## 4 The Temporal and Spatial Variability of the Soil and Vegetation Temperatures  
The two-dimensional plots of surface temperature-albedo throughout the growing season over the research site were derived from the nine scenes (see Figure 4). In the plots, \(T_{\rm s,max}\) and \(T_{\rm c,max}\) refer to the maximum soil temperature and minimum canopy temperature which were derived from maximum and minimum land surface temperatures, respectively, retrieved from the ASTER thermal images in the study area. However, the minimum soil temperature (\(T_{\rm s,min}\)) and maximum canopy temperature (\(T_{\rm c,max}\)) are the average values, which are computed from the two-dimensional plots according to the aforementioned procedure. According to Figure 4, it is clearly seen that these points vary considerably over the course of the season. The values of the four temperature end-members declined from the early season toward the later season, and the population of points moved gradually towards the x-axis except on DOY 240. In addition, the position and magnitude of the temperature end-members are closely related to changes in vegetation phenology over the course of the growing season. In conclusion, the correlation of the surface temperature-fractional cover and surface temperature-albedo spaces significantly enhances the capability to accurately derive the temperature end-members phenological changes. The soil and vegetation component temperatures were successfully separated using the \(T\)-\(\alpha\) space method in the study area from the early season to the later season (DOY 167, 176, 192, 215, 224, 231, 240, 247, 256) using the ASTER image data (Figure 5). Figure 4: The spaces of surface temperature-albedo and surface temperature-green vegetation fraction, the position of dry edges, wet edges, bare soil and the full vegetation cover edge in the spaces, established by using nine senses ASTER images. Obviously, the temporal and spatial distribution of the soil temperature and vegetation temperatures demonstrated a cooling-down process during the growing season, with the exception of DOY 240. Furthermore the variation has good consistency with maize phonological period in the oasis, the land surface temperature is higher in the early season when the plant start to grow and stable and medium in the mid-season when the fields are nearly full covered with vegetation. There is a high spatial  
Figure 5: The spatial distribution of soil and vegetation component temperatures from nine ASTER images during the growing season. heterogeneity not only of the soil temperature but also of the vegetation temperature during the early season when the row crops were partial canopy cover conditions, especially on DOY 167 and 176. The spatial variability of temperatures including soil and vegetation in the early season can be attributed to the different sowing dates of the crop, in which the different sowing dates contributed to the variation fraction canopy cover. This in turn leads to spatial variation in photosynthesis, water use and fraction of incoming solar radiation of the soil surface. Irrigation was scheduled on different days in the various fields during the four times periods with flood irrigation during the growing season. This means that the soil moisture content for the various fields differed when the satellite passed over the study area. The compounding effects from the spatial heterogeneity of the soil moisture content and the fractional vegetation cover contribute to the spatial variation in soil and vegetation component temperatures derived from the ASTER TIR images in the early growing season. However, when crops were near full canopy cover condition in the mid and late of crop growing season, the spatial distribution of the separated soil and vegetation component temperatures gradually became homogenous, especially on the DOY 231, 240 and 247, where the values of the vegetation temperature were nearly uniform over the croplands.