Calibration and Validation of the Cosmic Ray Neutron Rover for Soil Water Mapping within Two South African Land Classes

Abstract

Knowledge of soil water at a range of spatial scales would further our understanding of the dynamic variable and its influence on numerous hydrological applications. Cosmic ray neutron technology currently consists of the Cosmic Ray Neutron Sensor (CRNS) and the Cosmic Ray Neutron Rover (CRNR). The CRNR is an innovative tool to map surface soil water across the land surface. This research assessed the calibration and validation of the CRNR at two survey sites (hygrophilous grassland and pine forest) within the Vasi area with an area of 72 and 56 ha, respectively. The assessment of the calibrations showed that consistent calibration values (N 0 ) were obtained for both survey sites. The hygrophilous grassland site had an average N 0 value of 133.441 counts per minute (cpm) and an average error of 2.034 cpm. The pine site had an average N 0 value of 132.668 cpm and an average error of 0.375 cpm between surveys. The validation of CRNR soil water estimates with interpolated hydro-sense soil water estimates showed that the CRNR can provide spatial estimates of soil water across the landscape. The hydro-sense and CRNR soil water estimates had a R 2 of 0.439 at the hygrophilous grassland site and 0.793 at the pine site.

Opportunities and challenges in using catchment-scale storage estimates from cosmic ray neutron sensors for rainfall-runoff modelling

“…Nevertheless, at an intensely managed agricultural catchment, such as that of Elsick, regular agricultural management (e.g. harvesting, cattle grazing) makes installing and maintaining such a network impractical (Vather et al, 2019). For comparative reasons only, the CRNS station in Elsick was equipped with a single profile probe, at a convenient location on undisturbed soil.…”

“…Using a roving-type CRNS approach (e.g. Franz, 2018;Vather et al, 2019), could be one way to fully explore this scale representativeness. Nevertheless, despite the remaining spatial difference between the total catchment area and the CRNS footprint, we found that the CRNS signal detected the dynamics that are relevant for improving model calibration at the catchment scale.…”

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

“…In recent years, the Cosmic Ray Neutron Sensor (CRNS) has been developed and deployed across the globe to provide area-averaged soil water content estimates at an intermediate scale, which circumvents the shortcomings of currently existing soil water content measurement methods (at this scale) [6,7]. The CRNS is a novel non-invasive technique, which utilizes the low-energy cosmic ray neutron intensity above the ground surface, which is primarily moderated by the hydrogen content within the CRNS's measurement footprint.…”

“…The CRNS functions at its most fundamental level as a hydrogen mass detector, where the observed change in neutron intensity is dependent on all changes of hydrogen present in its support volume [7]. The abundance of hydrogen atoms in the air and soil largely controls the removal rate of low-energy neutrons from the support volume [14].…”

Towards disentangling heterogeneous soil moisture patterns in cosmic-ray neutron sensor footprints

“…Roving CRNS devices (e.g. McJannet et al, 2017;Schrön et al, 2018a;Vather et al, 2019) and dense sensor networks (e.g. Fersch et al, 2020;Heistermann et al, 2021) pose further opportunities for covering even larger areas.…”