## 1 Introduction
Regarding the dam displacements, these are rarely monitored in situ, as these activities are expensive and time demanding. In this framework, the dam structural surveillance can benefit from remote sensing techniques. Over the last few years, it has been shown that optical satellite technologies are able to determine the variation of water level by quantifying the extent of the water surface [13; 14; 15]. Using the methods implemented for optical images, SAR images can also be employed to monitor the extent of the water surface [16]. Some authors [17] have quantified the water extent from COSMO-SkyMed (CSK) SAR images using an automatic classification procedure, including clumping and segmentation preprocessing. The authors highlighted that surface backscatter is influenced by local wind conditions reduced the classification performance. Indeed, the segmentation algorithm removed most of the pixels outside the reservoir even though some misclassified pixels were kept, but the subsequent clumping algorithm was not able to remove all pixels not classified as water within the water body. They concluded that the reservoir surface can be monitored accurately by discarding SAR images in which wind enhances surface roughness in relation to their intensity and direction. Despite limits related to wind conditions and surrounding morphology, SAR sensors allow also the detection of dam displacements by applying interferometric procedures [18; 19; 20; 21; 22]. Regarding temperatures, there are several sensors providing thermal data at adequate spatial resolutions, such as Landsat 8 (acquired at 100 m although resampled here to 30 m, [23]) and Sentinel 3 (SLSTR 0.75 km, suitable only for large reservoirs). For instance, applying a single-channel algorithm to Landsat 4, 5, and 8 thermal images can extend monitoring to the past (from 2018 back to 1984) to perform a lake surface temperature analysis [24]. The Landsat 8 satellite, for example, orbits the earth over the sunlit side of the earth in a sun-synchronous orbit, crossing every position on the earth's land surface at least once every 16 days. Thermal images allow the capture at the same local time of the instantaneous brightness temperature of the water surface in clear sky conditions, allowing retrieval of any trend in surface water temperature. In the case of suddenly changing cloudiness conditions, the temperature that would characterise the water surface in absence of clouds might be estimated [25; 26], thus enhancing the actual temporal resolution. Nowadays, remote sensing SAR and thermal images are available over three decades (e.g., the Landsat 4 images from 1982 and the European Remote Sensing (ERS) satellite images from 1991), allowing the interpretation of the behaviour of any structure over long time periods. According to [27], measuring displacements with traditional instruments (such as pendulums) are nominally more accurate than those estimated via GNSS techniques. However, the latter allows the identification of the displacements' periodicity with denser observations. [PERSON] et al. [27] fitted an analytical model designed for describing deformations of buttress gravity dams [28] to time series of a pendulum and GNSS-estimated displacements of a dam structure, obtaining submillimetre standard deviation of residuals along the cross-crest direction (<0.88 mm for GNSS displacements, <0.37 mm for pendulum displacements). [PERSON] et al. [29] compared the displacements estimated via a Differential SAR (DInSAR) interferometry technique, the \"coherent pixels\" technique [30] with those recorded by a network of conventional ground-based sensors to monitor an earth dam. The authors pointed out the unique opportunity to perform extensive monitoring of the earth dam, including the embankment and the slopes surrounding the reservoir, with higher density and coverage than that provided by in situ sensors. Recently, DInSAR and GNSS have been applied to monitor dam displacements.