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.
Highlights
Soil water content is one of the key state variables in the soil–vegetation–atmosphere continuum, which is due to its role in the exchange of water and energy at the soil surface [1,2]
The Cosmic Ray Neutron Sensor (CRNS) is a novel technique of soil water content estimation that is affected by all hydrogen sources within its measurement footprint
The first section evaluates the CRNS soil water content estimates before, during, and after the clear-felling using the CRNS at the bottom of the tower mast, which has been operational in the Two-Streams Catchment since 2014
Summary
Soil water content is one of the key state variables in the soil–vegetation–atmosphere continuum, which is due to its role in the exchange of water and energy at the soil surface [1,2]. The abundance of hydrogen atoms in the air and soil largely controls the removal rate of low-energy neutrons from the support volume [14] These hydrogen atoms are present as soil water, lattice water, water in soil organic carbon, below and above-ground biomass, atmospheric water vapour, snow, water intercepted by vegetation, and water on the ground (ponding) [15,16,17]. Soil water is generally the major source of hydrogen at the land surface (60–80% of the mass in the CRNS support volume [19]) and is suitable to be estimated from the changes in neutron intensity. Since the inception of the CRNS, there have been several studies that have focused on the estimation of biomass using the CRNS These initial studies investigated the correction of site biomass, to account for the attenuation of the cosmic ray neutron intensity by biomass, and to improve the accuracy of the CRNS’s soil water content estimates. There have been several CRNS studies involving biomass (Table 1)
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