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Abstract
Land surface soil moisture (SSM) is crucial to research and applications in hydrology, ecology, and meteorology. To develop a SSM retrieval model for bare soil, an elliptical relationship between diurnal cycles of land surface temperature (LST) and net surface shortwave radiation (NSSR) is described and further verified using data that were simulated with the Common Land Model (CoLM) simulation. In addition, with a stepwise linear regression, a multi-linear model is developed to retrieve daily average SSM in terms of the ellipse parameters x0 (horizontal coordinate of the ellipse centre), y0 (vertical coordinate of the ellipse centre), a (semi-major axis), and θ (rotation angle), which were acquired from the elliptical relationship. The retrieval model for daily average SSM proved to be independent of soil type for a given atmospheric condition. Compared with the simulated daily average SSM, the proposed model was found to be of higher accuracy. For eight cloud-free days, the root mean square error (RMSE) ranged from 0.003 to 0.031 m3 m−3, while the coefficient of determination (R2) ranged from 0.852 to 0.999. Finally, comparison and validation were conducted using simulated and measured data, respectively. The results indicated that the proposed model showed better accuracy than a recently reported model using simulated data. A simple calibration decreased RMSE from 0.088 m3 m−3 to 0.051 m3 m−3 at Bondville Companion site, and from 0.126 m3 m−3 to 0.071 m3 m−3 at the Bondville site. Coefficients of determination R2 = 0.548 and 0.445 were achieved between the estimated daily average SSM and the measured values at the two sites, respectively. This paper suggests a promising avenue for retrieving regional SSM using LST and NSSR derived from geostationary satellites in future developments.
Acknowledgement
The authors thank FLUXNET for supplying data and advice.
Funding
The work described in this articel was supported in part by the National Nature Science Foundation of China [grant number 41271379]; partly supported by the CAS (Chinese Academy of Sciences) Action-Plan for West Development [grant number KZCX2-XB3–15]; and also partly supported by the Exploratory Forefront Project for the Strategic Science Plan in IGSNRR, CAS [grant number 2012QY006]. Mr Pei Leng was financially supported by the China Scholarship Council for his stay at ICube, Strasbourg, France.