Research significance

Karst water is the general name of groundwater existing in karst rock mass [1]. The research of this project is limited to groundwater in carbonate caves, pipelines and cracks. In the karst mountainous areas of southwest China, karst aquifers generally have huge water storage and water conduction space, and groundwater is abundant. Therefore, karst water is the most valuable groundwater type.

The coexistence of surface and underground karst space in karst areas is the fundamental reason for the formation of special karst environment [2]. Due to the spatial characteristics of the double-layer structure of karst environment, the uneven development of karst and the diversity of natural environmental conditions in karst areas, the karst water system is complex and changeable, which also leads to the uneven distribution of water resources in time and space and the serious shortage of surface water in karst areas, forming the water resources characteristics of "groundwater rolling, surface water as expensive as oil"

China is the country with the widest karst area in the world, with a karst area of 346× 104km2, accounting for 15.73% of the global karst area of 2200× 104km2. Carbonate rocks are mainly hard carbonate rocks formed in Mesozoic and before, which are easy to develop and form uneven underground karst cave system, and the population distribution density in karst areas is very high. Therefore, the problem of drought and water shortage is very serious. The karst rocky mountain area in southwest China covers eight provinces (cities, districts) in southern China centered on Yunnan-Guizhou Plateau, including Guangxi (hereinafter referred to as Guangxi), Guizhou (Yunnan), Sichuan (Sichuan), Chongqing (Chongqing), Hubei (Hubei), Hunan (Hunan) and Guangdong (Guangdong).

There are 46 ethnic groups living in karst mountain areas in southwest China, and most ethnic minorities live in rocky desertification mountain areas. The population of the whole region is about 8000× 104, which is dominated by traditional decentralized agricultural economy. The backward situation of "the old and the young living in poverty" has not been completely changed, and it is a key area for the development and poverty alleviation in western China. The climate in this area is humid, and the annual average rainfall exceeds 1000mm, but about 80% of the rainfall is concentrated in the rainy season. The infiltration coefficient of bare karst area is generally 0.3 ~ 0.4, and the high one can reach 0.5 ~ 0.6, and most of the rainfall is converted into underground runoff. In addition, soluble rocks have low soil-making ability, thin and scattered soil, sparse vegetation and poor water retention; The wind is strong and the temperature is high, and the ground evaporation is strong. Therefore, soon after the rain, drought can occur in these areas, with less rainfall in the dry season (1~ May) and severe drought [4]. Although the total amount of groundwater resources is rich, it is unevenly distributed, and many places mainly exist in underground karst caves. Due to the complexity of karst water storage space, it is very difficult to find water, which generally presents the characteristics of water shortage in the project. Because of the above reasons, about 1, 700× 104 people have difficulty drinking water. In Yunnan, Guizhou and Guangxi provinces alone, 253 1× 104 mu of cultivated land is seriously drought.

In Yunnan-Guizhou karst plateau area, on the one hand, on the surface or planation plane of karst plateau (elevation 1500~3500m ~ 3500 m), the terrain is generally flat and open, and there are many inter-mountain basins of different sizes distributed inside. On the other hand, Wujiang River (elevation 400 ~ 100~200m), Jinsha River (elevation 700 ~ 1500m), Yuanjiang River (elevation 100~200m) and Xijiang River (elevation 200 ~/kloc-0) In these valley areas where water resources are concentrated, the distribution of cultivated land and population is less, and there are not many important towns, factories and mines. Take Wujiang as an example. From the lower reaches of Wujiang River to the county seat along the river, the 400-kilometer-long main stream is basically cliffs and covered with rocks. Only in relatively open areas, such as Sinan and Yanhe, some platforms have cultivated land, which is tens to hundreds of meters wide, generally higher than the river surface 15 ~ 30m, and is formed by weathering small pieces of exposed clastic rocks. Along Nanpanjiang River and Yuanjiang River, it is basically a canyon terrain with only some slopes. However, there are dozens of karst fault basins and karst basins on the karst plateau or planation plane at an altitude of about 1300m or 1600m or higher, with thousands to tens of thousands of hectares of cultivated land and tens of thousands to millions of residents. The water level of major rivers is much lower than that of major cultivated land distribution areas, densely populated areas and economic activity areas, resulting in the problem of "incompatibility between water and soil". Faced with this objective problem and its background, with the deepening of exploration and development of karst water resources, more and more people realize that many karst water storage structures are like natural reservoirs in the upper reaches of rivers, and scientific investigation, planning, exploration, development and dispatching will play a very important role in solving the problem of "incompatibility between water and soil" and alleviating the increasingly tense contradiction between supply and demand of water resources.

Rational development and utilization of karst water resources and improvement of the regulation ability of groundwater resources are not only the need to solve the problem of drought and water shortage, but also the basic work to control rocky desertification and improve the ecological environment. In the karst mountainous area of southwest China, the area of rocky desertification has been determined as11.15×104km2. Serious rocky desertification leads to the decrease of cultivated land area, the decline of soil fertility, the weakening of water conservation ability, frequent droughts and floods, the deterioration of microenvironment climate, and the destruction of natural landscape and biodiversity, which seriously restricts the socio-economic development and the pace of getting rid of poverty and running towards a well-off society in southwest karst mountainous areas, and has become the most harmful slow-changing geological disaster in karst mountainous areas. There are 109 poverty-stricken counties in Yunnan, Guizhou and Guangxi provinces, 73 of which are located in rocky desertification areas, and the per capita annual income of farmers is only 55% of the national average. Because of rocky desertification, they can't maintain basic rations, and it is also difficult to use domestic water. In recent decades, the rapid population growth, deforestation and unreasonable development and utilization of natural resources such as land in this area have led to large-scale soil erosion and ecological environment destruction, which has become the main driving force for the intensification of rocky desertification [5][6]. Drought and water shortage are the most important natural factors that form rocky desertification. Rocky desertification makes the surface vegetation lose the function of coordinating water, drought makes the regeneration ability of vegetation decline, the biological chain (plant chain) is interrupted, the vegetation coverage rate is reduced, and the ecological environment is fragile. Therefore, rocky desertification is easy to occur and leads to the degradation or even loss of human living environment. The evolution process of karst rocky desertification is mainly reflected in: the intense destruction of surface vegetation → the intensification of surface erosion and soil erosion, the destruction of topsoil → the restoration of residual vegetation and soil seed bank and the loss of natural regeneration ability, the obstruction of benign ecological cycle, the large-scale rocky desertification → the destruction of hydrous vegetation and geological structure, the change of hydrological and water circulation system characteristics → the imbalance, metamorphosis and deterioration of water resources system. This imbalance and transformation not only includes the transformation form, runoff path, occurrence space, specific characteristic quantity and its distribution in time series of precipitation in karst rocky mountainous areas, but also includes the changes in the quality and function of water resources [7]. Thereby aggravating the degree of drought and water shortage and forming a new vicious circle. Therefore, according to the perfect minimum law of environment developed by British scientist blakeman in the early 20th century, which was put forward by German chemist J.V. Justus von Liebig in 1840, the quality of the whole environment is not determined by the average state of environmental factors, but is controlled by the one with the largest gap from the optimal state among environmental factors [8]. Severe drought and water shortage on the surface are the main limiting factors for ecosystem reconstruction in rocky desertification areas [9]. To control rocky desertification, we must put water resources planning and management in the first place, break the unfavorable situation of single-layer dispatching of double-layer runoff, speed up the research and implementation of advanced and applicable karst water development technologies and schemes, rationally and effectively develop and utilize karst water resources, cooperate with surface water conservancy projects, realize joint dispatching of surface and underground double-layer runoff in karst areas, and improve water resources regulation and control ability. Only by giving full play to the ecological function of karst water, solving the problem of drought and water shortage and ensuring ecological water demand can ecological construction be effectively carried out [10 ~ 14]. As the saying goes, "the key to beautiful mountains and rivers lies in water".

There is a huge underground water storage space in karst area, which forms a natural regulation function for runoff and plays a role in regulating precipitation on annual and inter-annual scales. Moreover, the groundwater quality is good, the water quantity is guaranteed to be high, the water conveyance distance is short, and the development project occupies little land. It has unique advantages and irreplaceable role in solving the problems of scattered production and domestic water in rural areas and drought-resistant and seedling-protecting water in forest and grass cultivated land in karst areas with serious water shortage. To give full play to the advantages of karst water resources and improve the drought and water shortage in karst areas depends on the degree of hydrogeological investigation, the improvement of understanding of karst water occurrence law and the progress of karst water exploration and development technology. In recent years, relevant departments have actively explored various methods such as "water cellar project", "resettlement project" and "water diversion project" to alleviate the drinking water difficulties of people and animals in the western region. Water cellar water collection is simple and easy, and it can store water in normal years, but it is difficult to play a role in dry years. Moreover, the water quality is very poor, and long-term drinking will bring harm to people's health. Resettlement project can fundamentally solve the problem of drinking water for people and livestock, but it needs huge financial and material resources, and the limited cultivated land, housing and environmental conditions in karst areas seriously restrict resettlement. The water diversion project is suitable for centralized water supply in towns, mines and other places. However, the karst rocky mountainous areas in southwest China have complex topography, and the rural residents in areas with serious water shortage live in scattered areas. In many areas, long-distance water transfer is not economical, and water sources are also unsustainable in dry season. Moreover, the development and utilization rate of surface water in many areas has been very high, and some river basins have caused ecological and environmental problems, with serious surface water pollution and increasingly serious water shortage. In addition, the surface leakage in karst rocky mountainous areas is serious, the conditions for building reservoirs are poor, and the cost of foundation treatment is expensive. Compared with the northern plains and southern hilly areas in China, the cost of building a billion-dollar large reservoir can only be (2000 ~ 3000) × 100 in the karst mountainous areas in southwest China. Therefore, it is very necessary and urgent to carry out the research on the exploration and development technology of karst water, provide reliable technical support for the effective development and utilization of karst water, and greatly improve the success rate and benefit of karst water exploration and development.

Various surface and underground karst spaces in karst areas have become the occurrence places of water resources, but the uneven development of karst has brought great difficulties to the exploration and development of karst water [16]. Determined by the uneven development of karst, the buried distribution of karst water is also uneven, which shows the spatial difference of water abundance and the anisotropy of hydraulic connection. In the engineering practice in karst areas, the following problems are often encountered: (1) The water output of a single well varies greatly, from tens to hundreds of cubic meters per hour to less than one cubic meter from the adjacent water supply hydrogeological exploration hole; Curtain grouting holes in water conservancy projects, some of which are tens of meters long, are basically impermeable. When the cave is exposed, the permeability can be increased by dozens of times, and some leakage channels will be distributed between two boreholes less than a few meters apart. In some mines, the water inflow is very small, but once the underground river passage is exposed, a huge water flow will immediately rush into the mine. These phenomena are all manifestations of uneven distribution of karst water. The heterogeneity of karst water often brings great difficulties to the exploration and development of karst water, so that the average pore-forming rate of water supply exploration holes in bare karst areas still hovers around 30%. But everything is divided into two parts. As long as we know its nature, determine the degree of unevenness and master its distribution law, we can make use of it. For example, in the development and utilization of karst water, as long as the pipeline distribution is determined and the location of the pipeline is found out, a small number of projects can be used to block the underground river, form a storage reservoir, raise the water level, reduce the head of water, and even divert water by itself. This is difficult to achieve for uniform pores and fractured aquifers. Because of the greatest resource potential, economic and social development demand and development benefit of karst water, the study of karst water occurrence law and effective exploration and development technology have always been the main work of hydrogeology, engineering geology and environmental geology research in karst areas. With the deepening of karst geological survey and the progress of detection technology, the success rate of karst water exploration and development will be improved in different degrees [17]. In addition, according to the requirements of sustainable development, karst water must be protected while being developed. Therefore, in the research of karst water exploration and development technology, the study and evaluation of karst water vulnerability at the same time not only provide scientific basis for the rational utilization and protection of karst water resources, but also promote the progress of environmental geology research, which meets the requirements of sustainable development.