Abstract
The VI-Ts diagram determined by the scatter points of the vegetation index (VI) and surface temperature (Ts) has been widely applied in land surface studies. In the VI-Ts diagram, dry point is defined as a pixel with maximum Ts and minimum VI, while wet point is defined as a pixel with minimum Ts and maximum VI. If both dry and wet points can be obtained simultaneously, a triangular VI-Ts diagram can be readily defined. However, traditional methods cannot define an ideal VI-Ts diagram if there are no full ranges of land surface moisture and VI, such as during rainy season or in a period with a narrow VI range. In this study, a new method was proposed to define the VI-Ts diagram based on the subpixel vegetation and soil information, which was independent of the full ranges of land surface moisture and VI. In this method, a simple approach was firstly proposed to decompose Ts of a given pixel into two components, the surface temperatures of soil (Tsoil) and vegetation (Tveg), by means of Ts and VI information of neighboring pixels. The minimum Tveg and maximum Tsoil were then used to determine the wet and dry points respectively within a given sampling window. This method was tested over a 30 km × 30 km semiarid agricultural area in the North China Plain through 2003 using Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) and MODerate-resolution Imaging Spectroradiometer (MODIS) data. The wet and dry points obtained from our proposed method and from a traditional method were compared with those obtained from ground data within the sampling window with the 30 km × 30 km size. Results show that Tsoil and Tveg can be obtained with acceptable accuracies, and that our proposed method can define reasonable VI-Ts diagrams over a semiarid agricultural region throughout the whole year, even for both cases of rainy season and narrow range of VI.
Highlights
The VI-Ts diagram determined by the scatter points of remotely sensed vegetation index (VI) and land surface temperature (Ts) has been widely used to retrieve information on the partitioning of available surface energy [1,2,3] and surface moisture status [4,5,6,7,8,9]
MODerate-resolution Imaging Spectroradiometer (MODIS)-Tveg and MODIS-temperatures of soil (Tsoil) were obtained using the approach proposed in Section 3 across the whole year of 2003
The accuracies of MODIS-Tveg and MODIS-Tsoil depend on the isoline slope of land surface moisture based on the linear regression from 9 neighboring pixels
Summary
The VI-Ts diagram determined by the scatter points of remotely sensed vegetation index (VI) and land surface temperature (Ts) has been widely used to retrieve information on the partitioning of available surface energy [1,2,3] and surface moisture status [4,5,6,7,8,9]. All these applications are under the condition of homogenous atmospheric forcing such as solar radiation and air temperature over a sampling window for defining a VI-Ts diagram, so the size of the sampling window cannot be too large. It is difficult to find enough ideal pixels within a limited sampling window, especially when using a satellite data with moderate / coarse resolution, such as 1 km MODerate-resolution Imaging Spectroradiometer (MODIS)
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