## 5 Conclusions  
The monitoring of geo-hydrological processes is a fundamental tool for preparedness and mitigation efforts toward the aim of reducing vulnerability to geohazards. It is of great scientific interest to better understand and measure the complex dynamics--still not completely known--of these processes. Within this framework, DOFSs have gained much attention since their early introduction. However, applications of DOFS technology to geo-hydrological monitoring are still in an early stage. In this paper, we have reviewed and discussed the use of distributed fibre-optic sensors in geo-hydrological applications, and in particular, for the monitoring of levees, soil slope/landslides, and ground subsidence.  
The distributed sensors techniques currently used in geo-hydrological monitoring have been briefly discussed, and Rayleigh-, Raman-, and Brillouin-based sensing techniques described. To summarise the features and characteristics of current technology, we have included an overview of the performance of commercially available instruments for each of the sensing mechanisms addressed (Tables 1-3). The paper then has focused on a general review of the state-of-the-art applications grouped by the geo-hydrological processes (levees and embankments failure, slopes and landslides, and ground subsidence) and the physical fields of interest. Both field applications and tests in physical models have been considered, providing features and issues of the implemented sensor systems. To support the reader in the comprehension of the monitoring approaches, we have provided some general information regarding the underlying driving mechanisms for each natural process here addressed.  
Finally the paper has been concluded with an analysis of open issues, where we have also discussed potential practical future directions. To extend the sensitivity to new measurands by novel transduction mechanisms integrated into fibre cables, to further improve the performances of interrogators, to introduce data analysis/interpretation models, and to implement installation tools and guidelines are among the most urgent actions to be undertaken.  
The author acknowledges the European Commission (Horizon 2020) and the Italian Ministry of Instruction, University and Research for financial support within the Water JPI and the WaterWork2014 Cofunded Call (project DOMINO). The author also thanks [PERSON], [PERSON] and [PERSON] for fruitful discussions.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
BOTDA Brillouin optical time domain analysis  
BOTDR Brillouin optical time domain reflectometry  
CW Continuous wave  
DAS Distributed acoustic sensor  
DOFS Distributed optical fibre sensor  
DSS Distributed strain sensor  
DTS Distributed temperature sensor  
DVS Distributed vibration sensor  
FBG Fibre Bragg grating  
I-OFDR Incoherent optical frequency domain reflectometry  
MMF Multi-mode fibre  
OFDR Optical frequency domain reflectometry  
OFS Optical fibre sensor  
OTDR Optical time domain reflectometry  
POF Polymer optical fibre  
POFDR Polarisation-sensitive optical frequency domain reflectometry  
\(\
u\)-OTDR Photon-counting optical time domain reflectometry  
\(\phi\)-OTDR Phase-optical time domain reflectometry  
POTDR Polarisation-sensitive optical time domain reflectometry  
RF Radio frequency  
SBS Stimulated Brillouin scattering  
SMF Single-mode fibre  
SNR Signal-to-noise ratio