Soil moisture measurements by Cosmic-Ray neutron sensing: A critical review
Markus Köhli
Abstract
Soil moisture information is crucial for water management in agricultural and hydrological systems. Several measurement methods exist on multiple scales, however, poorly harmonized. Despite its significance this lack of precision marginalized the topologically complex assessment of soil water. In addition, there is an urgent need for real-time, high-quality, temporally and spatially consistent data on soil moisture. Such are needed to optimize water management strategies as well as climate change monitoring, modeling and mitigation. Over the last decade, Cosmic-Ray Neutron probes have been recognized as a promising tool for soil moisture measurements due to the large footprint of several hectares and half a meter in depth. Using this technique one can relate the flux density of above-ground neutrons generated by cosmic-ray induced air showers to the amount of water in the environment. Its strengths are the non-invasive nature of the method and the low-maintenance sensors. National and international monitoring networks like COSMOS, COSMOS-UK, ADAPTER and TERENO sites are on the forefront of integrating these sensors into their monitoring program. To address the need of harmonization of this new methodology, the ‘Soil Moisture Metrology’ (SoMMet) consortium establishes the metrological basis of the Cosmic-Ray Neutron Sensing and its chain of traceability. Initially, Cosmic-Ray Neutron instruments had relied on the use of the scarce helium-3. In order to scale up the method and to reduce costs, research groups in Europe have developed instruments using alternative technologies including readout electronics and data acquisition systems. With a more cost-efficient instrumentation the initial focus on hydrological research Cosmic-Ray Neutron instruments emerge into applied contexts. Examples are irrigation management and soil moisture mapping for climate-resilient agriculture. Still, challenges arise especially for irrigated fields with a size smaller than the footprint or heterogeneous conditions, especially biomass distribution.