## 4 Discussion on Practical Open Issues and Future Development  
The application of DOFS to the monitoring of geo-hydrological processes is still hampered by open issues. Figure 22: (**a**) Deployment setup of a cable for ground fissure monitoring. (**b**) Monitoring site in Yangshuli, Wuxi, China. (**c**) Strain distribution along the cable: the two peaks locate the main ground fissure E and W, respectively. Reproduced with permission from [222]. From a technological point of view, the current generation of DOFSs represents a revolution in terms of number of sensing points if compared to conventional single-point contact sensors technology. Spatial resolution and distance range are now adequate for most of the applications dealing with static soil movements, but they may be not for levees monitoring, where precursor infiltration flows may be very small. On the contrary, reduced strain range is intrinsically limited to the maximum elongation the fibre can withstand (i.e., a few percent), and represents a severe issue for the long-term monitoring of soil mass movements. Dynamic strain sensing (by means of DAS or DVS) is still in a very early developmental stage, and at the moment there are only a few examples in geo-hydroological monitoring. Nonetheless, distributed dynamic strain sensing is the only technology that has the potential to replace and advance accelerometers' and geophones' array, widely used in geo-hydroological monitoring and therefore is expected to become more popular in the short-term. As a general comment, all DOFSs covered by this review require more field applications to become a well-established and mature technology. One of the major practical concerns is represented by the installation: proper precautions must be taken to avoid damaging the cable during the installation, and the cable must survive in a very hostile environment for years to recover initial installation efforts. Often it must be deployed on steep slopes and hazardous sites, already heavily compromised by the hazard; therefore, the installation can be very complex and must be minimally invasive to avoid further jeopardizing the stability of the site. Similar considerations apply to levee monitoring: the installation of the cable should not create preferential filtration paths in the cross-sections; otherwise, the cable itself will drive the erosion, leading to the collapse. Paradoxically, the possibility of deploying the interrogator at remote sites--which is a sound feature in other fields of application--may not always be a viable solution, because it comes with additional invasive and expensive work. Current cable technology claims survivability over several years, but there exist only a few long-term monitoring campaigns that partially support these claims. Some authors also highlighted the evolution of fibre-soil coupling over time, making repeated and periodic calibration impelling. Furthermore, DOFSs must face the massive penetration of FBG technology into geo-hydroological applications. Fibre Bragg grating technology indeed has an intrinsic advantage over DOFSs in practical monitoring: it is a technology that can mimic conventional sensors (e.g., strain gauges and extensometers), both in operative principle and installation; therefore, they can potentially replace the conventional existing technology, with the advantages of common optical sensors. Instead, DOFSs enable new paradigms of measurement in geo-hydroological applications that cannot be replicated by conventional instruments or FBGs. In this sense, we may say that it is a \"transformative\" technology. This terminology was originally introduced by [PERSON] et al. regarding the disruptive nature of DTS technology for hydrologic systems (e.g., lakes, snow-covered glacier, water stream) [224].