Quantitative Detection of Bosten Lake Area Using NOAA/AVHRR Data

To estimate the area of Bosten Lake and the area change since 1990, the threshold values of the channel reflectance and bright temperature of NOAA/AVHRR data are used to distinguish water body from other objects. For channel 2, water body indicates lower reflectance than vegetation and other land covers. The ratio of Channel 2 to Channel 1 can enhance the difference between water body and land, and decrease the effects of cloud and vegetation to a certain extent. For the image in April, the water body is identified by the reflectance of Channel 2 and the ratio of channel 2 to channel 1. The pixels are identified as water body if the reflectance in channel 2 is lower than the threshold T₁, as the land if it is higher than T₂ (threshold), and as mixed pixels if it is between T₁ and T₂. Based on the histograms of channel 2 and the ratio of channel 2 to channel 1, the threshold values of the T₁ and T₂ are found, and they are fixed on the different satellite images. In October, the water body in the south-east of the Lake is identified with bright temperature of channel 4 because the region is covered by vegetation. But, in the east of the Lake, the water body is still identified by the reflectance of channel 2 and the ratio of channel 2 to channel 1. The threshold value of channel 4 is also determined according to the histogram. The linear mixed model is used to calculate the area of mixed pixels at the edge of lake. The area of Bosten Lake in April and October from 1990 to 1998 are calculated, and the area changes of Bosten Lake during the nine years are detected. The result shows that the area of Bosten Lake has been increasing from 1990 to 1998 due to the increasing of the temperature/precipitation and the decreasing of evaporation, and the water utilization by mankind and the water exiting has been stable, even has decreased since 1988. Furthermore, the area of Bosten Lake is bigger in spring than that in autumn due to the supplying of the melt water of snow after the winter.