Applicability of cosmic-ray neutron sensor for measuring soil moisture at the agricultural-pastoral ecotone in northwest
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plicability of cosmic-ray neutron sensor for measuring soil moisture at the agricultural-pastoral ecotone in northwest China 1
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Xingyan TAN , Lanhui ZHANG , Chansheng HE 1 1 Zhibo HAN & Xuliang LI 1
1
, Yuzuo ZHU ,
Key Laboratory of West China’s Environmental System (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou 2
University, Lanzhou 730000, China; Department of Geography, Western Michigan University, Kalamazoo, MI 49008, USA
Received January 30, 2020; revised April 26, 2020; accepted June 23, 2020; published online August 24, 2020
Abstract Accurate monitoring of soil moisture is crucial in hydrological and ecological studies. Cosmic-ray neutron sensors (CRNS) measure area-average soil moisture at field scale, filling a spatial scale gap between in-situ observations and remote sensing measurements. However, its applicability has not been assessed in the agricultural-pastoral ecotone, a data scarce semiarid and arid region in Northwest China (APENC). In this study, we calibrated and assessed the CRNS (the standard N0 method) estimates of soil moisture. Results show that Pearson correlation coefficient, RP, and the root mean square error (RMSE) between 3 –3 the CRNS soil moisture and the gravimetric soil moisture are 0.904 and less than 0.016 m m , respectively, indicating that the CRNS is able to estimate the area-average soil moisture well at our study site. Compared with the in-situ sensor network measurements (ECH2O sensors), the CRNS is more sensitive to the changes in moisture in its footprint, which overestimates and underestimates the soil moisture under precipitation and dry conditions, respectively. The three shape parameters a0, a1, a2 in the standard calibration equation (N0 method) are not well suited to the study area. The calibrated parameters improved the accuracy of the CRNS soil moisture estimates. Due to the lack of low gravimetric soil moisture data, performance of the calibrated N0 function is still poor in the extremely dry conditions. Moreover, aboveground biomass including vegetation biomass, canopy interception and widely developed biological soil crusts adds to the uncertainty of the CRNS soil moisture estimates. Such biomass impacts need to be taken into consideration to further improve the accuracy of soil moisture estimation by the CRNS in the data scarce areas such as agricultural-pastoral ecotone in Northwest China. Keywords Soil moisture, Cosmic-ray neutron sensor, In-situ observations, The agricultural-pastoral ecotone in Northwest China (APENC) Citation:
Tan X, Zhang L, He C, Zhu Y, Han Z, Li X. 2020. Applicability of cosmic-ray neutron sensor for measuring soil moisture at the agricultural-pastoral ecotone in northwest China. Science China Earth Sciences, 63, https://doi.org/10.1007/s11430-020-9650-2
1. Introduction The near-surface soil moisture has a profound influence on the partitioning of precipitation into surface runoff, evapotranspiration and infiltration, and thus on the surface energy balance (Brunetti et al., 2019). Obtaining
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