Advancing the use of InSAR measurements for groundwater management and science in California's San Joaquin Valley
Keywords: Sustainability.
Geophysics.

In California's San Joaquin Valley, groundwater extraction has caused widespread land subsidence during the past 15 years. Many locations have seen sinking at 30 cm/yr, but just a few cm/yr of subsidence is sufficient to cause adverse impacts. Damage has therefore been extensive, with loss of capacity in major aqueducts a particular concern. In recent years, two significant events have transformed the way subsidence is understood and managed. First is the widespread availability of interferometric synthetic aperture radar data (InSAR), enabling accurate, high-resolution monitoring of subsidence across the Valley. The second was the passing in 2014 of the Sustainable Groundwater Management Act (SGMA), putting a legal requirement on groundwater agencies in the Valley to devise and implement groundwater management plans to avoid the "significant and unreasonable" results of subsidence. The central tenet of this thesis is to use the opportunity presented by InSAR data to meet the need encapsulated by SGMA. My objective is to use InSAR data to advance our scientific understanding of subsidence, with an emphasis on the development of innovative management practices that aid the implementation of SGMA. In the first Chapter, I focus on a numerical tool for simulating subsidence known as a 1D compaction model. A 1D compaction model simulates the delayed drainage and subsequent compaction of clays in response to head declines in coarser-grained materials, and these models are the primary numerical tool used to simulate subsidence in the Valley. Our ability to make insights with existing 1D compaction models has been restricted by the models' short temporal coverage and treatment of the subsurface as a single layer. Through careful data preparation in collaboration with local partners, I was able to develop a 1D compaction model that covered a 65-year period from 1952-2017 and resolved the subsurface into three layers. The model, which was calibrated using InSAR data, enabled me to identify that the majority of subsidence originates as deep compaction, and that timescales of residual compaction are in the decades-to-centuries range, as opposed to the 1--3-year timescales suggested in some literature. I then expanded this work by applying the 1D compaction model to a new location and extending it to project subsidence until 2080. I found that current groundwater management plans will lead to very high subsidence rates, but also concluded that simplifications in 1D compaction modelling mean there are substantial limitations in our ability to make predictive models. Overall, the first Chapter of this thesis demonstrates the ability, and limitations, of 1D compaction models to improve our understanding and management approach to subsidence. In the second Chapter, I consider the link between subsidence and storage. Monitoring storage changes is essential for sustainable groundwater management. Storage changes can be quantified by considering the two main components through which they are expressed: saturation changes and deformation of aquifer materials. I quantified these components in the Tule and Kaweah subbasins of the San Joaquin Valley. To quantify the component expressed through saturation changes, I followed existing observational approaches by identifying head measurements from shallow wells and scaling by specific yield. When considering the deformation component, I noted that standard approaches in the Valley are to ignore it or approximate it with a simple linear relation to measured head. However, by considering head and deformation measurements made at extensometers, I found that assuming a linear relationship between deformation and head might provide a poor estimate. Instead, I used InSAR data to quantify the deformation component of storage changes. I showed that the two components -- saturation and deformation -- accounted for storage declines of equal magnitude over 2015-2021, suggesting that the practice of ignoring the deformation component can result in large errors when estimating storage changes in regions with subsidence. Summing the two calculated components gave a new estimate of the total storage change that captured the major trends seen in independent estimates, while better accounting for the deformation component. An additional benefit is that this method accounts for the deformation component in the unconfined aquifer. This method to quantify total storage change can be a practical and effective tool to support groundwater management. In the final Chapter, I take a big-picture view of Valley-scale subsidence. Subsidence in the Valley has occurred in two periods, 1925-1970 ("the historic period") and during a series of droughts from 2007-present, causing adverse impacts during both periods. Our subsidence record during the 2007-present period is incomplete due to a 2011-15 gap in Valley-wide observations, making it difficult to develop an appropriate management response. To meet this need, I used satellite geodesy to obtain measurements of subsidence and quantify the Valley-wide subsidence volume during the 15-year period 2007-present. I found a total subsidence volume of 14 km3, comparable to the 19 km3 that occurred during the 55-year historic period. Considering the 2007-present subsidence rate and projected future overdraft reductions, I conclude that halting Valley-wide subsidence is not a realistic goal. Instead, I recommend focusing on localities where subsidence impacts are greatest and reducing overdraft from the deeper regions of the aquifer system where subsidence originates.