(INVITED)Advances in fiber optic sensors for soil moisture monitoring: A review

Real time measurements of soil water content play a critical role in many fields of science as agronomy, geology, engineering, and hydrology. In the last years, agriculture has become one of the most important application fields of soil water sensors technology in order to optimize the irrigation process and to guarantee sustainable water resources management. This work provides a review on the latest emerging methodologies based on optical fiber sensing for soil moisture monitoring for agricultural and hydrological applications. In particular, the main studies referring to optical fiber sensors based on a variation of the refractive index of the external medium, sensors based on heated distributed temperature sensor (HDTS) and sensors based on Fiber Bragg Gratings (FBG) or Long Period Gratings (LPG) are here explored. Finally, some approaches based on the NIR absorbance spectroscopy are proposed for measuring the soil water content. Most of these approaches and technologies are still in a prototyping phase and only few of them are properly evaluated in situ real context.