Evaluation of the soil water content of two managed ecosystems using cosmic-ray neutron sensing on China's Loess Plateau

Cosmic-ray neutron sensing (CRNS) is a promising method for the continuous monitoring of soil water content (theta(v)) at the hectometre scale. However, few studies have validated its applicability within the complex terrains of various ecosystems in semiarid loess regions. In this study, CRNS-based theta(v) was measured on China's Loess Plateau under two managed ecosystems (Caragana korshinskii K. shrubland [KOS] and Medicago sativa L. grassland [SAG]). The accuracy of CRNS measurement was assessed with the traditional oven-drying method, while its sensitivity to theta(v) variation during the study period was compared with typical point-scale monitoring sensors (EC-5, Decagon Devices, Inc.). CRNS was found to be strongly correlated with the oven-drying based theta(v) under both ecosystems, with coefficients of determination (R-2) of 0.99 and 0.93, and root mean square errors of 0.006 and 0.007 cm(3)/cm(3), for KOS and SAG, respectively. The mean monitoring radius of the CRNS was approximately 204 and 231 m for KOS and SAG, with a mean measurement depth of approximately 39 and 45 cm, respectively. Moreover, the theta(v) determined by the CRNS and EC-5 sensors exhibited similar trends during the observation period, excluding that for April. However, the CRNS-based theta(v) had a higher spatial representation for both managed ecosystems compared with that for EC-5. No significant difference was detected in the response range of theta(v) to precipitation between the CRNS and EC-5 sensors at a depth of 0-10 cm, whereas the response time of CRNS for KOS was much shorter than that of EC-5 sensors (p < 0.01). Overall, the CRNS measured hectometre-scale theta(v) had acceptable accuracy and sensitively traced the precipitation response under different ecosystems with high theta(v) heterogeneity in the semiarid loess regions.