, who provided an overview of the foundational concepts of ISW depletion. His presentation focused on how groundwater pumping alters streamflow and the need for effective models and management strategies to address these interactions. The link to the article is here.
Next, Dr. Nicholas Murphy, senior groundwater scientist with The Nature Conservancy’s California Water Program, introduced the concept of groundwater‑dependent ecosystems (GDEs). He discussed their role in ecological health and the importance of considering GDEs within the framework of groundwater sustainability planning.
What are groundwater-dependent ecosystems?
As defined under SGMA, groundwater-dependent ecosystems are ecological communities that depend on groundwater emerging from the aquifer or occurring near the ground surface for some or all of their ecosystem needs.
SGMA requires that all beneficial uses and users be considered in groundwater sustainability plans, including groundwater-dependent ecosystems (GDEs). GDEs are diverse, with many different types existing across the state. While many groundwater sustainability plans under SGMA currently focus on vegetative GDEs (such as groundwater-dependent plant communities) and their relationship to the undesirable result of lowering groundwater levels, for this presentation, Dr. Murphy focused mainly on instream aquatic species. These organisms, such as salmon and steelhead, benefit from groundwater and serve as strong indicators of groundwater-dependent ecosystems.
Five steps to assess potential impacts to GDEs from ISW depletion
Step 1: Identify and map beneficial users.
The first step is to understand where the environmental beneficial users occur within the subbasin, and then to discretely identify and map them. For groundwater-dependent ecosystems, this could include threatened and endangered aquatic species. The map on the left shows the current spawning range of salmonids. This is a good starting point for subbasins with salmon and steelhead in their rivers and streams.
Dr. Murphy noted that statewide datasets map the habitat of rare plants and animals, and the Nature Conservancy’s 2019 critical species lookbook also categorizes species that have a connection to groundwater.
Step 2: Identify protective streamflow thresholds
The second step is to identify protective stream flow thresholds that preserve ecological function. The goal is to identify thresholds that, if exceeded, would be expected to cause adverse impacts to beneficial users under SGMA.
The California Environmental Flows Framework can be used to evaluate ecological flow needs. The diagram outlines five functional flow components: fall pulse flow, wet-season peak flow, wet-season baseflow, spring recession flow, and dry-season baseflow.
Dr. Murphy noted that the potential ecological impacts of streamflow depletion from groundwater pumping often manifest in the dry-season baseflow component, so that’s a good functional flow to focus on. TNC tools, such as the California Natural Flows Database and the Functional Flows Calculator, can be used to estimate unimpaired natural flows in surface waters across the state or to calculate functional flow metrics using real-world observed stream gage data.
When available, local studies and datasets will be among the most valuable information at the subbasin level. If minimum instream flow recommendations exist for local watersheds, these site-specific studies can provide a high-resolution picture of ecologically protective stream flows.
Step 3: Assess existing conditions of streamflow
Once protective, ecologically functional flows have been identified, the next step is to assess existing stream flow conditions in the area of interest. To evaluate existing impacts from groundwater pumping, we need an accurate understanding of streamflow across different water-year types.
Resources available to complete this step include observed streamflow gauge data. Dr. Murphy noted that TNC’s Functional Flows Calculator includes a mapping tool that compiles publicly available stream gage data in California. If additional monitoring is necessary in the subbasin, programs such as the DWR stream gage improvement program could provide funding mechanisms for it. In the absence of comprehensive monitoring data, existing integrated surface-water-groundwater models may be used to estimate streamflow conditions.
Step 4: Assess streamflow depletion due to groundwater pumping
Step four is to assess existing and future impacts of groundwater pumping on streamflow. These impacts are best assessed through a unified modeling approach. Multiple types of models may be appropriate at different times to assess streamflow depletion from groundwater pumping.
Well-developed numerical models help understand surface water and groundwater dynamics in the subbasin. Numerical models use mathematical equations to simulate complex water systems. However, the cost and technical expertise required for these models can be high. Groundwater managers may need additional decision-support tools. Later, Dr. Murphy discussed new analytical modeling frameworks that estimate streamflow depletion and support SGMA efforts.
Step 5: Develop Sustainable Management Criteria
Finally, step five involves consolidating all previous information to develop ISW Sustainable Management Criteria (SMCs) that prevent exceedance of protective streamflow thresholds. These criteria must be designed to avoid surpassing protective streamflow thresholds and, once established, should directly inform projects and management actions at the subbasin level to achieve 2040 sustainability goals.
Emerging analytical methods to estimate streamflow depletion
Recognizing the heavy lift required to comprehensively address streamflow depletion due to groundwater pumping, The Nature Conservancy has been researching how lower-cost analytical modeling frameworks can support SGMA implementation efforts.
At the management scales needed for SGMA implementation, streamflow depletion can’t be measured directly; it must be modeled. Different types of models exist, from analytical models to fully integrated numerical models of surface water and groundwater systems.
“Pros and cons exist for every approach, and I highly recommend the paper, Quantifying streamflow depletion from groundwater pumping: A practical review of past and emerging approaches for water management, by Zipper et al.,” he said. “It’s a great primer for where and when groundwater managers might want to utilize different tools in their portfolio of available solutions.”
An emerging analytical methodology, called analytical depletion functions (ADF), allows expansion of simple analytical models to a regional scale. Previously, these models could only estimate impacts between a single well and stream, but now, using ADF, cumulative impacts from multiple wells across a subbasin can be estimated using traditional analytical models.
ADF models are spatially distributed analytical models. However, they rely on several simplifying assumptions. One of these is the assumption that instream water is available for depletion.
However, as the work on streamflow depletion has shown, this is not always a realistic condition in many California watersheds. Non-perennial stream dynamics can result in limited water availability. Researchers at the University of Kansas developed a stream-drying and depletion-redistribution function to more accurately represent streamflow depletion dynamics in ADF modeling frameworks. By combining an unimpaired flow data set with analytical estimates of stream flow depletion, it is now possible to estimate at the stream reach scale when surface water is available for depletion in a watershed, or when a stream reach is expected to go dry during the summer months.
In an upcoming publication that’s currently in revision, this research framework is applied to the Scott Valley as a case study. Dr. Murphy noted that because Dr. Harter’s group at UC Davis has developed an established numerical model, we can compare model performance across methodologies. The analytical modeling framework allows us to distribute the impacts of groundwater pumping as estimates of streamflow depletion at the stream-reach scale.
“When we look at the results between the numerical and analytical models, we have been really pleased to see comparable model performance in this geography. The incorporation of drying considerations and the implementation of ADF modeling frameworks appear to be reasonable approaches in this geography for assessing streamflow depletion from groundwater pumping.”
Dr. Murphy said this is the first time the research framework allows a comparison of analytical modeling results with real-world observed streamflow data. “Previously, analytical models produced estimates of streamflow depletion as a value that could only be compared to other modeling estimates. Pairing this analytical modeling framework with unimpaired flow estimates, we can compare model-estimated streamflow from the numerical and analytical modeling approaches to a USGS gage at the outlet of the Scott Valley.”
The modeling tools allow us to simulate management scenarios and compare them against various regulatory requirements. In the Scott, there are emerging minimum instream flow recommendations from CDFW, and we can examine the percentage of time that the analytical and numerical modeling workflows correctly predict stream flow relative to exceedance of these management recommendations.
“Because we see comparable model performance, we’re really interested in how numerical and analytical modeling approaches can be leveraged together to provide a portfolio of tools for groundwater managers,” said Dr. Murphy. “When we need low-cost, low-lift screening tools, ADF models show promise for implementation. When we really need to get into the nuts and bolts of management scenarios involving complex dynamics, land-use changes, groundwater recharge, and water-availability modeling, numerical models will continue to be a powerful tool for SGMA implementation.”
