Tracing the Footprint of Overuse: InSAR Multi-Temporal Monitoring of Aquifer Deformation Prior to Groundwater Regulation in Farafra Oasis, Egypt

This study employs advanced Synthetic Aperture Radar Interferometry (InSAR) to quantify pre-regulatory aquifer deformation in Egypt hyper-arid Farafra Oasis (2003–2010), a critical period of unmanaged groundwater exploitation. Leveraging reprocessed ENVISAT ASAR C-band data (32 SLC images) optimized for arid environments, we implement a differential InSAR (DInSAR) workflow with Small Baseline Subset (SBAS) time-series analysis. The methodology includes rigorous co-registration, Goldstein-Werner filtering, SRTM DEM-assisted phase correction, and Minimum Cost Flow phase unwrapping, validated against GPS (RMSE = 0.8 cm) and hydrogeological data. Results reveal spatially heterogeneous subsidence of 1.5–5.2 cm, with peak rates of 1.1 cm/yr (2003–2008) concentrated in northern (Zone A: 4.8 ± 0.9 cm) and southwestern (Zone B: 3.2 ± 0.7 cm) wellfields. Deformation patterns correlate strongly with groundwater abstraction, demonstrating direct causality between intensive pumping and aquifer compaction. A significant decline in spring discharge (e.g., Ain Tenin: 156 to 13 m³/day) paralleled subsidence trends. Post-2008 rate reductions (0.4 cm/yr in Zone A) align with early regulatory interventions but confirm irreversible compaction. This work pioneers retrospective InSAR analysis as "environmental forensics" for diagnosing pre-policy aquifer stress, addressing a critical gap in groundwater governance literature. By quantifying the legacy impacts of incentivized agricultural expansion in a policy vacuum, we establish Farafra as an archetype for unsustainable fossil aquifer exploitation. The findings underscore InSAR dual utility as a geophysical tool and policy catalyst, with direct relevance to arid regions globally facing similar pressures of groundwater dependency and institutional inertia. We advocate integrating archival SAR data into sustainable aquifer management frameworks to prevent irreversible resource degradation.