Effects of Biomass and Soil Water Content Distribution on Cosmic-ray
Neutron Probes Measurement
Abstract
The cosmic-ray neutron probe (CRNP) provides continuous monitoring of
average near-surface soil water content (SWC) on a hectometer scale.
However, the performance of the CRNP in surfaces of highly heterogeneous
vegetation and SWC caused by the highly eroded terrain remains
uncertain. This study evaluated three different vegetation calibration
methods developed for CRNP on the Loess Plateau, compensating for the
privious lack of vegetation correction for CRNP in this area. The
landscape biomass and its distribution within the CRNP footprint of a
small watershed on the Chinese Loess Plateau were measured using an
unmanned aerial vehicle (UAV) equipped with a visible light camera and a
Rededge multispectral camera. Three CRNPs were installed at three
different locations including loamy sand hills (LSH) plot, silty loam
dam (SLD) plot, and a sandy loam slope (SLS) plot and electronic sensors
of SWC were buried at various depths in each of the three plots. We
found the parameter, N0, was best represented by the biomass at the
average growth conditions of the monitoring period, yielding the lowest
RMSE (0.068). The variation of spatio-temporal distribution of SWC
reduced the representativeness of the CRNP to the regional average SWC.
The CRNP SWC calibrated using the three methods at three sites yielded
similar temporal changes with precipitation. The three methods each
reduced the RMSE between the estimated CRNP SWC and the kriging weighted
SWC with the BWE (biomass water equivalent) corrected method, Veg-N0
method having the lowest RMSE. The correction of the spatial
distribution of vegetation using the three methods further reduced the
RMSE, in the order: LSH< SLS< SLD. The cooperation
between CRNP and UAV could obtain regional averaged SWC accurately,
which should be useful for formulation sustainable vegetation management
strategies on the Loess Plateau and regions with water scarcity possibly
around the world.