Toward Large‐Scale Soil Moisture Monitoring Using Rail‐Based Cosmic Ray Neutron Sensing

Abstract

Cosmic ray neutron sensing (CRNS) has become a promising method for soil water content (SWC) monitoring. Stationary CRNS offers hectare‐scale average SWC measurements at fixed locations maintenance‐free and continuous in time, while car‐borne CRNS roving can reveal spatial SWC patterns at medium scales, but only on certain survey days. The novel concept of a permanent mobile CRNS system on rails promises to combine the advantages of both methods, while its technical implementation, data processing and interpretation raised a new level of complexity. This study introduced a fully automatic CRNS rail‐borne system as the first of its kind, installed within the locomotive of a cargo train. Data recorded from September 2021 to July 2022 along an ∼9 km railway segment were analyzed, as repeatedly used by the train, supported by local SWC measurements (soil samples and dielectric methods), car‐borne and stationary CRNS. The results revealed consistent spatial SWC patterns and temporary variation along the track at a daily resolution. The observed variability was mostly related to surface features, seasonal dynamics and different responses of the railway segments to wetting and drying periods, while some variations were related to measurement uncertainties. The achieved medium scale of SWC mapping could support large scale hydrological modeling and detection of environmental risks, such as droughts and wildfires. Hence, rail‐borne CRNS has the chance to become a central tool of continuous SWC monitoring for larger scales (≤10‐km), with the additional benefit of providing root‐zone soil moisture, potentially even in sub‐daily resolution.

Incoming Neutron Flux Corrections for Cosmic‐Ray Soil and Snow Sensors Using the Global Neutron Monitor Network

“…(2021) and Altdorff et al. (2023)), a spatially varying correction could be applied to account for atmospheric and geomagnetic variations, with the added advantage that the input data required for the new neutron intensity correction (i.e., latitude, longitude and elevations) will already be collected as part of the standard rover measurement procedures.…”

“…The approach developed also lends itself to application in roving CRNS studies. In rover surveys where vast distances and varying landscapes can be covered (e.g., the transregional, train-based CRNS surveys of Schrön et al (2021) and Altdorff et al (2023)), a spatially varying correction could be applied to account for atmospheric and geomagnetic variations, with the added advantage that the input data required for the new neutron intensity correction (i.e., latitude, longitude and elevations) will already be collected as part of the standard rover measurement procedures.…”

Improved representation of soil moisture simulations through incorporation of cosmic-ray neutron count measurements in a large-scale hydrologic model

“…The next step in this research is to evaluate the ability of this CRNS module in mHM for estimating soil moisture through a large-scale soil moisture monitoring initiative, e.g. by utilizing more stationary CRNS networks or the novel rail-based CRNS data from Altdorff et al (2023). Improving the model predictions will contribute to reducing the uncertainties associated with drought and flood management strategies and informed agricultural decisions.…”

Fifteen Years of Integrated Terrestrial Environmental Observatories (TERENO) in Germany: Functions, Services, and Lessons Learned

“…The CRNS field application is now a worldwide standard. TERENO research projects advanced the use of CRNS by further developing the theory and applications: redefine the sensor footprint (Köhli et al., 2015; Schrön et al., 2023); assess “road effects,” which can lead to an underestimation of soil moisture at complex sites or in mobile CRNS operations (Schrön et al., 2018); assess the influence of water in the litter layer or biomass on the CRNS signal (Baatz et al., 2015; Bogena et al., 2013); develop a new CRNS sensor downhole method (Rasche et al., 2023); or assess soil moisture measured along transects using permanent CRNS installations on trains for the first time (Altdorff et al., 2023). Since 2008, ISI Web of Science (WoS) listed a total of 186 published articles with German Helmholtz Association members contributing the largest share of publications and citations (29%, according to WoS where “abstract” includes “cosmic” and “ray” and “neutron” and “soil” or “snow,” accessed 18 November 2023).…”