Connecting Scales of Soil-Moisture Measurements by Cosmic Rays Neutron Sensing

Cosmic Rays Neutron Sensing (CRNS) emerged among proximal sensors as a reliable option for filling the scale gap between point probes and remote sensing. CRNS consists in detecting ambient neutrons close to the land surface, generated by the flow of high-energy particles from space. By placing a non-invasive detector above ground, the albedo of ambient neutron is collected. Thanks to the strong absorption of epithermal neutron by hydrogen in water molecules, the neutron count rate can be converted into a value of Soil Moisture (SM) averaged in the large volume of soil probed by the neutrons. A single CRNS probe can effectively monitor SM in an area up to a dozen hectares and a depth up to 50 cm, which defines an intermediate spatial scale in between point measurements and remote sensing, while also granting continuous measurement and a sub-daily temporal resolution. The measurement provided by a single probe can be representative of a whole agricultural or environmental monitoring site. However, it is the synergy between different technologies across scales that can boost the overall value of measurement systems. Local measurements like point-scale probes and soil sampling campaigns, together with a deep knowledge of the site, allow to interpret and validate the CRNS observed dynamics and in turn validate hydrological modelling. On the other hand, the horizontal spatial scale of the CRNS footprint is fairly comparable with remote sensing pixel, opening the possibility to interpolate data between satellite passes and, if a CRNS network is available, between multiple ground validation points. We here report observations from a set of sites in Northern Italy where CRNS probes manufactured by Finapp were installed. The sites span a variety of landscapes, soils and land uses (including agricultural, vineyard, forest, high-altitude grassland) and are monitored by a variety of approaches including capacitive probes, manual sampling campaigns, hydrological modelling (CLM5 and HYDRUS-1D model), and remote sensing products (ERA-5-Land, SMAP). The observation and comparison of different study cases show how the integration of CRNS with the other techniques can provide a more complete and reliable understanding of soil moisture dynamics at a local scale, making it possible to extend these understandings to a regional scale.