A universal calibration function for determination of soil moisture with cosmic-ray neutrons

Abstract. A cosmic-ray soil moisture probe is usually calibrated locally using soil samples collected within its support volume. But such calibration may be difficult or impractical, for example when soil contains stones, in presence of bedrock outcrops, in urban environments, or when the probe is used as a rover. Here we use the neutron transport code MCNPx with observed soil chemistries and pore water distribution to derive a universal calibration function that can be used in such environments. Reasonable estimates of pore water content can be made from neutron intensity measurements and by using measurements of the other hydrogen pools (water vapor, soil lattice water, soil organic carbon, and biomass). Comparisons with independent soil moisture measurements at one cosmic-ray probe site and, separately, at 35 sites, show that the universal calibration function explains more than 79% of the total variability within each dataset, permitting accurate isolation of the soil moisture signal from the measured neutron intensity signal. In addition the framework allows for any of the other hydrogen pools to be separated from the neutron intensity measurements, which may be useful for estimating changes in biomass, biomass water, or exchangeable water in complex environments.

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A universal calibration function for determination of soil moisture with cosmic-ray neutrons

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-9-10303-2012 ◽

2012 ◽

Vol 9 (9) ◽

pp. 10303-10322 ◽

Cited By ~ 4

Author(s):

T. E. Franz ◽

M. Zreda ◽

R. Rosolem ◽

T. P. A. Ferre

Keyword(s):

Soil Moisture ◽

Water Distribution ◽

Neutron Transport ◽

Cosmic Ray ◽

Urban Environments ◽

Calibration Function ◽

Transport Code ◽

Probe Site ◽

Universal Calibration

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Field testing of the universal calibration function for determination of soil moisture with cosmic-ray neutrons

Water Resources Research ◽

10.1002/2014wr015513 ◽

2014 ◽

Vol 50 (6) ◽

pp. 5235-5248 ◽

Cited By ~ 29

Author(s):

David McJannet ◽

Trenton Franz ◽

Aaron Hawdon ◽

Dave Boadle ◽

Brett Baker ◽

...

Keyword(s):

Soil Moisture ◽

Cosmic Ray ◽

Field Testing ◽

Calibration Function ◽

Universal Calibration

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The Effect of Atmospheric Water Vapor on Neutron Count in the Cosmic-Ray Soil Moisture Observing System

Journal of Hydrometeorology ◽

10.1175/jhm-d-12-0120.1 ◽

2013 ◽

Vol 14 (5) ◽

pp. 1659-1671 ◽

Cited By ~ 83

Author(s):

R. Rosolem ◽

W. J. Shuttleworth ◽

M. Zreda ◽

T. E. Franz ◽

X. Zeng ◽

...

Keyword(s):

Soil Moisture ◽

Water Vapor ◽

Neutron Transport ◽

Cosmic Ray ◽

Atmospheric Water Vapor ◽

Fast Neutrons ◽

Atmospheric Conditions ◽

Atmospheric Water ◽

Neutron Measurement ◽

Neutron Intensity

Abstract The cosmic-ray method for measuring soil moisture, used in the Cosmic-Ray Soil Moisture Observing System (COSMOS), relies on the exceptional ability of hydrogen to moderate fast neutrons. Sources of hydrogen near the ground, other than soil moisture, affect the neutron measurement and therefore must be quantified. This study investigates the effect of atmospheric water vapor on the cosmic-ray probe signal and evaluates the fast neutron response in realistic atmospheric conditions using the neutron transport code Monte Carlo N-Particle eXtended (MCNPX). The vertical height of influence of the sensor in the atmosphere varies between 412 and 265 m in dry and wet atmospheres, respectively. Model results show that atmospheric water vapor near the surface affects the neutron intensity signal by up to 12%, corresponding to soil moisture differences on the order of 0.10 m3 m−3. A simple correction is defined to identify the true signal associated with integrated soil moisture that rescales the measured neutron intensity to that which would have been observed in the atmospheric conditions prevailing on the day of sensor calibration. Use of this approach is investigated with in situ observations at two sites characterized by strong seasonality in water vapor where standard meteorological measurements are readily available.

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Can canopy interception and biomass be inferred from cosmic-ray neutron intensity? Results from neutron transport modeling

10.5194/hess-2016-226 ◽

2016 ◽

Cited By ~ 1

Author(s):

Mie Andreasen ◽

Karsten H. Jensen ◽

Darin Desilets ◽

Marek Zreda ◽

Heye Bogena ◽

...

Keyword(s):

Soil Moisture ◽

Epithermal Neutron ◽

Neutron Transport ◽

Forest Biomass ◽

Cosmic Ray ◽

Ground Level ◽

Field Site ◽

Canopy Interception ◽

Soil Matrix ◽

Neutron Intensity

Abstract. Cosmic-ray neutron intensity is inversely correlated to all hydrogen present in the upper decimeters of the subsurface and the first few hectometers of the atmosphere above the ground surface. This method has been used for measuring soil moisture and snow water equivalent, but it may also be used to identify and quantify canopy interception and biomass. We use a neutron transport model with various representations of the forest and different parameters describing the subsurface to match measured profiles and time series of thermal and epithermal neutron intensities at a field site in Denmark. A sensitivity analysis is performed to quantify the effect of forest canopy representation, soil moisture, complexity of soil matrix chemistry, forest litter, soil bulk density, canopy interception and forest biomass on neutron intensity. The results show that forest biomass has a significant influence on the neutron intensity profiles at the examined field site, altering both the shape of the profiles and the ground level thermal-to-epithermal neutron ratio. The ground level thermal-to-epithermal neutron ratio increases significantly with increasing amounts of biomass and minor with canopy interception. Satisfactory agreement is found between measurements and model results at the forest site as well as two nearby sites representing agricultural and heathland ecosystems. The measured ground level thermal-to-epithermal neutron ratios of the three site range from around 0.56 to 0.82. The significantly smaller effect of canopy interception on the ground level thermal-to-epithermal neutron ratio was modeled to range from 0.804 to 0.836 for a forest with a dry and a very wet canopy (4 mm of canopy interception), respectively. At the examined field site the signal of the canopy interception is lower than the measurement uncertainty.

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Moisture and humidity dependence of the above-ground cosmic-ray neutron intensity

10.5194/egusphere-egu2020-17856 ◽

2020 ◽

Author(s):

Jannis Weimar ◽

Markus Köhli ◽

Martin Schrön ◽

Ulrich Schmidt

Keyword(s):

Monte Carlo ◽

Soil Moisture ◽

Neutron Transport ◽

Cosmic Ray ◽

Data Sets ◽

Neutron Intensity ◽

Alpine Regions ◽

Neutron Count ◽

Snow Water ◽

Humidity Dependence

The novel method of Cosmic-ray neutron sensing (CRNS) allows non-invasive soil moisture measurements at a hectometer scaled footprint. Using this technique one can relate the flux density of albedo neutrons, generated in cosmic-ray induced air showers, to the amount of water within a radius of several hundred meters. In the recent years the understanding of neutron transport by Monte Carlo simulations led to major advancements in precision, which have successfully targeted a manifold of use cases. For example the improvements in the signal interpretation have meanwhile also been applied to the determination of snow water in Alpine regions. Up to now, the conversion of soil moisture to a detectable neutron count rate relies mainly on the equation presented by Desilets and Zreda. While in general a hyperbolic expression can be derived from theoretical considerations, their empiric parameterisation needs to be revised as many groups have found site-specific calibrations, which are simply based on different empirical data sets.Investigating the above-ground neutron intensity by 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 sensor response function. The resulting formula significantly improves the soil-moisture-to-intensity conversion and allows for a more comprehensive instrument data quality, which especially closes the gap between observations of very dry and wet conditions.

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Cosmic-ray neutron transport at a forest field site: the sensitivity to various environmental conditions with focus on biomass and canopy interception

Hydrology and Earth System Sciences ◽

10.5194/hess-21-1875-2017 ◽

2017 ◽

Vol 21 (4) ◽

pp. 1875-1894 ◽

Cited By ~ 17

Author(s):

Mie Andreasen ◽

Karsten H. Jensen ◽

Darin Desilets ◽

Marek Zreda ◽

Heye R. Bogena ◽

...

Keyword(s):

Soil Moisture ◽

Epithermal Neutron ◽

Neutron Transport ◽

Estimation Method ◽

Cosmic Ray ◽

Ground Level ◽

Field Site ◽

Canopy Interception ◽

Neutron Intensity ◽

Moisture Estimation

Abstract. Cosmic-ray neutron intensity is inversely correlated to all hydrogen present in the upper decimeters of the subsurface and the first few hectometers of the atmosphere above the ground surface. This correlation forms the base of the cosmic-ray neutron soil moisture estimation method. The method is, however, complicated by the fact that several hydrogen pools other than soil moisture affect the neutron intensity. In order to improve the cosmic-ray neutron soil moisture estimation method and explore the potential for additional applications, knowledge about the environmental effect on cosmic-ray neutron intensity is essential (e.g., the effect of vegetation, litter layer and soil type). In this study the environmental effect is examined by performing a sensitivity analysis using neutron transport modeling. We use a neutron transport model with various representations of the forest and different parameters describing the subsurface to match measured height profiles and time series of thermal and epithermal neutron intensities at a field site in Denmark. Overall, modeled thermal and epithermal neutron intensities are in satisfactory agreement with measurements; however, the choice of forest canopy conceptualization is found to be significant. Modeling results show that the effect of canopy interception, soil chemistry and dry bulk density of litter and mineral soil on neutron intensity is small. On the other hand, the neutron intensity decreases significantly with added litter-layer thickness, especially for epithermal neutron energies. Forest biomass also has a significant influence on the neutron intensity height profiles at the examined field site, altering both the shape of the profiles and the ground-level thermal-to-epithermal neutron ratio. This ratio increases with increasing amounts of biomass, and was confirmed by measurements from three sites representing agricultural, heathland and forest land cover. A much smaller effect of canopy interception on the ground-level thermal-to-epithermal neutron ratio was modeled. Overall, the results suggest a potential to use ground-level thermal-to-epithermal neutron ratios to discriminate the effect of different hydrogen contributions on the neutron signal.

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Cosmic-ray neutron intensity measurements of soil moisture – A case study in the Skjern catchment, Denmark

10.1190/segam2014-1154.1 ◽

2014 ◽

Author(s):

Mie Andreasen ◽

Majken Caroline Looms* ◽

Karsten Høgh Jensen ◽

Torben O. Sonnenborg ◽

Heye Bogena ◽

...

Keyword(s):

Soil Moisture ◽

Cosmic Ray ◽

Neutron Intensity ◽

Intensity Measurements

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Investigating temporal field sampling strategies for site-specific calibration of three soil moisture–neutron intensity parameterisation methods

Hydrology and Earth System Sciences ◽

10.5194/hess-19-3203-2015 ◽

2015 ◽

Vol 19 (7) ◽

pp. 3203-3216 ◽

Cited By ~ 17

Author(s):

J. Iwema ◽

R. Rosolem ◽

R. Baatz ◽

T. Wagener ◽

H. R. Bogena

Keyword(s):

Remote Sensing ◽

Soil Moisture ◽

Satellite Remote Sensing ◽

Cosmic Ray ◽

Sampling Strategies ◽

Site Specific ◽

Neutron Intensity ◽

Soil Wetness ◽

Semi Arid

Abstract. The Cosmic-Ray Neutron Sensor (CRNS) can provide soil moisture information at scales relevant to hydrometeorological modelling applications. Site-specific calibration is needed to translate CRNS neutron intensities into sensor footprint average soil moisture contents. We investigated temporal sampling strategies for calibration of three CRNS parameterisations (modified N0, HMF, and COSMIC) by assessing the effects of the number of sampling days and soil wetness conditions on the performance of the calibration results while investigating actual neutron intensity measurements, for three sites with distinct climate and land use: a semi-arid site, a temperate grassland, and a temperate forest. When calibrated with 1 year of data, both COSMIC and the modified N0 method performed better than HMF. The performance of COSMIC was remarkably good at the semi-arid site in the USA, while the N0mod performed best at the two temperate sites in Germany. The successful performance of COSMIC at all three sites can be attributed to the benefits of explicitly resolving individual soil layers (which is not accounted for in the other two parameterisations). To better calibrate these parameterisations, we recommend in situ soil sampled to be collected on more than a single day. However, little improvement is observed for sampling on more than 6 days. At the semi-arid site, the N0mod method was calibrated better under site-specific average wetness conditions, whereas HMF and COSMIC were calibrated better under drier conditions. Average soil wetness condition gave better calibration results at the two humid sites. The calibration results for the HMF method were better when calibrated with combinations of days with similar soil wetness conditions, opposed to N0mod and COSMIC, which profited from using days with distinct wetness conditions. Errors in actual neutron intensities were translated to average errors specifically to each site. At the semi-arid site, these errors were below the typical measurement uncertainties from in situ point-scale sensors and satellite remote sensing products. Nevertheless, at the two humid sites, reduction in uncertainty with increasing sampling days only reached typical errors associated with satellite remote sensing products. The outcomes of this study can be used by researchers as a CRNS calibration strategy guideline.

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Error estimation for soil moisture measurements with cosmic-ray neutron sensing and implications for rover surveys

10.5194/egusphere-egu2020-8488 ◽

2020 ◽

Author(s):

Jannis Jakobi ◽

Johan Alexander Huisman ◽

Martin Schrön ◽

Justus Fiedler ◽

Cosimo Brogi ◽

...

Keyword(s):

Soil Moisture ◽

Measurement Uncertainty ◽

Survey Design ◽

Time Windows ◽

Inverse Correlation ◽

Cosmic Ray ◽

Environmental Water ◽

Neutron Measurement ◽

Neutron Intensity ◽

Computationally Intensive

The cosmic ray neutron (CRN) probe is a non-invasive device to measure soil moisture at the field scale. This instrument relies on the inverse correlation between aboveground epithermal neutron intensity (1eV &#8211; 100 keV) and environmental water content. The measurement uncertainty of the neutron detector follows Poisson statistics and thus decreases with decreasing neutron intensity, which corresponds to increasing soil moisture. In order to reduce measurement uncertainty (e.g. < 0.03 m3/m3), the neutron count rate is often aggregated over large time windows (e.g. 12h or 24h). To enable shorter aggregation intervals, the measurement uncertainty can be reduced either by using more efficient detectors or by using arrays of detectors, as in the case of CRN rover applications. Depending on soil moisture and driving speed, aggregation of neutron counts may also be necessary to obtain sufficiently accurate soil moisture estimates in rover applications. To date, signal aggregation has not been investigated sufficiently with respect to the optimisation of temporal (stationary probes) and spatial (roving applications) resolution. In this work, we present an easy-to-use method for uncertainty quantification of soil moisture observations from CRN sensors based on Gaussian error propagation theory. We have estimated the uncertainty using a third order Taylor expansion and compared the result with a more computationally intensive Monte Carlo approach and found excellent agreement. Furthermore, we used our method to quantify the dependence of soil moisture uncertainty on CRN rover survey design and on selected aggregation time. We anticipate that the new approach helps to quantify cosmic ray neutron measurement uncertainty. In particular, it is anticipated that the strategic planning and evaluation of CRN rover surveys based on uncertainty requirements can be improved considerably.

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