The Link Between Head, Storage and Subsidence
The Place
We focus on California’s Central Valley, which is a large, clay-rich alluvial aquifer system which is fundamental to the successful continuation of a multi-billion dollar agricultural industry.
The Problem
Obtaining reliable, timely estimates of the amount of groundwater stored in the subsurface is a crucial component of groundwater management. One of the most common methods to estimate storage changes is to take measurements of hydraulic head and convert them to storage estimates using the equation dSgw = S * dhmeasured, where S is the storativity which takes the value of specific yield or the product of aquifer thickness and specific storage depending on whether the aquifer is confined or unconfined. However, in aquifer systems where there are large quantities of clay present, there are a number of challenges in applying this equation: head measurements are mostly sensitive to conditions in sands whereas major storage changes are known to occur in the clays; storativity varies spatially and temporally; and semi-confined aquifers fall outside the traditional theory. In this project we investigate circumstances in which the relation dSgw = S * dhmeasured gives erroneous answers by using subsidence as a proxy for the storage change in clays. We are developing new methods of integrating head and subsidence - observed through InSAR - to provide improved estimates of groundwater storage changes.
Our Approach
We return to a basic equation for the volume of stored groundwater, which leads us to consider storage changes as two components: the change in storage due to changing saturation of pores, and the change in storage due to changing porosity (i.e., compaction/expansion) of the aquifer system. We explore how we can use water table measurements to quantify the first component and surface deformation measurements, using InSAR, to quantify the second component. This gives us the integration of head, subsidence and storage. We anticipate that this method can provide reliable estimates of storage changes with high spatial resolution and high latency.
Schematic illustrating the two intrinsic sources of storage change in the San Joaquin Valley aquifer system: drainage of pores and compaction of aquifer materials.
Project Publications and Presentations
Lees, M., & Knight, R. (2023). Integrating shallow head measurements and InSAR data to quantify groundwater-storage change in San Joaquin Valley, California (USA). Hydrogeology Journal. https://doi.org/10.1007/s10040-023-02705-7
Lees, M. (2022, October). A New Approach to Estimating Groundwater Storage Changes in California’s Central Valley. Talk presented at the SustainValencia2022, Valencia.
Lees, M. (2022, September). A new approach to monitoring storage: a case study in the Tule and Kaweah subbasins. Poster presented at the Groundwater Resources Association of California Western Groundwater Congress 2022.
Lees, M., Knight, R., Smith, R. (2021): Linking Subsidence to Changes in Stored Water in California’s San Joaquin Valley. Association of Environmental & Engineering Geologists 65th Annual Meeting, September 20-26 2021.
Project Sponsors
The Gordon and Betty Moore Foundation
NASA