**4. Results**

#### *4.1. Variation for Lake Area*

After the above stage, construction of a yearly time-series of the water surface areas on Daihai Lake and Huangqihai Lake are displayed in Figure 5.

**Figure 5.** Time-series for variation of two lakes.

In this experiment, 22 pictures were used and a set of time-series data was made for each lake during from 1984 to 2018. Overall, the trend of surface area in Daihai Lake is declining. Daihai Lake area reached a maximum level of 133.5km<sup>2</sup> in 1984 and then declined at a rate of about three per cent. The entire lake area shrank to a staggering 82.05 km2, which accounted for 61.5% of the water surface in 1984. This shrinkage data is more than 30.3% of the total shrinking area of lakes in Inner Mongolia for nearly 30 years [6].

If Daihai Lake is a typical example of serious shrinkage of the water surface as considered, then the complete drying up of the Huangqihai Lake is regrettable. Overall, the water area of the Huangqihai Lake showed a downward trend until it ran dry completely. The area reached a peak of 71.25 km<sup>2</sup> in 1984, but declined sharply in 1989, then rose to a rising trend from 1994 to 1998. After 1999, the lake fell sharply again until it dried up completely from 2006 to 2013. In the following years, water area has recovered slightly, but it is limited to the region that was located in the northwest and northeast of the lake. The recovery of the water area is mainly owing to the inflow of the river. There is a contrast between the two adjacent lakes. Daihai Lake still maintains a certain water quantity, but Huangqihai Lake has experienced drying up. This is important evidence that there are other sources of water supply in Daihai Lake and the decline of Daihai has been alleviated.

In the above paragraphs, we described the changes in the two lakes in terms of time distribution. Then, we continued to observe the characteristics of water surface changes for each lake from a spatial perspective. We selected some year images with significant changes and drew diagrammatic sketch of the water surface change on Daihai Lake and Huangqihai Lake respectively. Figures 6 and 7 show the spatial changes of each lake.

**Figure 7.** Spatial variation of Huangqihai Lake surface.

Figure 6 shows that the whole Daihai Lake mainly shrank from south to north, while the north and northeast also showed a small shrinkage. Before 1989, the lake shrank mainly in the south, and the water surface in the north and northeast was stable. During the later 1990s, the south and northeast of Daihai Lake began to shrink sharply. After 2004, the main shrinking parts of water surface have gathered around in the south and northeast of the lake.

Compared with the steady shrinkage of Daihai Lake, the water surface of Huangqihai Lake fluctuates more violently. Before 1994, the surface of the lake shrank only slowly inward along the shoreline. However, between 1994 and 2002, the southern and northeastern parts of Huangqihai Lake showed drastic contraction. From 2002 to 2008, the lake dried up for the first time. This result is basically consistent with the previous results [30]. Although the lake recovered in the northwest region between 2014 and 2018, as shown in Figure 1, it was just the result of the river flowing into the lake.
