Drought-driven transient aquifer compaction imaged using multitemporal satellite radar interferometry

In unconsolidated, heterogeneous aquifer systems, low rates of pore-pressure diffusion of applied effective stresses due to the drainage of thick, low-permeability, clay-rich layers with time constants of decades to centuries cause delayed, residual permanent compaction and land subsidence. Current application of satellite differential radar interferometry (DInSAR-differential interferometric synthetic aperture radar) to estimate aquifer hydraulic properties (compressibility and/or storage) in these systems is limited by the temporal availability of synthetic aperture radar data (1992-present). In this paper we study the degree of aquifer compaction due to water extraction using DInSAR through an example in southeast Spain. Ground deformation data indicate large-scale deformation and in particular the discovery of the highest rates of groundwater-related land subsidence recorded in Europe (>10 cm/yr), affecting the Guadalentin River basin (>200 km(2)), the largest tributary of the Segura River. Modeling of the ground surface time series of the Guadalentin Basin indicates that deformation is mainly driven by nonlinear time-delayed flow processes in the underlying aquifer. After a drought period (1990-1995), the aquifer responded with an exponential decay of the land subsidence (lasting similar to 8 yr), suggesting transient groundwater pore-pressure flow. We show that multitemporal satellite radar interferometry analysis and its modeling can be a stimulating way to study nonlinear soil mechanics and groundwater flows at aquifers. A deeper understanding of such processes could help the management of water resources and land subsidence of unconsolidated coastal and Quaternary alluvial aquifers in a highly evolving climate region (the Mediterranean Sea and elsewhere).