Cosmic-ray neutron probes on the high plains of Nebraska: applications to large scale agriculture

Cosmic-rays have some surprising applications in precision agriculture. The cosmic-ray neutron probe (CRNP), when implemented as a roving instrument, can be used to create spatial maps of soil moisture, and from these maps soil hydraulic properties can be inferred. In this work, we combine data from a mobile CRNP with laboratory samples to make spatial predictions of soil hydraulic properties for select field sites around the state of Nebraska. These maps, which focus on wilting point and field capacity, can, in turn, be used to determine the optimal timing and application rates for irrigation farmers, many of whom have the capability to finely tune the spatial distribution of water applied on a field, but currently lack the requisite data to support such management practices. We find that 4 CRNP soil moisture maps are adequate to describe the dominant underlying spatial structure of the field (\u3e75% of variability) using Empirical Orthogonal Functions. The CRNP soil moisture maps combined with an elevation layer provided strong statistical predictors of laboratory measured soil hydraulic properties. The economic viability of the method depends on numerous local cost factors but rising demand for water resources may dictate the need for innovative approaches such as this one to reduce future water use

Elevation dependence of cosmogenic 36Cl production in Hawaiian lava flows

Abstract We measured an elevation profile of cosmogenic 36 Cl in two well-preserved lava flows on Mauna Kea, Hawaii (19.8°N, 155.5°W ) in order to directly constrain the elevation dependence of cosmogenic nuclide production rates. The flows are verticallyextensive hawaiites erupted at 40.1 ± 0.6 and 62.2 ± 1.0 ka from point-vents on the upper flanks of Mauna Kea. The average paleo cutoff rigidity (a measure of geomagnetic shielding of cosmic rays) for these flows is 11 GV and their paleo-elevation range is 2100-3700 m. Production of 36 Cl is dominated by neutron reactions, with the high-energy 39 K(n,x) and 40 Ca(n,x) mechanisms accounting for nearly half of the 36 Cl production and the low-energy reaction 35 Cl(n,γ) responsible for the remaining half. Production by negative muons is small at the elevations of our samples, accounting for less than 2% of the total production in the lowest elevation samples. The elevation dependence of 36 Cl production measured in these lava flows is described by an effective attenuation length of 138 ± 5 g cm − 2 . This result is close to the value of 140 g cm − 2 determined from neutron monitor surveys of high-energy nucleon fluxes, but significantly below the value of 149 g cm − 2 determined from measurements of low-energy neutrons. The predicted atmospheric attenuation length for these lava flows, incorporating both high-and low-energy mechanisms, is 144 g cm − 2 . The good agreement between the 36 Cl elevation profile and cosmic-ray surveys validates the use of neutron flux measurements to scale 36 Cl production rates when production by muons is negligible

Using Cosmic-Ray Neutron Probes to Monitor Landscape Scale Soil Water Content in Mixed Land Use Agricultural Systems

With an ever-increasing demand for natural resources and the societal need to understand and predict natural disasters, soil water content (SWC) observations remain a critical variable to monitor in order to optimally allocate resources, establish early warning systems, and improve weather forecasts.However, routine agricultural production practices of soil cultivation, planting, and harvest make the operation andmaintenance of direct contact point sensors for long-termmonitoring challenging. In this work, we explore the use of the newly established Cosmic-Ray Neutron Probe (CRNP) and method to monitor landscape average SWC in a mixed agricultural land use systemin northeastAustria.Thecalibrated CRNP landscape SWC values compare well against an independent in situ SWC probe network (MAE = 0.0286m3/m3) given the challenge of continuous in situ monitoring from probes across a heterogeneous agricultural landscape. The ability of the CRNP to provide real-time and accurate landscape SWC measurements makes it an ideal method for establishing long-term monitoring sites in agricultural ecosystems to aid in agricultural water and nutrient management decisions at the small tract of land scale as well as aiding in management decisions at larger scales

The global distribution of secondary cosmic rays and applications to cosmogenic dating

Methods of surface exposure dating usmg terrestrial cosmogemc nuclides require accurate knowledge of the spatial variability of nuclide production rates. The nucleon component of the secondary cosmic-ray flux is responsible for a major fraction of terres trial nuclide production and this component is particularly sensitive to variations in alti tude and position in the geomagnetic field. To be applied at widely different locations, calibrated production rates must be scaled to account for variations in cosmic-ray intensity due to these two factors. Current scaling models are based on a small number of nuclear emulsion and cloud chamber measurements and data from BF3 proportional counters that are mostly limited to altitudes above 3,000 m. Over the past 50 years, however, there have been numerous alti tude and latitude surveys with neutron monitors as well as additional measurements with proportional counters. These surveys not only describe more precisely how the nucleon flux depends on altitude and geomagnetic position, but also provide valuable data on the energy spectrum and on the effects of solar activity. This work utilizes more recent and more extensive measurements of nucleon intensity along with an improved understanding of cosmic-ray phenomena to derive scaling models for thermal neutron absorption reactions and high-energy spallation reactions. Latitude data are ordered according to effective vertical cutoff rigidity [GV] and altitude data according to mass shielding depth [g cm2 ]. Neutron monitor data are corrected for instrumental biases and parameterized using polynomials. Attenuation lengths for thermal neutrons are greater than for high-energy neutrons by 15% at 3800 m and 14 GV, whereas the difference at sea level is estimated to be negligible at all latitudesDigitized from paper copies provided by the Department of Hydrology & Atmospheric Sciences

Cosmogenic nuclides as a surface exposure dating tool: improved altitude/latitude scaling factors for production rates

Applications of in situ cosmogenic nuclides to problems in Quaternary geology require increasingly accurate and precise knowledge of nuclide production rates. Production rates depend on the terrestrial cosmic-ray intensity, which is a function of the elevation and geomagnetic coordinates of a sample site and the geomagnetic field intensity. The main goal of this dissertation is to improve the accuracy of cosmogenic dating by providing better constraints on the spatial variability of production rates.In this dissertation I develop a new scaling model that incorporates the best available cosmic-ray data into a framework that better describes the effects of elevation and geomagnetic shielding on production rates. This model is based on extensive measurements of energetic nucleon fluxes from neutron monitor surveys and on more limited data from low-energy neutron surveys. A major finding of this work is that neutron monitors yield scaling factors different from unshielded proportional counters. To verify that the difference is real I conducted an airborne survey of low-energy neutron fluxes at Hawaii (19.7° N 155.5° W) to compare with a nearby benchmark neutron monitor survey. Our data confirm that the attenuation length is energy dependent and suggest that the scaling factor for energetic nucleons is 10% higher between sea level and 4000 m than for low-energy neutrons at this location. An altitude profile of cosmogenic 36Cl production from lava flows on Mauna Kea, Hawaii, support the use of neutron flux measurements to scale production rates but these data do not have enough precision to confirm or reject the hypothesis of energy-dependent scaling factors

Radius of influence for a cosmic-ray soil moisture probe : theory and Monte Carlo simulations.

The lateral footprint of a cosmic-ray soil moisture probe was determined using diffusion theory and neutron transport simulations. The footprint is radial and can be described by a single parameter, an e-folding length that is closely related to the slowing down length in air. In our work the slowing down length is defined as the crow-flight distance traveled by a neutron from nuclear emission as a fast neutron to detection at a lower energy threshold defined by the detector. Here the footprint is defined as the area encompassed by two e-fold distances, i.e. the area from which 86% of the recorded neutrons originate. The slowing down length is approximately 150 m at sea level for neutrons detected over a wide range of energies - from 10{sup 0} to 10{sup 5} eV. Both theory and simulations indicate that the slowing down length is inversely proportional to air density and linearly proportional to the height of the sensor above the ground for heights up to 100 m. Simulations suggest that the radius of influence for neutrons >1 eV is only slightly influenced by soil moisture content, and depends weakly on the energy sensitivity of the neutron detector. Good agreement between the theoretical slowing down length in air and the simulated slowing down length near the air/ground interface support the conclusion that the footprint is determined mainly by the neutron scattering properties of air

Cosmic-ray neutron probes on the high plains of Nebraska: applications to large scale agriculture

Cosmic-rays have some surprising applications in precision agriculture. The cosmic-ray neutron probe (CRNP), when implemented as a roving instrument, can be used to create spatial maps of soil moisture, and from these maps soil hydraulic properties can be inferred. In this work, we combine data from a mobile CRNP with laboratory samples to make spatial predictions of soil hydraulic properties for select field sites around the state of Nebraska. These maps, which focus on wilting point and field capacity, can, in turn, be used to determine the optimal timing and application rates for irrigation farmers, many of whom have the capability to finely tune the spatial distribution of water applied on a field, but currently lack the requisite data to support such management practices. We find that 4 CRNP soil moisture maps are adequate to describe the dominant underlying spatial structure of the field (\u3e75% of variability) using Empirical Orthogonal Functions. The CRNP soil moisture maps combined with an elevation layer provided strong statistical predictors of laboratory measured soil hydraulic properties. The economic viability of the method depends on numerous local cost factors but rising demand for water resources may dictate the need for innovative approaches such as this one to reduce future water use

Continuous and autonomous snow water equivalent measurements by a cosmic ray sensor on an alpine glacier

Snow water equivalent (SWE) measurements of seasonal snowpack are crucial in many research fields. Yet accurate measurements at a high temporal resolution are difficult to obtain in high mountain regions. With a cosmic ray sensor (CRS), SWE can be inferred from neutron counts. We present the analyses of temporally continuous SWE measurements by a CRS on an alpine glacier in Switzerland (Glacier de la Plaine Morte) over two winter seasons (2016/17 and 2017/18), which differed markedly in the amount and timing of snow accumulation. By combining SWE with snow depth measurements, we calculate the daily mean density of the snowpack. Compared to manual field observations from snow pits, the autonomous measurements overestimate SWE by +2 % ± 13 %. Snow depth and the bulk snow density deviate from the manual measurements by ±6 % and ±9 %, respectively. The CRS measured with high reliability over two winter seasons and is thus considered a promising method to observe SWE at remote alpine sites. We use the daily observations to classify winter season days into those dominated by accumulation (solid precipitation, snow drift), ablation (snow drift, snowmelt) or snow densification. For each of these process-dominated days the prevailing meteorological conditions are distinct. The continuous SWE measurements were also used to define a scaling factor for precipitation amounts from nearby meteorological stations. With this analysis, we show that a best-possible constant scaling factor results in cumulative precipitation amounts that differ by a mean absolute error of less than 80 mm w.e. from snow accumulation at this site