## 4 Results and Discussion
### Influence Factors  
#### 4.2.1 Climate Factors  
According to water balance equations, the water-surface area variations of the Honghu Lake are closely related to climate factors, including precipitation within its basin and evaporation over its surface. To investigate the process by which these climate factors influence the lake's water-surface area, we examine the relationship between the water-surface area and the monthly precipitation (P) and monthly evaporation (E), as well as the difference in the monthly precipitation and monthly evaporation (P-E), in the period between the 1990s and 2013. Considering the months in which the annual maximum and minimum areas occurred, we divided a whole year into three periods: April-May, June-October and November-March. To obtain improved analytical results, we calculate the monthly precipitation by summing up the daily precipitations for a month before the dates of the remote sensing images. As shown in Figure 5a, the water-surface areas and their corresponding monthly precipitations present have similar fluctuation patterns in the April-May period, thus indicating their statistically significant, correlation with a correlation coefficient (\(R\)) of 0.677 (\(p<0.01\)). This correlation suggests that the water-surface area variations are partly driven by the local precipitation during this period. Meanwhile, not all water-surface area variations are related to monthly evaporation (curve abbreviated). The difference between the monthly precipitation and evaporation and the water-surface area also shows similar fluctuation patterns in this period and statistically significant correlations (\(p<0.01\)). With the correlation coefficient (\(R\)) of 0.743, the two factors show strong relativity that suggests the significant contribution of the comprehensive effect of precipitation and evaporation to the water-surface area variations in the April-May period. Figure 4: Curve of water-surface area variations of the Honghu Lake from the 1990s to 2013. Similar to the foregoing analysis, a correlation analysis is performed for the June-October period, in which the water-surface areas and their corresponding monthly precipitations indicate somewhat similar fluctuation patterns, especially given their relatively poor correlation (Figure 5b) with a correlation coefficient (\(R\)) of 0.384 (\(p<0.05\)). This correlation suggests that local precipitation has minimal influence on the water-surface area variations during this period. Meanwhile, not all of the water-surface area variations are related to monthly evaporation (curve abbreviated). The water-surface area and the difference between monthly precipitation and evaporation in this period present similar fluctuation patterns with a statistically significant correlation (\(p<0.05\)). With the coefficient of 0.329, the two factors demonstrate a weak correlation that suggests the significant contribution of monthly precipitation to the water-surface area variations during this period. The curve shows that the water-surface areas in specific months, such as in April 2009 and April 2013, do not perfectly coincide with the corresponding monthly precipitations and the differences in the monthly precipitation and evaporation. The water-surface area has an accumulated effect; thus, a lag may exist between the two climate factors in the April-May period. The same effect can be  
Figure 5: Relationship between water area and climate factors in the (**a**) April–May and (**b**) June–October periods. observed in the June-October period. When the climate factors demonstrate a slight change, the water-surface area is almost unaffected. In the April-May period, the minimum area occurred in April 2011, but the monthly precipitation and difference in the monthly precipitation and evaporation in that month were not at the minimum, thus demonstrating that the water-surface area during this period was simultaneously affected by other factors.