Exploring the potential of using low-energy cosmic-ray neutrons to monitor soil moisture dynamics in wetlands

In past two decades, Cosmic-Ray Neutron Sensing (CRNS) has evolved to a useful and promising approach to monitor soil moisture as well as snow water equivalents and biomass non-invasively at the hectometre-scale. Its large integration radius and average sensitive measurement depth of 20 to 30 cm allows for overcoming small-scale heterogeneities and for estimating soil moisture at spatio-temporal scales to e.g., inform environmental models or validate soil moisture products from remote sensing data.

CRNS relies on the inverse relationship between environmental hydrogen e.g., stored in soil moisture and the intensity of naturally occurring low-energy cosmic-ray neutrons. The relationship between soil moisture and neutron intensity is strongly non-linear which leads to larger uncertainties when the soil moisture is high. At the same time, neutron-to-soil moisture conversion functions have been developed for homogeneous soil moisture distributions which leads to larger uncertainties in soil moisture estimates for strongly heterogeneous conditions. Therefore, CRNS is expected to provide most accurate soil moisture estimates at monitoring sites with generally drier soils and homogeneous soil moisture distributions while knowledge gaps remain with respect to wet and heterogenous observation sites e.g., due to partial water cover.

Against this background, we investigate the signal dynamics of observed low-energy cosmic-ray neutron intensities at a wetland site in north-eastern Germany in order to gain understanding of the local background neutron flux and the potential to estimate soil moisture in water-free areas of wetland sites. Therefore, we monitor neutron intensities at two locations in the wetland with different partial water cover in the sensitive measurement radius of the individual neutron detectors, apply Monte-Carlo based neutron transport simulations and use field measurements of soil moisture to test and adjust existing neutron-to-soil moisture conversion functions to the specific conditions of the observation site.

Our analyses underline the potential of non-invasive CRNS for monitoring soil moisture dynamics in water-free areas of wetland sites which are generally considered unfavourable for the CRNS technique but also shed light on limitations at these observation sites.