## 2 Materials and Methods
### Satellite Data and Processing  
Tables 1 and 2 list the satellite data used for Halabiyeh dam and GERD, respectively. In particular, cloud-free Sentinel-2 multispectral products were selected and downloaded from the Copernicus Open Access Hub, all in Level-2A (L2A) Bottom Of Atmosphere (BOA) format in the case of Halabiyeh dam and Level-1C (L1C) Top-Of-Atmosphere (TOA) and L2A formats for GERD prior to and after 2021, respectively. In the latter case, preliminary tests on sample data suggested that no particular advantage could be achieved by means of atmospheric correction of L1C products for the purposes of site detection. Of the 13 bands, we concentrated on the visible to near-infrared bands and their combinations as per the methodology developed by [PERSON] and [PERSON] [62]. COSMO-SkyMed StripMap HIMAGE and Enhanced Spotlight images were processed from Level 1A, single-look complex slant products (SCS) up to geocoded products according to the methodology described in [PERSON] et al. [84]. In both use-cases, the geocoding accuracy of ellipsoid-corrected scenes and products was refined using tens of ground control points (GCPs) to make the scenes overlap with the available ESRI World Imagery base map. Change detection maps were generated as per the procedure by [PERSON] and [PERSON] [68; 72]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
\begin{tabular}{c} Dry \\ Humid \\ Humid \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Archive and newly acquired satellite imagery used for documenting archaeological sites in the prospective Halabiyeh dam. In the case of Halabiyeh dam, the region was poorly covered by archive satellite imagery of adequate spatial resolution collected prior to the beginning of our surveys. Only one COSMO-SkyMed Enhanced Spotlight scene, vertical-vertical (VV) polarization, ascending geometry, incidence angle of 50.85\({}^{\circ}\), dating back to 14 January 2012 was available over the area where the dam arch was supposed to be placed. Therefore, a robust acquisition strategy of SAR imagery from the COSMO-SkyMed constellation was necessary, to complement the rich and consistent cloud-free open access Sentinel-2 dataset, and allow a sufficient integration of both active and passive satellite systems. A new set of Enhanced Spotlight scenes was tasked to concatenate with the 2012 image, while other three Enhanced Spotlight frames were tasked to provide a full coverage of the Euphrates section where the reservoir should have been formed after the impoundment of the planned dam. These scenes allowed a spatial upgrade of the regional-scale assessment and site identification made at 3 m spatial resolution by means of a bespoke time series of StripMap HIMAGE images that was also tasked to cover the whole study area within the same frame. On the contrary, the area of the GERD reservoir was already documented, with a rich archive of optical and SAR imagery available, at least since 2013. Therefore, we decided to integrate the available open access Sentinel-2 dataset, with few targeted acquisitions of COSMO-SkyMed at 3 m spatial resolution matching with the archive StripMap HIMAGE scenes. The new data were collected during the dry season of May 2021 and the humid season of October-November 2021 (Table 2), in order to address the main environmental variable (i.e., the seasonality and its impact on the visibility of anthropogenic features). For the purposes of our methodology, Google Earth was helpful to select existing data and task new acquisitions based on the three main phases, i.e., pre-, during and post-construction. Although the dataset is discontinuous, the Google Earth time-lapse highlights the key construction phases of GERD, but also triggers some hypotheses about the timing of the filling-up process (Figure 5).