Climate change is a global environmental problem which will lead to a series of phenomena, such as accelerating surface evaporation rate, increasing atmospheric water retention capacity, accelerating water circulation rate and decreasing system stability(Held and Soden, 2006, Huntington, 2006). Two exemplary climate extremes that are closely associated with the expected changes in hydrologic cycle are drought and pluvial flood(Shao and Kam, 2020).The frequent occurrence of extreme weather phenomena, such as drought, flood, and the alternation of drought and flood, has caused negative effects on natural ecosystem and social system from many aspects(Li and Ye, 2015). The phenomenon of climate disasters has attracted increasingly attention due to the frequency and severity of its occurrence.
There are many methods used to study the temporal and spatial distribution characteristics of drought and flood. Scholars combined some commonly used methods, such as MK mutation test method, trend analysis method, correlation analysis method, moving average method, cumulative anomaly method, ensemble empirical mode decomposition method (EEMD), inverse distance weighted interpolation method (IDW), empirical orthogonal function method (EOF) and other methods to further analyze the temporal and spatial characteristics of regional drought and flood(Lu et al., 2018, Min et al., 2017, Wang et al., 2017). Based on precipitation data from 1960–2019, existing studies have analyzed the drought and flood disasters in the Huang-Huai-Hai plain, an important commercial grain production base in China. Methods such as Mann-Kendall test, wavelet analysis, EOF and center of gravity model were used to analyze the temporal and spatial distribution characteristic of total precipitation at different time scales in the Huang-Huai-Hai plain(Ling et al., 2021). In the study of the changes in extreme drought and flood events in Iran, the Mann-Kendall test was used to assess the changing trends of flood disasters. The standardized precipitation index was used to study regional drought disasters(Modarres et al., 2016). Therefore, the Mann-Kendall method, as a non-parametric test method recommended by the World Meteorological Organization, has the advantages of a wider detection range, fewer human factors, a higher degree of quantification, and can intuitively show the trend of change and has been widely used.
It is necessary to compare and analyze different drought and flood indexes to carry out subregional research. In the study of comparing and evaluating the time and space dynamics of drought in 6 climatic regions of Iran, scholars have conducted a comparative analysis of 6 drought indexes including the Z index. The research suggest that the Z index can still be used as one of the better drought prediction indicators in Iran considering the dry season, duration, and climate conditions(Shahabfar and Eitzinger, 2013). In order to evaluate the sensitivity of drought index based on precipitation to different record length, 7 drought indexes were discussed, including the China Z Index (CZI), the Modified China Z Index (MCZI), Percent of Normal Precipitation Index (PNPI), Deciles Index (DI), the Z-score Index (ZSI), Effective Drought Index (EDI), and Standardized Precipitation Index (SPI). The research found that the Z index has good time stability and high sensitivity to different record lengths(Mahmoudi et al., 2019). With the strengthening of the severity of flood disasters, research on flood disasters gradually increases. Based on the method of artificial bee colony algorithm and back-propagation neural network, the precipitation was forecasted in Wujiang River basin which not only reveals the interannual variation trend and abnormal situation of precipitation in the basin, but also uses Z index to identify the flood and drought years. The results provided a new idea for climate prediction, flood prevention and drought relief(Wang et al., 2020a). In summary, the Z index is used frequently to study drought and flood disasters because the Z index involves parameters such as skewness coefficients and standard variables in the calculation process, which enables the Z index to better consider the temporal and spatial distribution of rainfall. Among them, for extreme precipitation, the larger the skewness coefficient, the better the Z-index analysis effect, and the more it can reflect the degree of extreme drought and flood(He Yanhu et al., 2014).
Many scholars have conducted various studies on drought and flood disasters. However, most studies aimed at regional droughts and floods, and most of them evaluated regional droughts and floods from the perspective of a single factor (precipitation), while ignoring the impact of the characteristics of the underlying surface on the formation of drought and flood. The sensitivity of drought and flood is different based on different carriers. Among them, the most prominent carrier affected by drought and flood disaster is agricultural production(Zhang et al., 2015). It is generally believed that in the process of interaction between meteorological conditions and underlying surface factors, when the distribution of moisture on the underlying surface has direct or indirect adverse effects on human survival, production and life, it is called a drought and flood disaster. The formation of the underlying surface is the result of the long-term action of various geological forces. Therefore, geological action has led to the formation of regional landforms, thereby affecting the distribution of flood and drought disasters. Scholars have studied the spatial and temporal distribution characteristics of drought and flood in Hebei Province, and put forward the influence of underlying surface factors on the formation of drought and flood. It revised the index method and established a drought and flood assessment system suitable for the actual situation in Hebei Province(Shao et al., 2001). According to the survey data of rainstorm and flood disasters in 637 counties (districts) in eastern China, the distribution and change patterns of rainstorm and flood disasters in eastern China are analyzed(Shi et al., 2020). There are many analyses of drought and flood disasters in China, however, most studies on drought and flood has not taken account the underlying surface factors.
Hainan Island is mainly affected by tropical cyclones, tropical storms and typhoons, but there is little research on its drought and flood disasters. Zhang Lei et al., analyzed the drought and flood disasters in Hainan Province from 1998 to 2011 based on analytic hierarchy process and entropy method. The scholars aim to help farmers and policy makes reduce the risk of red pepper from major meteorological disasters(Zhang et al., 2017). It focuses on the assessment of the risk, sensitivity, vulnerability and prevention capability for flood, chilling and drought disaster. Hainan Province is one of the provinces which is affected by meteorological disasters. Natural disasters have reduced agricultural production by more than 10% in Hainan, and have reduced the crop planting area by more than 30%(Xu et al., 2015). The climatic characteristics of tropical regions are relatively unique, and there are many studies on the drought and flood of disasters in tropical regions(Firoz et al., 2018, Yamamoto et al., 2021). For example, in Indonesia's rivers, which are also tropical regions, the extent of inundation is predicted caused by future flood events, so as to better manage the watershed(Yamamoto et al., 2021).There are many kind of flood risk assessment method should be adopted to evaluate the flood risk caused by extreme flood(Kvočka et al., 2016). The underlying surface has a major impact on the occurrence and development of drought and flood disaster. Consequently, on the basis of using the traditional meteorological index to evaluate the regional drought and flood, it is necessary to correct the initial situation according to the underlying surface condition.
Therefore, the goals of this study were: (1) to evaluate the vulnerability of drought and flood disasters, and consider the influence of potential factors such as unique topography and land use/cover on the formation of drought and flood events. (2) to zone drought and flood areas using land use data sets and DEM data base on the Z index of precipitation. (3) to analysis the spatial and temporal distribution characteristics of drought and flood disasters in Hainan Island, and to put forward targeted measures to prevent drought and flood.