Abstract
During the past decade, cosmic-ray neutron sensing technology has enabled researchers to reveal soil moisture spatial patterns and to estimate landscape-average soil moisture for hydrological and agricultural applications. However, reliance on rare materials such as helium-3 increases the cost of cosmic-ray neutron probes (CRNPs) and limits the adoption of this unique technology beyond the realm of academic research. In this study, we evaluated a novel lower cost CRNP based on moderated ultra-thin lithium-6 foil (Li foil system) technology against a commercially-available CRNP based on BF3 (boron trifluoride, BF-3 system). The study was conducted in a cropped field located in the Konza Prairie Biological Station near Manhattan, Kansas, USA (325 m a.s.l.) from 10 April 2020 to 18 June 2020. During this period the mean atmospheric pressure was 977 kPa, the mean air relative humidity was 70%, and the average volumetric soil water content was 0.277 m3 m−3. Raw fast neutron counts were corrected for atmospheric pressure, atmospheric water vapor, and incoming neutron flux. Calibration of the CRNPs was conducted using four intensive field surveys (n > 120), in combination with continuous observations from an existing array of in situ soil moisture sensors. The time series of uncorrected neutron counts of the Li foil system was highly correlated (r2 = 0.91) to that of the BF-3 system. The Li foil system had an average of 2,250 corrected neutron counts per hour with an uncertainty of 2.25%, values that are specific to the instrument size, detector configuration, and atmospheric conditions. The estimated volumetric water content from the Li foil system had a mean absolute difference of 0.022 m3 m−3 compared to the value from the array of in situ sensors. The new Li foil detector offers a promising lower cost alternative to existing cosmic-ray neutron detection devices used for hectometer-scale soil moisture monitoring.
Highlights
The strong inverse relationship between epithermal neutron intensity 1–2 m above the ground and the hydrogen pool in the upper decimeters of the soil has provided the basis for an emerging non-invasive method for quantifying soil moisture at the hectometer horizontal scale
Cosmic-ray neutron probes (CRNPs) have a large (∼12 hectares) footprint that fills the spatial niche between remote sensing soil moisture products and traditional point-level soil moisture sensors (Zreda et al, 2008; Desilets et al, 2010; Bogena et al, 2015)
The field test of a novel Li foil CRNP for soil moisture monitoring spanned a wide range of environmental conditions from 10 April to 18 June 2020 (∼69 days)
Summary
The strong inverse relationship between epithermal neutron intensity 1–2 m above the ground and the hydrogen pool in the upper decimeters of the soil has provided the basis for an emerging non-invasive method for quantifying soil moisture at the hectometer horizontal scale. Cosmic-ray neutron probes (CRNPs) have a large (∼12 hectares) footprint that fills the spatial niche between remote sensing soil moisture products (i.e., several kilometers) and traditional point-level (i.e., several decimeters) soil moisture sensors (Zreda et al, 2008; Desilets et al, 2010; Bogena et al, 2015). Stationary CRNPs have been widely adopted to quantify soil moisture conditions in forests (Bogena et al, 2013; Lv et al, 2014; Heidbüchel et al, 2016; Vather et al, 2019, 2020), cropland (Rivera Villarreyes et al, 2011; Zhu et al, 2015) and grassland (Hawdon et al, 2014; Montzka et al, 2017), and have been used in data assimilation studies to improve soil moisture estimates of a land surface model at the watershed level (Shuttleworth et al, 2013). A primary barrier for the widespread adoption of CRNPs for soil moisture sensing in and beyond the realm of academic research is the cost of the instrument
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