Journal Article

Harnessing the Power of Geophysical Imaging to Recharge California's Groundwater

Knight, R., Goebel, M., Kang, S.

2025

Overview

Motivated by the availability of 20,000 line‐km of airborne electromagnetic (AEM) data covering the Central Valley of California, we developed a workflow that uses resistivity profiles from electromagnetic (EM) data to assess the suitability of areas for groundwater recharge. We defined a suitable area as one where “fastpaths” of coarse‐grained material could efficiently move water from the ground surface to the water table. We defined recharge metrics and generated the corresponding maps by integrating resistivity profiles from AEM data, sediment type (from driller's logs), water level measurements, and water quality measurements. The workflow is publicly available through a web‐based application, fastpath (https://fastpath.stanford.edu). We produced maps displaying recharge metrics on a 400 m × 400 m grid covering the Central Valley, with 80% of the cells sufficiently close to an AEM resistivity profile (within ∼3 km) to be assessed for recharge. Various decisions are made in the workflow that result in a range of values for determined metrics at any given location. The maps summarizing all metrics show that between 19% (2,000,000 acres) and 56% (7,000,000 acres) of the total area in the valley is land suitable for recharge. The landcover with the largest total area of land classified as suitable is cultivated crops. We estimated the total space available for recharge water to be ∼170 km3 which is two orders of magnitude greater than an estimate of the total volume of water likely to be available for recharge. Plain Language Summary The changing climate in California is raising concerns about sustaining the supply of groundwater. One proposed solution is to use excess water during years of flooding to recharge or replenish the groundwater systems. At a site that is suitable to use for recharge, water can be spread on the ground surface and will efficiently move downward through connected pathways of sand and gravel to reach the water table. We have available in the Central Valley of California 20,000 km of geophysical data, acquired using a helicopter, that images sediment type between the ground surface and the water table. We developed a workflow that allowed us to analyze these geophysical data and produce maps of the Central Valley displaying recharge metrics, measures of the suitability of an area for recharge. The maps summarizing all metrics show that between 19% (2,000,000 acres) and 56% (7,000,000 acres) of the total area in the valley is land suitable for recharge. The landcover with the largest total area of land classified as suitable is cultivated crops. We estimated the total space available for recharge water to be ∼170 km3 which is two orders of magnitude greater than an estimate of the total volume of water likely to be available for recharge. Key Points Developed a workflow to assess areas for groundwater recharge using electromagnetic data to generate maps displaying metrics Recharge metric maps of the Central Valley, using airborne electromagnetic data, classify 19%–56% of the land suitable for recharge The landcover with the largest total area of land classified as suitable for recharge is cultivated crops

• Publisher: John Wiley & Sons, Inc,American Geophysical Union (AGU)
• Subject: airborne electromagnetic method / Aquifers / Groundwater / Groundwater availability / Groundwater recharge / managed aquifer recharge / Sediments / Site selection / Software / subsurface characterization / Sustainability / Water levels / Water quality measurements / Water table