## 4 Results and Discussion  
### Variation Characteristics of Honghu Lake  
The monthly maximum (blue plus cyan) and minimum (blue) water-surface areas and their distributions from the 1990s to 2013 are shown in Figure 2, while the occurrence years and maximum/minimum ratios are given in Table 1. Both the monthly maximum and minimum water-surface areas indicate seasonal variations. The monthly maximum areas range from 293.75 km\({}^{2}\) to 303.26 km\({}^{2}\), with small fluctuations occurring between November and March, and exceed 280 km\({}^{2}\) in other months. Compared with the maximum areas, the minimum areas in all months have a larger fluctuation range between 140.20 and 258.65 km\({}^{2}\). Hence, the maximum/minimum ratios mainly depend on the minimum areas. The monthly minimum area in April-May is approximately 100 km\({}^{2}\) less than that in November-March, thus indicating the possible occurrence of spring drought in this area. The monthly maximum and minimum areas appear in different years, and the maximum/minimum ratios range from 1.14 (January) to 2.00 (April), thus suggesting significant inter-annual variability across all calendar months. Moreover, two-thirds (8/12) of the months observed with the maximum areas mainly occurred in 1996, 2003 and 2010. According to hydrological records, widespread flooding events occurred in the Yangtze River basin during these years [35]. In particular, the most serious flooding in the last 60 years occurred in the summer of 2010 and caused significant losses for thousands of local residents [36]. Meanwhile, the majority of the months observed with minimum areas occurred in 2001 and 2011, during which some degree of drought might have occurred. From the late spring to the early summer of 2011, regions of the middle and lower reaches of the Yangtze River in China suffered the most serious drought [37]. The total precipitation from January to May 2011 was 51.6% lower than the average rainfall between 1951 and 2011 over the same period [38]. Figure 2: Monthly minimum and maximum area distributions of the Honghu Lake from the 1990s to 2013. Table 2 and Figure 3 present the annual maximum (blue plus cyan) and minimum (blue) water-surface areas from the 1990s to 2013. Intra-annual variations are presented in each panel, along with the inter-annual variability among the panels. For all years, the majority of the water areas were connected when the annual maximum area was observed. In contrast, when the annual minimum occurred, the lake was divided irregularly, even exposing the lakebed. In addition, the spatial distributions of the annual maximum area generally changed around the edges of the lakes beginning in the 1990s, but the distributions of the annual minimum area changed irregularly after 2000. The results of the annual analysis are similar to those of the monthly analysis, _i.e._, pronounced seasonality and inter-annual variability occurred. The annual maximum and minimum area varied widely between different years and occurred in different months. The annual maximum areas were greater than 290 km\({}^{2}\) from the 1990s to 2006, except in 2001, and were less than 285 km\({}^{2}\) from 2006 to 2013, except in 2010. These values suggest a decrease in the maximum area. The annual minimum areas fluctuated in a wider range compared with the maximum areas, _i.e._, between 140.2 and 277.4 km\({}^{2}\) from the 1990s to 2013. Therefore, the annual maximum/minimum ratios mainly depend on annual minimum areas. In the present work, the annual maximum/minimum ratios range from 1.07 to 1.93. Evidently, the smallest minimum area among those recorded occurred in 2011, during which the corresponding annual maximum/minimum ratio reached 1.93. Additionally, more than half (12/15) of the annual maximum area occurred in the winter months (November to March). In contrast, the majority of the minimum areas (9/15) occurred in the spring season (between April and May), proving yet again that spring drought is frequent in the Honghu watershed.