## 1 Introduction  
The monitoring of dam displacements plays a crucial role in the detection of unsafe conditions, requiring appropriate management actions and/or counteractive measures from the early stages [1]. Indeed, the early detection of a dam instability should constitute a key component of any dam maintenance plan and could lead to structural stabilisation to prevent any initial warning from turning into a disaster [2]. Dam displacements can be due to several factors, including the consolidation of the embankment materials, the dam composition itself (including mineralogy, grain size, density of the compacted material, etc. ), and forcing factors (including temperature, water level fluctuations, etc.). Usually, concrete dams exhibit a nonlinear trend of the displacement related to thermal and hydrological loads [3], while irreversible displacements are assumed to behave linearly over time [4]. In these structures, indeed, the linear component of displacements is assumed to be the irreversible part of the displacements themselves, and the corresponding velocity is strongly connected to the stability of the structure [5] and is interpreted as the aging term. On the other hand, the oscillatory component of displacements is generally due to external seasonally varying forcing, such as thermal dilation and hydrostatic pressure (ibid.). In this study, an earth dam is monitored since the structures of this type are the most frequent, although relatively small in size. Indeed, about 65% of the dams reported in the International Commission on Large Dams (ICOLD) World Register of [6] are earth dams, corresponding to the oldest type, followed by rock-filled dams (13%) and gravity dams (13%) (the full list of the acronyms used in the manuscript is reported in Appendix A, Table A1). Although at the national scale of Italy, the monitoring of large dams is managed by Registro Italiano Dighe [7], a public body under the supervision of the Ministry of Infrastructure and Transport, at the regional scale (viz. Sicily), the monitoring of water reservoirs is the responsibility of the Osservatorio delle Acque office [8]. For Sicilian reservoirs, water storage data are available since 2010 with monthly frequency. However, for most of the dams, no information is available about dam deformations or displacements. Dam structure can be monitored via traditional contact sensors (extensometers, accelerometers, tiltmeters); ground-based methods (ground-based SAR, ground-based photogrammetry, terrestrial laser scanning, robotic total stations); and global navigation satellite system (GNSS). Remotely based methods include airborne Laser Imaging Detection and Ranging (LiDAR) and space-borne InSAR. The pros and cons of these methods are summarised in [9]. More recently, [PERSON] et al. [10] reviewed the available technologies for dam deformation monitoring, including geodetic and GNSS sensors for precise measurements at specific locations, remote sensors for areal deformation measurements, and integrated monitoring systems, data processing, and methods for deformation analyses. The forcing factors include (_i_) reservoir water levels can be monitored in situ via traditional instruments including staff (head) gauges, water pressure measurement devices, stage encoders, ultrasonic devices, and electromagnetic gauges; (_ii_) the dam-reservoir system temperature can be monitored via thermo-hygrometers (air humidity and temperature) [11], noncontact infrared thermometers (target surface temperature), and fibre optic distributed temperature sensing (temperature distributions within the dam) [12]. For in situ water level observations over small reservoirs, these are discontinuously collected by several caretakers by means of staff gauges resulting in inhomogeneous time series (i.e., not accurate and subjective measures). Additionally, lack of information can occur for in situ temperature, as in situ monitoring stations require maintenance which sometimes does not follow the requested time scheduling.