## 5 Discussion  
The overall distribution of drought events in Hunan Province was determined by analyzing the SPEI-12 time series values of all stations (Figure 4). From 1965 to 2015, the province experienced eight drought events that lasted for more than 10 months (Figure 4). The interval between drought events from 1965 to 2000 was seven years (1970, 1977, and 1984), and the drought period decreased by two years from 2000 to 2015 (2004, 2006, 2008, 2010, and 2012). [PERSON] et al. (2017) analyzed the drought in Hunan Province using the rotated empirical orthogonal function and discovered three main drought cycles, namely, 2, 7, and 18 years. The drought time distribution is mainly affected by the atmospheric circulation, in which two- to three-year periodic oscillation is related to the quasi-two-year oscillation (QBO) of the tropospheric atmospheric circulation, and the six-year cycle is related to the seven-year quasi-period of the five-year related ENSO event. On average, the drought at each station lasts for about two months, and summer and winter droughs are prone to occur. [PERSON] et al. (2017) analyzed the annual and seasonal distributions of droughs in Hunan Province between 1989 and 2008. During this period, nine summer droughs, six winter droughs, and five spring and autumn droughs occurred, a result that is consistent with the conclusion of this study. This study proved this conclusion on a longer time scale. A possible reason Hunan Province is prone to summer drought is that it is affected by the Pacific subtropical high-pressure air mass. When the airflow does not go from south to north, a strong summer drought emerges, and this phenomenon occurs in the Yangtze River Basin. This condition once again proves the strong influence of atmospheric circulation on regional drought. This study proposed an effective framework for dividing meteorological stations through an SPEI time series by using the K-means clustering method to analyze the spatial distribution characteristics of partial drought. The results showed that Hunan Province can be divided into three clusters. Cluster 1 stations are mainly distributed in the high-altitude areas of Hunan Province, and Cluster 2 stations are mainly distributed in southern Hunan. Cluster 3 stations are mainly distributed in basins and hilly areas and prone to severe drought. An interesting phenomenon is that Nanjing Mountain serves as the dividing line between Clusters 1 and 2, which further proves the influence of topographical factors on drought ([PERSON] et al., 2017). [PERSON] et al. (2017) reported that in plateaus and mountainous areas with complex terrain, the order of drought spread varies with altitude. Agricultural droughs in high-altitude areas evolve into hydrological droughs. The opposite is true for low-altitude areas. Altitude plays a key role in the spread of drought in plateau areas. [PERSON] et al. (2017) \begin{table}
\begin{tabular}{c c c c} \hline  & **Cluster 1** & **Cluster 2** & **Cluster 3** \\ \hline August 1967–June 1969 & Continuous wet & Slightly wet to mildly dry & Mildly dry to slightly wet \\ December 1998–August 1999 & Mildly dry & Severely dry & Slightly wet \\ September 2009–August 2011 & Slight recovery & Continuous drought & Wet to drought \\ \hline \end{tabular}
\end{table} 
Table 5: Three periods for the transition of the dry/wet status of the three clusters. studied the spread of drought in 16 sub-basins of a river basin from 1980 to 2014, and the results showed that the topographic index and hydrological drought have a significant correlation. These experimental results show that topographical factors are important content in drought research whether in plateau or watershed areas. The impact of Hunan Province on drought due to its special topography is also obvious. The reason is manifested in two aspects: (1) The high altitude affects the distribution of airflow, which in turn affects precipitation. (2) The second aspect is directly affected by photosynthesis.