Application and accuracy of cosmic-ray neutron probes in three soil textures on the Loess Plateau, China

Abstract

The cosmic-ray neutron probe (CRNP) is a new tool for continuously monitoring mean soil–water content (SWC) on a hectometer scale, but the performance of CRNPs in highly heterogeneous distributions of SWC, vegetation and especially soil texture caused by the broken surface on the Loess Plateau in China remains uncertain. The study evaluated the application and accuracy of the method for three soil textures in a small watershed on the plateau to provide a reference and scientific basis for its application. Three CRNPs were installed in a loamy sand hills (LSH) plot, a silty loam dam (SLD) plot, and a sandy loam slope (SLS) plot, soil moisture sensors were buried at various depths in three plots. The footprints of the CRNPs for SWC observation had radii of 127–170, 126–174 and 148–174 m in SLD, SLS and LSH, respectively, and the mean measuring depth was about 20 cm for SLD and SLS and 40 cm for LSH based on general calculation equations. The accuracy of the CRNP method was high when validated using oven-dried method (RMSE = 0.068 cm3 cm−3). The mean SWCs estimated from the other types of sensors were consistent with the mean SWCs from the CRNP probes in the three plots. The coefficient of determination of the linear relationship was in the order: SLS > LSH > SLD. The coefficient of variations (CV) were higher in LSH than SLD and SLS, indicating that SWC varied most in LSH. During periods of rain, the variations were larger for SLD than SLS and LSH, with a CV of 32.2%. The CRNP for SLD had the highest rates of change during periods with and without rain for both increasing and decreasing SWC, but this plot held more water than LSH and SLS, consistent with the high clay and organic-matter contents in SLD. The rates of change were significantly correlated with the antecedent SWCs (P < 0.01), and the higher the antecedent SWC, the faster the rate of change for each plot. This study verified the application of the CRNP for monitoring field-average SWC of the broken surface on the Loess Plateau and represents a new approach for the study of the spatial distribution of SWC on the plateau. CRNP can measure SWC of the complex underlying surface accurately, so as to provide a more reliable assessment of the impacts of landscapes and vegetation changes, such as conversion of cropland to forest and grassland on the Loess Plateau on water resources.