Abstract
Investigations of neutron transport through air and soil by Monte Carlo simulations led to major advancements toward a precise interpretation of measurements; they particularly improved the understanding of the cosmic-ray neutron footprint. Up to now, the conversion of soil moisture to a detectable neutron count rate has relied mainly on the equation presented by Desilets and Zreda in 2010. While in general a hyperbolic expression can be derived from theoretical considerations, their empiric parameterization needs to be revised for two reasons. Firstly, a rigorous mathematical treatment reveals that the values of the four parameters are ambiguous because their values are not independent. We found a three-parameter equation with unambiguous values of the parameters that is equivalent in any other respect to the four-parameter equation. Secondly, high-resolution Monte-Carlo simulations revealed a systematic deviation of the count rate to soil moisture relation especially for extremely dry conditions as well as very humid conditions. That is a hint that a smaller contribution to the intensity was forgotten or not adequately treated by the conventional approach. Investigating the above-ground neutron flux through a broadly based Monte-Carlo simulation campaign revealed a more detailed understanding of different contributions to this signal, especially targeting air humidity corrections. The packages MCNP and URANOS were used to derive a function able to describe the respective dependencies, including the effect of different hydrogen pools and the detector-specific response function. The new relationship has been tested at two exemplary measurement sites, and its remarkable performance allows for a promising prospect of more comprehensive data quality in the future.
Highlights
Techniques for determining the environmental water content are mostly bound to local instrumentation or remote sensing products, neither of which meet the typical correlation lengths for soil moisture
The provided ratios cover a range of soil moisture values at a fixed air humidity and a range of air humidity at a fixed soil moisture value, while both quantities have been increased from the lower to the upper bound
Within the parameter space of this study, the relative intensity change for scaling water vapor lies in all cases in the order of 20% for 1 g/m3 → 35 g/m3. This observation can be attributed to the fact that, for dry conditions, neutrons travel much longer paths and start with higher energies, both of which increase the transport through air, which, in case of a decreasing vertical humidity profile, is even amplified
Summary
Techniques for determining the environmental water content are mostly bound to local instrumentation or remote sensing products, neither of which meet the typical correlation lengths for soil moisture. This lack of spatial coverage makes the interpretation of available data difficult (Vereecken et al, 2008), and it is called the intermediate scale gap (Robinson et al, 2008). CRNS is based on the principle that neutrons in the epithermal-to-fast energy range (1–105 eV) are highly sensitive to hydrogen, which turns neutron detectors into efficient proxies for changes of the environmental water content It follows an inverse relationship between the above-ground epithermalto-fast cosmic-ray neutron intensity N and the surrounding amount of hydrogen atoms, i.e., predominantly the volumetric water content θ (cm3/cm). The originally proposed N0 method by Desilets et al (2010)
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