jpl.nasa.gov/). As the ��zoom lens�� for Terra, ASTER data can be used by other Terra and space-borne instruments for validation and calibration. Since both MODIS and ASTER are on the same satellite, ASTER provides an opportunity to validate MODIS observational data.The purposes of this study were: (1) to propose a new approach to obtain Tsoil and Tveg within a given pixel; (2) to propose a new practical method to define a VI-Ts diagram using the information of vegetation and bare soil components within pixels; (3) to validate the proposed method by using ASTER, MODIS and ground-based data; (4) to compare the proposed method with the traditional method across a semiarid agricultural region in the North China Plain through 2003.2.

?Study Area and Data Collection2.1.

Study Area and Ground Data CollectionThe North China Plain (NCP) is one of main crop regions in China. The region displays a typical continental monsoon climate. The yearly mean air temperature is 13.1 ��C, and the annual precipitation is about 610 mm, of which about 70% falls between June and August. Therefore, the NCP is zoned as a semiarid agricultural region. Our study area (30 km �� 30 km) locates at the center of the NCP (Figure 2). The light, temperature and water conditions support a 1-year 2-harvest cropping system (winter wheat (Oct.-Jun.) – summer maize (Jul.-Sep.) in this study area. Winter wheat is mainly dependent on irrigation. Usually, about four irrigations are required in the whole growing lifecycle of winter wheat.

The Yucheng Experimental Station (YES, Latitude 36��49��51�� N, Longitude 116��34��18�� E, 26m above the sea level) of the Chinese Academy of Sciences GSK-3 locates in the study area.Figure 2.Study area. On the right is the ASTER false-color image (UTM-N50, WGS-84, 15 m, Band 3, Brefeldin_A 2, 1). The red part is vegetation, mainly winter wheat. The green rectangle in the ASTER image is the Yucheng Experimental Station (YES).Regular meteorological data recorded at the time when the EOS-Terra satellite overpassed our study area were collected from a flux station in the YES through 2003, including air temperature, humidity, wind speed, precipitation, downward and upward shortwave solar radiation, and downward and upward long-wave radiation [19]. The bulk temperature of an infinitely thick vegetation canopy is close to ambient air temperature [10], so observed air temperature can be used to validate Ts of wet points (Ts_wet, it corresponds to the minimum Tveg) in this study.