by Christina Buck, Jim Blanke, Reza Namvar, and Thomas Harter

The Sustainable Groundwater Management Act (SGMA) presents many new challenges and opportunities.  One challenge is accounting for ‘interbasin flow,’ or subsurface groundwater movement between subbasins, a piece of the overall water budget required in Groundwater Sustainability Plans (GSPs).

Interbasin flow as part of the groundwater budget.

The Department of Water Resources is tasked with evaluating whether groundwater management in one subbasin will undermine an adjacent subbasin’s ability to reach sustainability.  Recognizing that subbasins throughout the Central Valley are interconnected, it’s much better to address this technical and management challenge up front rather than have each subbasin individually submit their GSP and hope for the best.

To tackle this issue, the Water Foundation funded a project administered by Butte County Department of Water and Resource Conservation that gathered a group of Technical Collaborators (TC) to discuss and provide recommendations on quantification of interbasin flow in GSPs.  Since interbasin flows cannot be measured directly, the project reviewed available groundwater models to investigate how they may or may not be suitable in estimating interbasin flows within the northern Sacramento Valley.

Technical collaborators providing recommendations and report content.

Groundwater models will be a part of our future

The complexity of processes affecting interbasin groundwater flows make groundwater models effective and typically necessary tools for quantifying these flows. SGMA does not legally require the use of a groundwater model. Yet, successfully avoiding the six Undesirable Results defined by SGMA will require accounting for a complete surface water and groundwater budget. Models also enable GSAs to estimate the effects of groundwater management practices that affect the water budget (e.g., decreased pumping or increased recharge) on groundwater conditions over time. Models will be key to leveraging diverse local data sets and knowledge into a consistent science-based framework to guide groundwater management.  Models that can evolve with the Groundwater Sustainability Plan (GSP) process are key to efficient and informed adaptive management, to guide monitoring and to inform practice decisions, and as a learning tool for stakeholders.

Existing tools and model selection

The northern Sacramento Valley area is covered by three regional models including two Central Valley-wide models: 1) C2VSim developed by the Department of Water Resources (DWR) and 2) CVHM developed by the United States Geological Survey (USGS).  These models are both undergoing significant updates.  Another regional, Sacramento Valley-wide model is currently being developed by DWR called SVSim.  In addition, local groundwater models also exist (e.g., Butte Basin Groundwater Model).  None of the existing regional or local groundwater models were specifically developed for SGMA.

Although the regional models are a valuable starting point and are based on a shared physical understanding of groundwater flow, they differ in their representation of aquifer geology and in their approach to simulating hydrological processes (“boundary conditions”) that drive groundwater flow and storage.  They also contain inputs developed from different data sources. These differences partly stem from the diversity of objectives for which these models were originally created, but also from conceptual and data uncertainty about appropriately representing, for example, pumping, agricultural recharge, or stream-groundwater interaction, among others.  The three models therefore yield somewhat different water budgets and differing results in simulating groundwater level conditions.

Given these differences, agencies should consider the following question when considering which groundwater model to use for GSP development: How well does the model match my current understanding of the land surface layer and groundwater budgets in my area? This question can be answered by considering the quality and amount of data, supply and demand, boundary conditions, water budget results, and calibration.

The Technical Collaborators concluded there is not an obvious choice of one of the regional models for the northern Sacramento Valley.  Therefore, each subbasin should compare the model inputs and results to locally available historical data, if possible. An existing surface layer model or other water budget datasets should be used only to assist in selecting the appropriate groundwater model. It is not appropriate to mix output from the groundwater model with other local water budget sources. Groundwater model results should be presented in full to keep the results internally consistent. In addition, simulated groundwater elevations near the boundaries have the most effect on quantifying interbasin groundwater flows. Therefore, evaluating a model’s representation of groundwater levels in comparison to historical data is important, particularly in the areas along subbasin boundaries.

Cooperation and uncertainty

The most critical factor to address interbasin conditions will not come from a pure technical remedy, but rather from cooperation. Early cooperation with neighboring subbasins to compare interbasin flow estimates is important. Although the exact values may be different, the estimated interbasin flow magnitude and direction should be similar. Differences should  be expected and – if the models are well constructed – reflect our uncertainty in the modeled systems.  Differences in model outcomes need not disrupt progress on sustainable groundwater management and may help guide both, monitoring efforts, and management decisions.  Modeling the groundwater system and working towards sustainability is an iterative process and agencies should utilize adaptive management practices.  The uncertainty inherent in models needs to be anticipated and accounted for when making decisions based on their results.  Estimates and representation of the system in models will improve over time with a long term investment in these tools.

Final report available

The final report includes recommendations for the northern Sacramento Valley region, for GSAs statewide, and for DWR and USGS who are developing and maintaining the regional models.  For more background and details on the outcomes and recommendations of the Technical Collaborators, the final report is available from the project website: https://www.buttecounty.net/waterresourceconservation/SpecialProjects/InterbasinGroundwaterFlowProject.

The project was made possible through the generous support of the Water Foundation, an initiative of the Resources Legacy Fund.   

Christina Buck is the Water Resources Scientist for Butte County Department of Water and Resources Conservation.  Jim Blanke is a Senior Hydrogeologist at RMC, A Woodard & Curran Company.  Reza Namvar is a Senior Water Resources Engineer at RMC, A Woodard & Curran Company.  Thomas Harter is a groundwater expert at the University of California, Davis and an Associate Director of the UC Davis Center for Watershed Sciences.