## 2 Experimental Set Up
### Study Area  
The Heilongjiang River is one of the largest rivers in Northeast Asia, flowing through four countries (Mongolia, China, Russia and North Korea). The main stream has a total length of 2821 km, and also forms the boundary between China and Russia. There are abundant water resources in the Heilongjiang River, with a yearly runoff of 346.5 billion cubic meters. The main climate type in that region is monsoon. The precipitation distribution varies with the season. From April to October, the precipitation accounts for 90-93% of the annual precipitation, and the period from June to August accounts for 60-70%. From December, the winter dry season starts and the precipitation mainly falls in the form of snow. From 12 August 2013, several severe precipitation events continuously hit the northeastern part of Asia, leading to great flood in 39 rivers including part of the Heilongjiang River. Especially for the Tongjiang and Fuyuan Reaches of the Heilongjiang River, the flood had been the most serious one in the past 100 years. On 24 August 2013, more than 5 million people were affected in this disaster. The first case study analyses the event in this region. Two cloud-free scenes of HJ-1A CCD data are utilised. One image was obtained on 12 July 2013, which is around one month before the flood, and the other one was obtained on 27 August 2013 during the peak flow period. The dimension of the study region is 1082 \(\times\) 1321 pixels (around 1286 square kilometres). Its location and extent are shown in Figure 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Satellite Sensor** & **Band No.** & **Spectral Range (um)** & **Spatial Resolution (m)** & **Revisiting Time** \\ \hline \multirow{4}{*}{GF-4 PMS} & 1 & 0.45–0.90 & \multirow{4}{*}{50} & \multirow{4}{*}{20 s} \\  & 2 & 0.45–0.52 & & \\ \cline{1-1}  & 3 & 0.52–0.60 & & \\ \cline{1-1}  & 4 & 0.63–0.69 & & \\ \cline{1-1}  & 5 & 0.76–0.90 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Technical parameters of the GF-4 PMS data. Figure 1: (**a**) Location of the first study site near the border of Russia and China; (**b**) Extent of the HJ-1A CCD data used in this study. Dongting Lake is one of the most essential lakes in China, and one of the most important wetlands in the world as well. It is located on the southern bank of the Jinjiang section of the middle Yangtze River, and is one of the important dispatching lakes for the Yangtze River because of the strong ability of flood storage. The area of the lake is approximately 2690 square kilometres, across Hunan and Hubei provinces, and is roughly composed of East Dongting Lake, South Dongting Lake and West Dongting Lake. The water of Dongting Lake is clean and this area is one of the main freshwater fishery bases for commercial purposes. Due to its good environment and richness in water, soil and wildlife resources, it is one of the earliest birth places of Chinese rice raising agriculture. The basin area is of 262.8 thousand square kilometres, accounting for 14.6% of the Yangtze River basin area. In June and July 2016, heavy rains hit the middle and lower reaches of the Yangtze River basin, causing a catastrophic and wide flood in southern China. Eleven provinces and more than 10 million people were affected. On 3 July 2016, the water at the Chenglingji station in Dongting Lake also surpassed the warning level 32.50 m. A regional flood occurred in Dongting Lake. The second case study focuses on this area during this flood. Two cloud-free GF-4 PMS images are selected as the experimental data, which were obtained on 17 June 2016 (before the flood occurred) and 23 July 2016 (during the flood period), with a dimension of 2534 \(\times\) 2235 pixels (around 14,159 square kilometres). The location and extent of the second study site are shown in Figure 2.  
As can be seen from the figures, these two study sites are located in different geographic positions. The first case study mainly presents a river flood and the second one presents a lake flood.