Inferring the storage of aquifer systems from InSAR measurements via flow and geomechanical modelling

In hydrogeological science, it is widely acknowledged that the response of an aquifer to groundwater pumping is predominantly influenced by two key parameters characterizing the aquifer system: saturated hydraulic conductivity K_s and the oedometric bulk compressibility c_m. The response must be viewed in terms of changes of pore water pressure p and the deformation of the pore volume which traduces in a movement of the land surface. In a confined aquifer system subjected to groundwater pumping, the variation in groundwater pressure is linearly dependent on K_s, while the deformation of the pore volume is linearly dependent on c_m. However, c_m also impacts p, particularly the speed of pressure variation over time, as aquifer specific storage S_s is also dependent on c_m. The dependency p - c_m can be considered “weaker” than that p - K_s.  The RESERVOIR Project, funded by the EU-PRIMA Programme, aims to characterize aquifer properties, with a focus on S_s, by optimizing the use of the available measurements. Pressure measurements from piezometers provide fundamental information to quantify K_s through the groundwater flow equation. Additionally, displacement measurements of the land surface provided by InSAR can be optimally used in equilibrium equations to constrain c_m (and consequently S_s). This objective is achieved through a novel procedure utilizing a 3D groundwater flow simulator (MODFLOW) and a 3D geomechanical simulator (GEPS3D) in an iterative one way coupled approach. Spatial variations of S_s and K_s are mathematically described as stationary Gaussian random fields. The procedure is applied to characterize the properties of the alluvial aquifer system in the eastern portion of the Gediz River basin, Turkey. In this region, groundwater withdrawal for irrigation has led to a general decline in pore water pressure and land subsidence of up to 10 cm/year over the past decade. The convergence of the procedure was achieved after four iterations, highlighting the presence of considerable heterogeneity in the distribution of parameters. This heterogeneity cannot be effectively constrained without the aid of satellite-based earth observation measurements.