Water is the critical factor that constrain human survival and socio-economic development in karst areas (Apollonio et al., 2018; Mesnil et al.,2020; Castro, 2020). The karst features such as pipes, caves, cavities, sink holes and grooves, lead to frequent exchange of surface water and groundwater, which constructed an open system in epikarst zone. The difficulty of getting storage between surface water and groundwater is due to the complex and variable hydrological structure of open systems. This is the main reason for the frequent occurrence of water resources problems in karst areas. (Zverev and Kostikova, 2016). Epikarst zone is a natural water storage medium for the surface part of the strongly karst envelope in karst mountainous areas (Soglio et al., 2020; Fidelibus et al., 2017). Epikarst zone is an important critical zone for water resources transformation in the karst area (Jiang and Yuan, 1999; Williams, 2008), and it is also an important carrier for water resources and ecological environment. Recent years, with the global climate change, the water resources problems in karst areas become more and more prominent.
The transformation process of Five water in epikarst zone of karst area refers to the mutual transformation process among precipitation, karst groundwater, evapotranspiration water, soil water and overland flow. Among them, karst groundwater includes surface karst water and groundwater. In addition, evaporation water includes surface evaporation water and vegetation transpiration water. In the transformation process of Five water, part of the precipitation can be directly transformed into surface karst water, soil water, overland flow, groundwater, etc. (Qi et al., 2012). Precipitation can also be indirectly converted to groundwater indirectly through surface karst water (Jiang, 2009). At the same time, precipitation can be absorbed by vegetation, and finally converted to precipitation again by surface evaporation and vegetation transpiration (Carrière et al., 2019).
The current research on water resources is focusing on the three waters transformation, i.e. including overland flow, groundwater and precipitation (Hartmann, 2015). Wang (Wang and Shi, 2006) proposed corresponding rational water resources utilization on measures in the study of the three waters transformation process in the southwest karst mountains. Zhao (Zhao and Dong, 2015) analyzed the influencing factors of water resources transformation processes in karst areas. Jiang (Jiang and Guo, 2009) conducted various studies on water resources transformation and hydrological dynamics of the epikarst zone. These related studies revolved the macroscopic laws of overland flow, groundwater and precipitation. However, the overland flow, soil water and evapotranspiration in the epikarst zone are also important for water balance. The research on the epikarst zone focused on the monitoring study of hydrological and hydrodynamic processes and related research methods. However, the specific processes and laws of water resources transformation in different types of karst areas have rarely been studied by scholars.
Although researchers had conducted studies on karst water resources, future researches are still needed to reveal the mechanism of Five water. The blurred boundaries of different types of water in karst areas, the complex structure of water storage and the complicated and variable transformation process of Five waters are the difficulties in the current research. The transformation process of water resources of Five water was elucidates. In this article, the transformation process and response law of water resources in karst areas was also revealed. The efficiency of water use in karst areas can be effectively enhanced by the conclusions of this article.