Use of satellite remote sensing and independent component analysis to assess land subsidence and aquifer system properties over the Willcox Basin, USA

The Willcox Basin, located in southeast of Arizona, USA, covers an area of approximately 4,950 km² and is essentially a closed broad alluvial valley basin. The basin measures approximately 15 km to 45 km in width and is 160 km long. Long-term excessive groundwater exploitation for agricultural, domestic and stock applications has resulted in substantial ground subsidence in the Willcox Groundwater Basin. The land subsidence rate of the Willcox Basin has not declined but has rather increased in recent years, posing a threat to infrastructure, aquifer systems, and ecological environments.

In this study, an integrated analysis of remote sensing and in-situ groundwater observations is made to assess characteristics of land subsidence and response of the aquifer skeletal system to the change in hydraulic head in Willcox Basin. L-band ALOS and C-band Sentinel-1 SAR data acquired from 2006 to 2020 are analyzed using multi-temporal interferometric approach to derive subsidence deformation. We show that the overall deformation patterns are characterized by two major zones of subsidence, with the mean subsidence rate increasing with time from 2006 to 2020. This study also suggests that subsidence here is a result of human-induced compaction of sediments due to massive pumping in the deep aquifer system and groundwater depletion. 

Independent component analysis (ICA) a leading method for blind source separation to isolate signals without knowing a priori information about the signal sources, which was adopted to separate the mixed InSAR time series signal into a set of independent signals. On the one hand, the application of ICA filtered the residual errors in InSAR observations to enhance the deformation time series, and the deformation accuracy is improved by more than 13%. On the other hand, it also revealed that two different spatiotemporal deformation features exist in this area, indicating hydrogeological properties of aquifer systems are spatially variable in this basin.

In addition, the relationship between the observed land subsidence variations and the hydraulic head changes in a confined aquifer is analyzed. Using InSAR measurements and groundwater level data, the response of the aquifer skeletal system to the change in hydraulic head was quantified, and the hydromechanical properties of the aquifer system is characterized. The estimated storage coefficients, ranging from 6.0×10^-4 to 0.02 during 2006-2011 and from 2.3×10^-5 to 0.087 during 2015-2020, signify an irreversible and unrecoverable deformation of the aquifer system in the Willcox Basin. The reduced average storage coefficient (from 0.008 to 0.005) indicates that long-term overdraft has already degraded the storage ability of the aquifer system and that groundwater pumping activities are unsustainable in the Willcox Basin. Historical spatiotemporal storage loss from 1990 to 2020 was also estimated using InSAR measurements, hydraulic head and estimated skeletal storativity. The estimated cumulative groundwater storage depletion was 3.7×10⁸ m³ from 1990 to 2006. 

Understanding the characteristics of land surface deformation and quantifying the response of aquifer systems in the Willcox Basin and other groundwater basins elsewhere are important in managing groundwater exploitation to sustain the mechanical health and integrity of aquifer systems.