One Water is not a new concept. The idea that water, in all its various forms, is a valuable, circular resource to be protected is widely accepted in our industry. Legislation, such as California’s Sustainable Groundwater Management Act (SGMA), is forcing those in charge of managing water resources to think broadly about the connections from one form of water to another. Since SGMA’s promulgation in 2014, more than 350 basin-specific Groundwater Sustainability Agencies (GSAs) have been established to manage more than 100 groundwater basins and subbasins across the state. Under that same legislation, the Department of Water Resources (DWR) is authorized to provide technical assistance to GSAs to help with implementing or updating Groundwater Sustainability Plans (GSPs).
Based on Woodard & Curran’s SGMA, hydrogeology, and integrated hydrologic modeling qualifications, our firm was contracted by DWR to assist with developing three papers aimed at providing GSAs and the water resources community help in identifying, understanding, and communicating the nature and occurrence of depletions of interconnected surface water (ISW). The following papers were drafted by Woodard & Curran with DWR staff, in collaboration with S.S. Papadopulos & Associates, Inc. and with technical peer review assistance from outside consultants:
- Depletions of Interconnected Surface Water: An Introduction
- Techniques for Estimating Depletions of Interconnected Surface Water
- Examples of Approaches for Estimating Depletions of Interconnected Surface Water
DWR expects to publish the final document in this series — “Guidance for Establishing Sustainable Management Criteria (SMCs) for Depletions of Interconnected Surface Water” — in 2025.
How to estimate the quantity, timing, and location of depletions of interconnected surface water
The first paper in the series is designed to introduce GSAs and interested parties to the foundational concepts of ISW and depletions of ISW. This common understanding is necessary for the more complex approaches developed in the subsequent papers. SGMA defines ISW as surface water that is hydrologically connected at any point by a continuous saturated zone to the underlying aquifer and where overlying surface water is not completely depleted.
Rivers, streams, and other surface water bodies are an obvious component of our water resources. Less obvious is the groundwater beneath our feet. These surface water bodies and groundwater bodies interact. ISW is when the top of a saturated groundwater system reaches the bottom of a surface water body, resulting in a continuous saturated zone between the surface and the subsurface. ISW depletions occur when there is a change in groundwater levels that result in changes in the flow between surface water and groundwater. One way this occurs is through groundwater pumping, which lowers groundwater levels. Where ISW is present, this results in either more surface water recharging groundwater or less groundwater discharging to surface water. Depletion occurs when there is a change in surface water flow or volume. This common understanding provided by the introductory paper lays the groundwork for GSAs to start considering technical analyses and sustainable groundwater management activities to address depletions.
The second paper, which was recently released in conjunction with the third paper, dives into the data groundwater managers need to evaluate ISW depletion, the quantitative approaches to evaluate ISW depletion and factors to consider, and a high-level implementation process for quantifying ISW depletion using a numerical model.
The third paper expands on the concepts presented in the first two papers, along with hypothetical examples that utilize numerical models to evaluate ISW depletion. These numerical models are generally better at accommodating complex groundwater, land surface, and surface water conditions and have been leveraged by most agencies that have submitted groundwater sustainability plans (GSPs). Though hypothetical examples, the scenarios included are representative of actual conditions in California’s groundwater basins and subbasins.
Applying this guidance in California
Woodard & Curran’s water resources experts were excited to partner with DWR in the development of these three documents, which provide a framework for local agencies that must comply with SGMA. Our technical experts who supported this project, including myself, Ali Taghavi, and Reza Namvar, are now ready to work with our clients in developing approaches for GSPs to manage depletions of ISW. If you are reviewing these documents or considering updating your GSP and have any questions, we are here to help.
