Equilibrium analysis and prediction evaluation of the Heilongtan groundwater system in Lijiang of northwestern Yunnan Province.

The karst water system is a dynamic system controlled by the degree of karst development and the underlying geological conditions. It constantly evolves with changes in climate, human activities, and many other factors. As one of the main driving forces of environmental change, human activities have caused the change at the rate and intensity far exceeding that of natural succession in a short period of time. The environmental effects and induced hydrogeological problems resulting from human activities are extremely difficult to distinguish and quantify, and hence have been the working focus of local governments and scholars at home and abroad. The Lijiang basin is a typical covered and semi-covered karst water system developed in the strong uplift zone of the late Cenozoic crust, which has given birth to the Heilongtan spring, the Xiguanlongtan spring, the Jiudinglongtan spring and other karst springs (spring groups). These springs are not only important water sources for Lijiang City, but also the city's highlights of tourism resources. Among them, the Heilongtan groundwater system is the most important karst water subsystem in the groundwater system of the Lijiang basin. However, the primary discharge point of this system, the Heilongtan spring, has currently experienced a significant reduction in flow and increasing intermittent drying up, which have become important factors restricting local economic and social development. Based on the comprehensive analysis of data over the past 70 years, the investigation of hydrogeological conditions in the Lijiang basin area, and the dynamic monitoring of the Heilongtan groundwater system, we quantitatively evaluated the dynamic flow change of and its influencing factors on the Heilongtan groundwater system with the method of water equilibrium. Fully considering the impact of human engineering activities in socioeconomic development, we summarized factors for the drying-up of the Heilongtan groundwater system as follows. (1) From 1951 to 1990, human engineering activities that replaced and reduced regional vegetation increased natural evaporation capacity and continuously increased evaporative discharge by 10,827,400 m³, accounting for 87.00% of the total decrease in the resources of the Heilongtan groundwater system (12,445,400 m³). This is the main reason for the drying-up probability and the increasing span of Heilongtan spring. (2) From 1991 to 2018, with the recovery of vegetation, the evaporative discharge continuously decreased by 18,303,400 m³, compared with that in the 1980s, which was beneficial to the recovery of Heilongtan spring. However, the factors such as human mining, leakage from the surface river system, and urban development and rereconstruction have caused a total decrease of groundwater resources by 50,586,900 m³, which is unfavorable to the recovery of the Heilongtan spring and is also the cause for its drying up. Among these causes, urbanization and human groundwater extraction reduced groundwater resources by 45,805,200 m³, which accounted for 90.54% of the overall decrease and was another primary cause of the drying-up. According to the change of influencing factors of the Heilongtan spring in recent years, the future changes of its flow predicted by the method of water equilibrium shows that: (1) From 2021 to 2030, the Heilongtan spring will change into a seasonal spring and become drying up during median water year and low flow year. (2) After 2030, the Heilongtan spring groups will remain drying up for a considerable amount of time with a very slim chance of reproduction. This study research can serve as a foundation for the dynamic recovery of Heilongtan groundwater, and the development and environmental protection of the groundwater resources in the urban area of Lijiang. [ABSTRACT FROM AUTHOR]