The Central Valley of California is one of the most productive agricultural regions in the world. Covering 50,000 km2, bounded by the Sierra Nevada to the east and the Coast Ranges to the west, the valley yields a third of the produce grown in the United States valued at $17 billion dollars per year (U.S. Geological Survey 2016). In times of drought, when surface water deliveries were substantially reduced, the only way to meet irrigation needs has been through extensive pumping of groundwater. This has exacerbated an already serious problem in the Central Valley, where some areas have experienced declining water levels for several decades.
With the passage of the Sustainable Groundwater Act in 2014, there is recognized need for the sustainable management of groundwater throughout the Central Valley. The starting point for management is a groundwater model as it provides the modeling tools required to predict and assess changing conditions (e.g. climate, land use) and the outcomes of possible alternate water management actions; so is thus the foundation on which to build effective groundwater management. The challenge faced throughout California: the lack of adequate information about the subsurface to use as the basis for model development.
We are exploring the use of the airborne electromagnetic (AEM) method as a means of mapping the large-scale structure of the groundwater systems of the Central Valley. The AEM method is a helicopter-deployed system, which acquires data along planned flight lines to measure the electrical resistivity of the subsurface to depths of ~300 m. Through calibration with information taken from well data, electrical resistivity values are then transformed into sediment type, e.g. sand, gravel, silt, clay. In addition to producing the “most likely” model, we also capture information about the probability of various sediment types, and the uncertainty in the model.
The first completed AEM project was in the Tulare Irrigation District in Tulare County, in the San Joaquin Valley, the southern part of the Central Valley (Knight et al., 2018, in list of publications below). Data were acquired here on October 27, 2015. Shown above is the electrical resistivity model recovered from the AEM data. The electrical resistivity data are displayed on top of the Google Earth map, but actually display the changes in electrical resistivity that start at the ground surface and go to a depth of about 400 m. These data reveal, in great detail, the variation in the types of materials in the subsurface, which affects the way in which water moves and is stored below the ground. The cool colors (blues) show where there are fine-grained materials, such as clay. These materials contain water, but act as impediments to flow. The warm colors (reds) show regions where there are coarse-grained sands and gravels. These materials can hold a large amount of water and can move a large amount of water.
The figure below shows the 3-D electrical resistivity model recovered from the AEM data acquired in November 2018 in the Kaweah subbasin in the southeastern part of the Central Valley. Again we see, in the warm colors, regions of sand and gravel and, in the cool colors, regions of clay and silt. On the eastern side of this image, there are regions where the warm colors show resistive materials that are not sand/gravel, but the granite of the Sierra Nevada Mountains.
We are now working with water managers in this area to use both the AEM data and the interferometric synthetic aperture radar (InSAR) data to improve the local groundwater model. (More information about this study is here:Integration of InSAR with Airborne EM Data for the Development of Groundwater Models)
A new study of this area and north into Fresno County is looking in detail are the zone at the base of the Sierra foothills and out into the Central Valley, to understand where there are pathways for natural recharge and potential sites for managed recharge. The AEM data were acquired in December 2020. (More information about this study is here: Searching for Recharge Pathways Along the Edge of the Central Valley)
AEM data were acquired in Butte and Glenn Counties as part of the Groundwater Architecture Project (GAP). (More information about that study is found here: The Stanford Groundwater Architecture Project) In the Figure below is shown the 3D electrical resistivity model with the sand and gravel in warm colors (reds) and the clay and silt in the cool colors (blues). Working closely with the water managers in this area, we are interpreting these data to obtain information about the large-scale structure of the aquifer system, and the extent of vertical connectivity.
During this study in Butte and Glenn Counties, we developed an improved understanding of the impact of power lines on the AEM data (Kang et al., 2020 in list of publications below); this is important given the need to fly close to well locations so as to use the lithology information in wells to assist in the interpretation of the AEM data. We also developed a way to get information about the depth to the top of the saturated zone (close to the water table), so that we would account for the impact of saturation on the measured resistivity values (Dewar and Knight, 2020 in list of publications below).
The acquisition and processing of the AEM data in all study areas was handled by Aqua Geo Frameworks.