不同植被覆盖下宇宙射线中子法土壤水分反演

    Investigation on the applicability of cosmic ray neutron probe for observing soil moisture over diverse vegetation covers

    • 摘要: 土壤水分是影响农业生产和生态环境的关键变量,准确获知土壤水分空间分布信息具有重要意义。宇宙射线中子法(cosmic ray neutron probe, CRNP)可以实现对田块尺度土壤水分的连续观测,但其在不同地理环境及植被覆盖类型条件下观测土壤水分的能力还需要进一步加深理解。该研究在全球范围内选择了具有代表性的16个观测站点数据,采用统一的数据处理流程和土壤水分估算方法,开展了不同植被覆盖条件下的CRNP土壤水分观测能力的系统性综合比较。结果表明,总体而言,CRNP观测对土壤水分的动态变化敏感性强,且能够明确响应降水事件。在草地覆盖条件下,除1个站点结果相对较差外,其他站点表现出了非常高的土壤水分估算精度,RMSE和ubRMSE最优达0.03 cm3/cm3,Bias最优达-0.01 cm3/cm3R2高达0.93;对于农田和森林覆盖的站点,观测结果质量仍然较高,RMSE最优为0.05 cm3/cm3,ubRMSE最优为0.03 cm3/cm3,Bias最优为0.006 cm3/cm3R2最优为0.88;在灌木地区,所选站点的CRNP观测效果受局地环境强烈影响,土壤水分估算精度存在较大差异。研究有助于深入了解CRNP观测土壤水分的能力和潜力,为在不同植被覆盖下应用CRNP观测捕捉田块尺度土壤水分时空变化信息提供科学依据和参考。

       

      Abstract: Soil moisture is a key variable affecting agricultural production and ecological environment. It is therefore of great significance to acquire accurate spatial distribution information of soil moisture. Compare to conventional point-scale based measurements, the Cosmic Ray Neutron Probe (CRNP) provides an alternative approach for capturing soil moisture dynamics to address drought monitoring, plant water stress detection, as well as various hydrological applications. CRNP can realize continuous observation of soil moisture at the field scale and estimate average soil water content with a 0.1–0.2 km2 quasi-circular footprint by monitoring the neutron intensity near the ground. However, previous studies conducted soil moisture observation using CRNP are often limited to a single landscape, its ability to observe soil moisture in different geographical environments and vegetation cover types still needs to be further understood. In this study, data from 16 representative stations located in different climatic zones and under different vegetation cover conditions worldwide were selected, including seven grasslands covered stations, three farmlands covered stations, three forests covered stations and three shrubs covered stations, and a unified data processing procedure and soil moisture estimation method were used to invert the average soil moisture observation value in the observation area. The environmental factors affecting the neutron data were corrected, including atmospheric pressure correction, air humidity correction, incident neutron intensity correction and aboveground biomass correction to remove their influence to neutron counts. The N0 parameter method was used to invert the soil water value. Ground point-scale based soil moisture measurements within the CRNP footprint were aggregated to validate the soil moisture estimates based on CRNP observations. Therefore, a systematic and comprehensive comparison of soil moisture observation capabilities of CRNP over different climatic zones and vegetation cover conditions can be performed. The results indicate that, in general, CRNP observations under different vegetation covers and vegetation cover conditions were highly sensitive to soil water dynamics and can clearly respond to precipitation events. However, there were significant differences in the estimation accuracies. CRNP outperforms over grasslands, except result of one station was relatively poor, the other stations showed high reliability, , with the optimal RMSE of 0.05 cm3/cm3, the optimal ubRMSE of 0.03 cm3/cm3, the optimal Bias of 0.006 cm3/cm3, and the optimal R2 of 0.88. In shrub covered areas, the CRNP observation effect of selected sites was strongly affected by the local environment, and the accuracy of soil water estimation is quite different. In was shown R2 value varies greatly, from 0.91 at the best to 0.02 at the worst, while ubRMSE, RMSE and Bias are repectively of 0.01 cm3/cm3, 0.02 cm3/cm3, and -0.02 cm3/cm3 at the best condition. In terms of the CRNP sites under grassland cover, the differences in CRNP observations are mainly related to the growth of surface vegetation, etc.; at sites under crop cover, the differences in CRNP observations are mainly related to anthropogenic activities in the agricultural areas and surface heterogeneity; at sites under forest cover, the differences are mainly related to the high canopy, etc. With repsect to the sites under shrub cover, the differences are mainly related to climate, etc. The Yanco and Sidaoqiao sites have the worst observational accuracy among all the sites, with an R2 of only 0.46 at Yanco and 0.02 at Sidaoqiao, which have weak data consistency may be mainly related to special local soil moisture environments. In conclusion, CRNP observations of soil moisture are more likely to capture regional moisture fluctuations than point-scale in situ observations. This study helps us better understand the capabilities and potentials of CRNP in observing soil moisture, and provide a scientific basis and reference for applying CRNP observation to capture the spatio-temporal changes of soil water at field scale under different vegetation cover and climate conditions.

       

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