Over-Water Low-Energy Neutron Observations for Intensity Corrections Across Cosmic-Ray Soil Moisture Sensor Networks

Most studies using cosmic-ray neutron sensors (CRNS) for soil moisture estimation use high-energy neutron monitor observations to correct for changes in incoming neutron intensity, but there is interest in over-water CRNS observations and muon observations for such purposes. This study compares these approaches with a focus on observations from an over-water pontoon-based CRNS system. Pontoon and neutron monitor intensity comparisons showed similar responses with the best statistical agreement when neutron monitor observations were from locations of similar cutoff rigidity or when scaling for geomagnetic and elevational effects were applied. Comparison of historic variations in neutron monitor and muon detector intensity, and more recent observations from the pontoon, revealed temporal differences and weaker short-term responses from the muon detector. Time-delays in intensity correction for the pontoon and neutron monitors were observed during a Forbush decrease and through cross-correlation analysis over the comparison period with delays likely a result of longitudinal differences. Pontoon neutron intensity exhibited slightly higher amplitudes over the study period. Some of this was related to periods of irregular water vapour distribution in the atmosphere where current humidity corrections appear insufficient. Application of intensity corrections to soil moisture estimates illustrated the increasing importance of accurate corrections with decreasing cutoff rigidity and increasing elevation. The impact of neutron intensity correction was greatest for wet soil conditions at low cutoff rigidity sites at higher elevations. Over-water CRNS observations offer a means to correct CRNS observations with the advantages of being locally managed, locally applicable, and directly relevant to CRNS energy spectra.