In Iran, both local and regional scale land-surface deformation has resulted from the decline in groundwater levels \cite{Motagh_2008}. Moreover, the gap between groundwater extraction and renewal is so large that the resulting short-term impacts are likely to be irreversible \cite{Olen_2021}.

Here we use Earth Observation (EO) data to calculate vertical subsidence rates due to groundwater extraction in Tehran, Iran’s capital city. This data includes Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR); very high-resolution (VHR) Pléiades optical stereo imagery; and ICESAT-2 laser altimetry data.

The Centre for Observing and Monitoring Earthquakes and Tectonics (COMET) Looking into Continents from Space (LiCSAR) automated processing system is used to process six years (2015-2021) of Sentinel-1 SAR acquisitions \cite{Lazeck__2020} for interferometric (InSAR) analysis. The system generates short baseline networks of interferograms. We also correct for atmospheric noise using the GACOS system \cite{Yu_2018a,Yu_2018} and perform time-series analysis using open-source LiCSBAS software \cite{Morishita_2020}. Vertical and horizontal (east-west) velocities are calculated and mapped to produce nationwide and regional velocity fields \cite{Watson_2022}. Preliminary results indicate maximum subsidence rates in Tehran exceed 100 mm/year.

VHR Digital Elevation Models (DEMs) and laser altimetry ground returns are also used to calculate land subsidence rates in Tehran. By comparing rates calculated using all three EO techniques we aim to validate InSAR velocities whilst investigating and constraining the benefits, drawbacks, and biases associated with each technique. Phase bias \cite{De_Zan_2015}, for example, may be introduced to calculated InSAR velocities when using the LiCSBAS short baseline network strategy as subsiding regions in Iran are often vegetated cropland.

Publishing the open-source COMET-LiCS Subsidence Portal was the focus of previous work (https://comet-subsidencedb.org/). The portal presents automatically processed LiCSAR Sentinel-1 interferograms and LiCSBAS velocity timeseries for 99 subsiding regions across Iran. Interactive tools allow stakeholders to make quick, critical assessments related to extents and rates of subsidence. Validating portal data using DEMs and laser altimetry is essential before expanding the portal to have a global focus.