## 3 Discussion
### Monitoring and Displacements  
Since 2014, UNIPA researchers have carried out several studies on dam monitoring, using the Castello Bivona Earth Dam on the Magazzolo reservoir as a case study. The reservoir is located in Sicily and is mainly used for civil purposes, especially for irrigation of agricultural plots during the summer. The first application was reported in [111]; three GNSS geodetic receivers were installed on the top of the dam, to be used as control points in the sections of the dam most vulnerable to loads. The three receivers were able to transmit data to the department control centre of the University of Palermo via GSM. The main aim of this work was to test the GNSS post-processing accuracy for the determination of static results compared with other monitoring techniques, such as spirit levelling. Indeed, until today, Italian legislation did not provide for the use of GNSS to manage or monitor dam displacements but only for cadastral surveys. Various static monitoring methods using GNSS can be found in the literature, all of which are related to the fact that the geodetic receiver of the base station is permanently positioned close to the dam, thus not providing a guaranteed accuracy of the results independent of the displacement of the crown (top of the dam). So, in this first paper, a new approach compelling the GNSS receivers has been engaged to sort out these problems. The results obtained using the innovative GNSS approach reveal great performance, and the use of GNSS receivers at different sections of the dam allowed additional information about deformations over time that is not operationally possible to retrieve elsewhere. The post-processing positioning accuracy was approximately a few millimetres, and the results obtained with two different software, _Bernese_ (ver. 5.0) and _NDA_ (ver. 2009), were comparable in magnitude. As mentioned above, the monitoring of three different sections of the dam revealed different displacements behaviour (in both periodicity and time) that seems to be associated with water depth (and the water surface assessed via remote sensing) (Figure 4). A few years later, in 2017, the authors showed a comparison between different mathematical models for evaluating the displacements of the earth dam [112]. The mentioned models were hydraulic models, which have already been tested on concrete dams, and finite element models (FEMs), able to engage geotechnical parameters. The results obtained over a period of two years were satisfactory, and the comparison between the models revealed residuals of a few millimetres. Again, results from GNSS monitoring were compared with those obtained from the analysis of hydraulic and finite element methods to determine the most accurate one. Figure 3: Interference Station Position determination [111]. The results were compared with those obtained with different hydraulic models. Despite being generally referred to as concrete dams, the planimetric components of the displacement by the geotechnical FEM model were similar to those of the deterministic model with millimetre deviations, and in none of the cases, the threshold was higher than 5 mm. An article testing combined techniques for dam monitoring is, nowadays, one of the most cited in the literature in this field [112]. In this paper, both GNSS and remote sensing techniques were used; the latter, in particular, allowed monitoring of the water level of the reservoir. The water surface was estimated by different approaches, employing both SAR images (TerraSAR-X and Cosmo SkyMed), characterised by different acquisition spectral resolutions, modes, or geometric and optical (ASTER, OLI-TIRS, Landsat 8, Landsat 7 ETM+ SLC-Off, Landsat 5 TM). Two different techniques were tested for each type of image. Both were suitable to quantify the extension of the reservoir surface, as revealed by a discussion with water levels measured on the dam, allowing the extension of water level assessment from Earth Observation to ungauged reservoirs.