Research on genetic types and flow system characteristics of wetland in karst fault basin, China

As an important part of the ecosystem of Yunnan Plateau, karst wetlands are widely distributed in Yunnan fault basin. However, due to various reasons, lakes are silted up; the water surface area shrinks and even disappears. Lakes in eastern Yunnan are degraded significantly, and more than 20 natural lakes have prematurely disappeared in the past 30 years. In this paper, three typical types of karst wetland in the karst fault basins in eastern Yunnan are studied. These types are Tonghai lake degradation type (Qiluhu wetland), Luxi groundwater accumulation type (Huangcaozhou wetland) and Xuanwei underground river silting type (Geyi beaded wetland). Tonghai Qiluhu karst depression basin is located in the central part of Yunnan Province. It is a karst depression basin developed in the front arc of Yunnan mountain structure. The basin is a closed lake basin with a bottom length of about 22 km, a width of 9 km and a catchwater area of 345.95 km². The drainage basin of Luxi karst fault basin is located in the marginal slope area of southeast Yunnan Plateau, covering an area of 1,009.28 km². The bottom of the basin is distributed to the northeast in an irregular oval shape, with a longitudinal length of 16-20 km and a transverse width of 4-5 km. The overall terrain is high in the east and low in the west, and high in the north and low in the south. Xuanwei Geyi karst fault basin belongs to Beipanjiang third-level basin, Gexianghe fourth-level basin, and fifth-level Tianbian underground river system. The whole Tianbian underground river system is divided into a four-level discharge datum. One is Geyi Dalongtan on the edge of Rongqiu Depression, and the other is the beaded lake in Rongqiu Depression around Geyi town. The third is the bottom discharge of the Wujia family depression, and the fourth is the final discharge of the Gexiang river, with a watershed area of 160.82 km². The research contents include the genetic types and characteristics of wetlands, and the characteristics of "three fields" (runoff field, hydrochemical field and temperature field) of wetlands. The geological conditions, hydrogeological conditions and influencing factors of water environment of naturally formed wetland in karst fault basin are analyzed and the quality of water environment is quantitatively evaluated. The natural and human influencing factors, forms, intensity and effects of karst wetland, water source reduction, pollution intensification, water quality decline, wetland atrophy and ecological environment problems are also analyzed.

The research results indicate that the karst characteristics of the three types of karst fault basin wetlands are relatively similar, with frequent transformation of surface and groundwater, both of which take underground karst pipelines as the medium for transformation channels. Wetlands are discharge areas and also runoff areas that supply the outlet of underground rivers. All of the three types of wetlands are discharged to surface rivers through developed karst pipes. The lake degradation type (Qilu lake) collects surface water lakes at the bottom of the basin from the whole basin; the groundwater accumulation type (Huangcaozhou) is located in the area at shallow water level in the middle of the basin, and pore water and karst water are also recharge sources of wetlands. For the underground river silting type, the karst pipe blockage in the early groundwater runoff channel changes to the present karst fracture runoff, due to the topography of peak-cluster depression. Second, the wetland of lake degradation type (Qilu lake) covers a large area, which is greatly affected by natural and human factors. It is discharged by floods during the rainy season, and is used to irrigate during the dry season. Its severe water pollution is difficult to control with degradation of water quality and quantity. The groundwater accumulation type (Huangcaozhou) is controlled by the groundwater recharge of the upper spring, the underground river and the wetland itself, and the water quality is generally good. Although underground river of the silting type (Geyi beaded wetland) is affected by regional groundwater and human activities, the groundwater is generally abundant and relatively high in quality because the local population is relatively small with little impact. Thirdly, three types of water system structures are studied in terms of the characteristics of supplementation and drainage. The flow system structure of of Qiluhu degraded wetland in Tonghai is, surface collection→karst pipe drainage. The flow system structure of groundwater accumulation wetland in Huangcaozhou of Luxi is: surface runoff+underground seepage→surface runoff→karst pipe discharge. The flow system structure of Xuanweigeyi beaded wetland is, underground fissure seepage→spring drainage→karst pipe drainage.

The study findings suggest that we should protect and utilize wetland resources, and carry out long-term monitoring of wetland quality, water quantity and water level. We should protect wetlands and restore water environment by water transfer and physicochemical and biological measures. We should carry out afforestation and shrub planting around wetland to beautify wetland environment and conserve water source. We should improve farming patterns and use green pesticides. This study will provide a geological basis for the protection, scientific research, utilization and ecological restoration of wetlands in karst fault basins.