Neutrons at the dry end: sensing water content with cosmic rays in the Negev desert

The precise measurement of soil water content is a technological challenge. Traditional electromagnetic or remote-sensing approaches exhibit a range of problems, such as not covering the soil spatial heterogeneity, sensing only shallow soil horizons, exhibiting strong sensitivity to temperature, or losing accuracy under extremely dry conditions. A new approach based on cosmic-ray neutron sensing technology (CRNS) has promised to address most of these issues. For the first time, the method has been tested in the Negev desert in Israel to determine its potential and limitations under extremely dry conditions.

The soil moisture product of a stationary CRNS system agreed reasonably well with nearby TDR measurements during the rain and dry seasons between 2021 and 2023. However, it turned out that the sensor showed significant sensitivity to changes of air humidity and incoming radiation that were not covered by existing correction approaches. We found new correction parameters to remove these effects and thereby achieved a measurement precision of less than 0.25 vol. % at soil moisture values below 5 vol. %. In a mobile mode, the method also allowed to identify wet spots in soils in otherwise dry environments. Once the processing of CRNS data has been adapted to the special regional conditions of Israel, it has the potential for hydrological and agricultural applications.