Characteristics of temperature field of the Heilongtan groundwater system in Lijiang

The groundwater temperature is an important indicator of groundwater dynamic change. Therefore, comprehensively monitoring groundwater temperature, understanding its temporal and spatial distribution characteristics, and studying its change rule can not only reflect the hydrogeological characteristics of groundwater system, but also provide a basis for its identification and division. With an elevation of 3,500 m–2,400 m, the Heilongtan groundwater system of Lijiang, Yunnan Province is a typical mountain and basin landform with wide distribution of soluble rocks, strong development of karst, and combination of faults and folds. Its hydrogeological environment is complex, challenging the delimitation of spring area and groundwater system. Because the study area is located in the plateau mountainous area, and the temperature field of karst groundwater can also reflect the variation of groundwater temperature at different elevations in the mountain and canyon areas on the plateau, this study has a great referential significance for the research of groundwater in other similar areas. Based on previous research results as well as a large number of monitoring data on groundwater temperature obtained in the study area from June to November, 2018, the spatial-temporal variation characteristics of groundwater temperature in the spring area and the variation characteristics of borehole water temperature were compared and analyzed, and the indicative significance of these two types of variation characteristics were discussed. Meanwhile, the feasibility of system division of the Heilongtan spring area was verified. The results show that there is a good spatial correlation between the elevation of groundwater level and temperature, and the groundwater temperature in the recharge area at high altitude is low. From the recharge area to the discharge area, the water temperature experiences a gradual upward trend. The water temperature in the discharge area can also indicate the circulation of groundwater runoff. The lower the water temperature in the discharge area is, the smoother groundwater becomes. In terms of time, as groundwater receives a large amount of rainfall recharge during the flood season, the circulation of groundwater flow increases, which may result in a short storage time of water and the phenomenon that the groundwater temperature in the discharge area is closer to that in the recharge area and that the overall water temperature decreases. At the end of the flood season, as rainfall decreases, the circulation of groundwater flow weakens, and the underground water temperature in the discharge area rises in different degrees. In a certain depth below the surface, the groundwater temperature does not completely conform to the theoretical law that the temperature rises gradually with the increase of depth, On the contrary, the groundwater temperature will remain unchanged or decrease gradually with the increase of depth, the reasons of which can be summarized as follows. (1) The temperature of surface water formed by precipitation is higher than that of groundwater, and the groundwater will be heated during infiltration and recharge. (2) When a deep karst pipeline is developed, the groundwater will "wash" away the heat from the deep, and the groundwater temperature will decrease with the increase of depth. (3) If the vertical movement of groundwater is intense, with large up-and-down disturbances and good temperature exchange conditions, the temperature value will become relatively stable in this active zone. This phenomenon is found in all monitoring points in the discharge area, especially in the period of smooth groundwater flow. (4) Under the influence of solar radiation energy, the surface temperature shows diurnal and seasonal variations. The diurnal variation affects 1–2 m below the surface, and the seasonal variation affects 30 m below the surface. This study shows that there are similarities and differences in the groundwater temperature in the study area, and the groundwater temperature is a good indication for the delimitation of groundwater system. This study also verifies the rationality of the existing delimitation system in the study area.