# An Optical and SAR Based Fusion Approach for Mapping Surface Water Dynamics over Mainland China
## 4 Results  
### Water Occurrence Maps and Dynamics  
When looking at the multi-annual water occurrence map, it is clear that surface water resources are unevenly distributed in mainland China (Figure 3). Our results indicate  
Figure 2: The location of the validation sites displayed along with the RESOLVE eco-region map (**a**). The seasonal distribution of the validation months by site and shown along with Sentinel-2 quick looks (**b**).  
that about 1-3% of the land surface in mainland China is covered by open inland water (i.e., lakes, rivers and reservoirs) compared to a global average of around 3-4% [11]. Still, large regional disparities exist with the majority of surface water in the south-east and south-west. The southeastern region is the most densely populated region and the water extent and dynamics are largely controlled by anthropogenic factors [72]. The southwestern part of China, dominated by the Tibetan Plateau, is a vast, sparsely populated region where the water landscape and its dynamics are controlled by natural forces in particular, stream runoff from glacial meltwater [22].  
The current 4 year time series is too short to make any firm conclusions about long-term water transitions. However, the trends still provide useful illustrations of intra and inter-annual water dynamics.  
Poyang Lake, located in the northern part of Jiangxi Province, is the largest freshwater body in China. It receives its water primarily from five major rivers (i.e., the Ganjiang, Xinjiang, Raohe, Xiuhe, and Fuhe Rivers), and discharges into the Yangtze River at Hukou at its northern end. The lake resides in a subtropical monsoon region with a wet season from April to September, and a dry season from October to March. The temporal variations in the surface water area reflect this seasonality and high intra-annual fluctuations create an extensive wetland around the Lake, providing vital wintering habitats for migratory birds, many of which are critically endangered [73]. Nonetheless, the annual fluctuations are also a concern, especially in recent years where record high water levels have caused damaging floods in July 2017, 2019 and 2020 [74; 75; 76; 77]. These events are all clearly captured in the temporal development profile of the surface water area for Poyang Lake (cf. Figure 4).  
Surface water area also responds to human manipulations. Miyun Reservoir, located just north east of Beijing, is one of the largest water protection areas in the world [78]. The reservoir covers an area of 180 square kilometers, with a reservoir capacity of 4 billion cubic meters and an average depth of 30 m, making it the largest drinking-water supply for over 20 million inhabitants of Beijing [79]. Since water began to be channeled from central China's Hubei Province to the Capital in December 2014 via the south-to-north water diversion project, the reservoir's water level has gradually increased up until 2018 [80]. However, in recent years, the trend has leveled off as Miyun Reservoir has supplied over  
Figure 3: Maximum surface water extent in mainland China, and permanent and seasonal water summarized in zones of 0.5 degree latitude/longitude.  
4 million cubic meters of water to rivers running dry in its downstream area [81]. The time series consistently reflect these trends (Figure 5).  
Figure 4: The multi-annual surface water frequency (**a**) and the associated inter-annual monthly water dynamics (**b**) for Lake Poyang.  
Figure 5: The multi-annual surface water frequency (**a**) and the associated inter-annual monthly water dynamics (**b**) for the Miyun reservoir.