InSAR for the characterization of climate-related processes in Northwest Italy

Insights into hydrologically-induced deformations of the Earth surface, and particularly of aquifers, are crucial for a better understanding of water cycle dynamics and its interaction with solid earth processes and to provide useful information for the sustainable management of water resources. The high spatio-temporal resolution and millimeter/centimeter-scale accuracy of surface deformation data from satellite geodesy techniques such as Global Navigation Satellite System (GNSS) and Synthetic Aperture Radar Interferometry (InSAR) make it possible to measure and identify signals related to hydrological forcing. Elastic loading response has been primarily investigated using GNSS surface displacement to infer TWS variation at regional scales. The higher spatial resolution of InSAR measurements has made it possible to identify surface deformation patterns associated with the groundwater storage (GWS) variation of local aquifer systems.

In this work we leveraged Sentinel-1A Multi-Temporal InSAR observations from the European Ground Motion Service (EGMS) to analyse the deformation occurring in an area in North-Western Italy. This region hosts the Po valley, a large alluvial plain in northern Italy characterized by abundance of both surface and groundwater bodies, which are extensively exploited for farming and industrial activities. Recently, changing climatic conditions have imposed additional stress on water resources, culminating into a severe drought in 2022. GNSS data revealed an elastic response to the TWS variation associated to this drought at entire Po basin scale (Pintori & Serpelloni 2023).

We analysed InSAR time series (2018-2022) focusing on an area spanning from the low Lombardian plain to the foothills of the Alps, encompassing terrain that transitions from fine alluvial deposits in the south to coarser fan and glacial deposits in the north and including some main cities and two of the largest Italian lakes. We applied decomposition and clustering techniques in order to extract the signals contributing to the observed deformation and their spatio-temporal features. To identify the possible physical drivers, we compared our results with publicly available precipitation, rivers discharge and water head table piezometric data, and hydro-geological information. We found that different areas respond with different mechanical behaviours to the same forcing. We highlighted localized areas on the piedmont belt which are mainly characterized by a transient multiyear signal of up to 15 mm which results to be strongly correlated with precipitation, uplifting in wet periods and subsiding during drought periods. This is consistent with a poroelastic response which could be attributed to the higher localized concentration of coarse-grained material like gravel and sand in the piedmont belt. We applied models of poroelastic deformation, including, where available, hydraulic head data, to relate the identified poroelastic surface deformation to GWS variation, and characterize the aquifers properties. Outside these areas, the multiyear deformation pattern has a lower amplitude (up to 2mm) and is anticorrelated in time with precipitation, consistently with an elastic loading response. We computed the elastic deformation due to the estimated TWS variation from Pintori & Serpelloni (2023) and found agreement in order of magnitude and temporal trends with InSAR data.