Comparative Analysis of Different Methods for Extracting Water Body Area of Miyun Reservoir and Driving Forces for Nearly 40 Years

The Miyun Reservoir is the most important source of drinking water in Beijing, China. Based on the Landsat satellite series image, this paper compared four index methods (NDWI, RNDWI, MNDWI, AWEIsh-superimposed MNDWI) to extract the water body area of Miyun Reservoir from 1979 to 2018 to study its changes in the past 40 years. Furthermore, the internal and external driving forces of reservoir water body area change are studied by correlation analysis and catastrophe analysis. Compared to the characteristic curve of reservoir area calculation, it indicated that the AWEIsh-superimposed MNDWI method has the highest accuracy. The statistical, temporal and spatial analysis showed that the water body area changes of Miyun Reservoir are mainly divided into four distinct stages: (1) It increased in 1979-1997, which was mainly due to the increase in precipitation and the low level of economic development. In 1997, the maximum value (161.14 km(2)) was reached, which was 108.64 km(2) higher than the minimum value. (2) It shrank rapidly in 1997-2004. The areas had dropped by 101.61 km(2). By 2004, it was 59.61 km(2). The main reason is that the reservoir has suffered from persistent drought for many years after discharging water. (3) It maintained at a lower water level in 2004-2015, with an average annual area of 78 km(2). The reason is the low precipitation and rapid economic development. (4) It grew rapidly after 2015, mainly due to water from the South-to-North Water Transfer Project, coupled with the increased precipitation. By 2018, the water body area reached 125.42 km(2).