The Applicability of the Cosmic Ray Neutron Sensor to Simultaneously Monitor Soil Water Content and Biomass in an Acacia mearnsii Forest

Abstract

Soil water content is an important hydrological parameter, which is difficult to measure at a field scale due to its spatial and temporal heterogeneity. The Cosmic Ray Neutron Sensor (CRNS) is a novel and innovative approach to estimate area-averaged soil water content at an intermediate scale, which has been implemented across the globe. The CRNS is moderated by all hydrogen sources within its measurement footprint. In order to isolate the soil water content signal from the neutron intensity, the other sources of hydrogen need to be accounted for. The CRNS’s applications are not only limited to soil water content estimation, as it can potentially be used to monitor biomass. The Two-Streams clear-felling provided the unique opportunity to monitor the cosmic ray neutron intensities before, during, and after the clear-felling. The cadmium-difference method was used to obtain the pure thermal and epithermal neutron intensities from the bare and moderated detectors. The study concluded that the presence of biomass within the site reduced the epithermal neutron intensity by 12.43% and the N0 value by 13.8%. The use of the neutron ratio to monitor biomass was evaluated and changes in the neutron ratio coincided with biomass changes and resulted in a high correlation (R2 of 0.868) with the normalized difference vegetation index (NDVI) and (R2 of 0.817) leaf area index (LAI). The use of the CRNS to simultaneously monitor soil water content and biomass will be beneficial in providing more reliable soil water content estimates, provide biomass estimates at a field scale, and aid in understanding the dynamics between soil water content and vegetation.

Cosmic Ray Neutron Soil Moisture Estimation Using Physically Based Site‐Specific Conversion Functions

“…In addition, measuring the t/e ratio along the survey route could potentially also be used for biomass mapping. A recent study by Vather et al (2020) corroborates the t/e ratio‐biomass relationship. These applications would require the CRN rover to include bare neutron detectors.…”

Combining static and portable Cosmic ray neutron sensor data to assess catchment scale heterogeneity in soil water storage and their integrated role in catchment runoff response

“…lesser bulk density, greater soil organic matter content and greater porosity (Bruneau & Johnson, 2014;Meyles et al, 2001;Tetzlaff et al, 2014), compared to the mineral soils. For sites with greater organic matter content, it has been shown that applying CRNS technology can indeed be challenging (Bogena et al , 2013;Fersch et al, 2018;Heidbuchel et al, 2016) and accounting for the effect of high organic matter on the CRNS signal often requires additional sampling effort (Jakobi et al, 2018;Vather et al, 2020). In organic-rich, near-saturated soils, using the reference (typical) Neutron Count-SWC (N-SWC) equation can lead to unrealistically dynamic SWC estimates.…”

Cosmic-Ray neutron Sensor PYthon tool (crspy 1.2.1): an open-source tool for the processing of cosmic-ray neutron and soil moisture data

“…ERA5-Land is a data set, based upon the ERA5 reanalysis data and provided publicly by the European Centre for Medium-Range Weather Forecasts (ECMWF), that combines modelled data with real-world observations, resulting in a gridded, global hourly product at a 9 km resolution. Previous iterations of the ERA reanalysis data sets (such as ERA-Interim) have proven useful for other global networks for the task of gap filling missing data, such as in the FLUXNET community (Vuichard and Papale, 2015). We implement a similar approach to that used by the FLUXNET community in crspy and, consequently, to the global CRNS database, as we envision the potential of a merged database incorporating both flux tower and CRNS soil moisture data in the future.…”