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Mapping California's water resources

Using advanced geophysical imaging technologies in novel ways, we are mapping California's groundwater to enable sustainable groundwater management for the state's major agricultural areas and elsewhere. California's subsurface aquifers are being extensively pumped, especially during years of drought. In some areas, the subsidence, or compaction, caused by over-pumping means that even with abundant rain, the lost water-storage capacity may never be recovered. Negative changes in water quality are also being seen - the intrusion of saltwater into coastal aquifers and high levels of arsenic in wells in the Central Valley.

Mapping the groundwater system, ~300 m beneath the ground, in Butte and Glenn Counties.

But using the airborne electromagnetic method, we can identify freshwater aquifers and manage them carefully. In the same way that medical imaging revolutionized our approach to managing human health, geophysical imaging can revolutionize our approach to managing groundwater, giving us a roadmap to the sustainable management of one of our most precious resources.

Working with scientists from Aarhus University, Aqua Geo Frameworks, I•GIS, and Ramboll; the state of California Department of Water Resources; the State Water Resources Control Board; three regional water agencies; and the Danish Environmental Protection Agency (EPA), Stanford scientists are leading a $2 million project -- The Stanford Groundwater Architecture Project (GAP) -- to map the state's groundwater systems. Although legislation is in place requiring sustainable groundwater management, there is a critical need for data to support the development of plans. Without these data, "it's like managing a bank account and not knowing how much is going in and how much is coming out," says Stanford geophysicist Rosemary Knight, the lead scientist on the project.

Building on previous research led by Knight and other scientists the project commenced in October 2018 and will continue for two years. The purpose of the GAP is to determine the optimal workflow required to use the geophysical imaging method as the basis for sustainable groundwater management. Knight’s vision: that the geophysical imaging method will be adopted for groundwater management throughout the state.

The Problem

Dry riverbed

Without question, the state of California is suffering from a shortage of groundwater. Legislation is in place, but we do not have enough data to adequately manage the groundwater system -- to prevent excessive lowering of water levels, loss of groundwater storage, compaction of aquifers and associated subsidence of the ground surface, degradation of water quality, saltwater intrusion and impacts on surface water.

The 2014 state Sustainable Groundwater Management Act (SGMA) made local water agencies responsible for preparing groundwater sustainability plans (GSP) due by 2020 or 2022 for those agencies that oversee the 127 medium and high-priority basins in California.

A specific requirement for developing those plans is creating a 3D hydrogeological conceptual model of the subsurface. The conventional way to achieve that goal is the drilling of wells, a process that is slow, expensive, and provides insufficient spatial coverage.

The GAP provides a novel, technological approach to leverage geophysical data acquisition coupled with cutting edge modeling and analysis to create the hydrogeological models needed to sustainably manage groundwater.

The Places

Central Valley map

We are partnering in the GAP with three water agencies located in very different regions of California. Each agency has data needs to support the development of their sustainable groundwater management plans.

  • Indian Wells Valley is at the southern end of the Sierra Nevada. Information was needed about the large-scale structure of the groundwater basin and the location of zones of brackish water.
  • Butte County is in the northern Sacramento Valley. The key questions here involve the level of connectivity between various aquifer units, between those units and surface water, the vulnerability of different areas to subsidence and the variation in the depth to the water table.
  • The County of San Luis Obispo is along the central California Coast.  The community in the Paso Robles Area Subbasin needs a more complete picture of the geology and hydrology of this groundwater subbasin to make better decisions on sustainable groundwater management and groundwater supply in the future.

The Approach

At the core of our approach is the use of an airborne electromagnetic (AEM) method to map out the large-scale architecture of the groundwater system. The helicopter-deployed system can acquire data along ~300 km in a day, “seeing” up to ~400 m below the ground. The critical question motivating the Stanford Groundwater Architecture Project (GAP): “how to take this on in California where there has been no history of using the AEM method for groundwater management?”

Our goal is to determine the optimal workflow for the use of AEM data in California to support sustainable groundwater management, as required by SGMA. By working with our project partners, processes and methodologies will emerge that will be transferrable to other regions throughout California (and elsewhere in the world). The key elements of our approach are shown in the schematic below:

Ground water infographic

Our approach is described here as a 9-step workflow, links to results, recommendations, and useful materials for the various steps.

  1. Engage with the local agency to clearly identify the groundwater management questions linked to SGMA implementation.
    See results, recommendations, and useful materials.
  2. Develop the Data Management System that will enable data sharing among project participants and with stakeholders.
    See results, recommendations, and useful materials.
  3. Compile existing data such as well completion reports and geophysical logs along with geologic cross-sections and subsurface models.
    See results, recommendations, and useful materials.
  4. Design the AEM survey to fill in the data gaps so as to answer the groundwater questions.
    See results, recommendations, and useful materials.
  5. Acquire the AEM data using the helicopter-deployed system, with outreach in the local area to explain the objectives and procedures of the survey, and to address any health and safety concerns.
    See results, recommendations, and useful materials.
  6. Analyze the AEM data to obtain models of electrical resistivity, the geophysical property that is measured.
  7. Interpret the resistivity models to extract the information that we need, e.g. mapping of aquifer and aquitard units.
  8. Integrate all data to generate the hydrogeologic conceptual model using a computational framework that allows us to explore the range of possible models that are compatible with the data.
  9. Answer the management questions while acknowledging uncertainty so as to inform decision-making for sustainable groundwater management.


Helicopter carrying sensor


Our approach is described here as a 9-step workflow, links to results, recommendations, and useful materials for the various steps.

See the steps
Water coming out of a pipe

CA plans $12 million AEM project

The California Department of Water Resources (DWR) plans to conduct an aerial electromagnetic (AEM) survey of large areas of the state with funds from Proposition 68 in support of the Sustainable Groundwater Management Act (SGMA). The $12 million effort will provide groundwater agencies, related stakeholders, and the public with geophysical data, tools, and analysis for the California groundwater sustainability plan. The resulting information will provide standardized, statewide data and will have many uses, including identification of recharge areas and creation of a hydrogeologic conceptual model.

 The Stanford Groundwater Architecture Project is a two-year pilot project that is providing critical information about how best to use the AEM method to support groundwater management in California. 

Read the CA DWR fact sheet
Rosemary Knight

Our backstory: Putting knowledge into action

Stanford geophysicist Rosemary Knight has been working for 30 years to advance the use of imaging technologies to “see into Earth.” Her most recent work has used the airborne electromagnetic method to image and manage groundwater.

The Stanford work to map subsurface aquifers began in October 2015 when Knight partnered with Aqua Geo Frameworks (AGF) to acquire geophysical data using a helicopter-deployed system to image the groundwater system in Tulare County, in the Central Valley of California. That work revealed the complex distribution of sands and gravels holding the groundwater to a depth of 500 m (1600 ft) below the surface. Such images provide the basis for understanding the large-scale structure of the subsurface –essential for managing groundwater.

In Nov 2016, Knight, along with Professor Graham Fogg of UC Davis and Director Paul Gosselin of the Butte County Department of Water Resource Conservation, wrote a white paper that was presented to the Governor’s Office calling for a statewide program to map the groundwater systems of its groundwater basins using the geophysical imaging method.

Around the same time, the Danish government signed an agreement with California to collaborate on the topic of water management. Seeing an opportunity to work together on a new vision for the role of geophysics in groundwater management in California, Jacob Vind (Danish Ministry of Foreign Affairs) played a key role in pulling the Stanford, AGF and Danish research team members together. By the fall of 2018 the funding was in place to launch the Stanford Groundwater Architecture Project – the GAP.