Characteristics in the Spatiotemporal Variation and Periodic Evolution of Groundwater in the Xining Area of China, Eastern Qinghai–Tibet Plateau

Groundwater, an important source of water, has profound effects on global human survival and production. The spatiotemporal characteristics of groundwater and the periodic variation law of different time scales must be analyzed to grasp the dynamic situation of groundwater and provide scientific guidance for the rational utilization and management of groundwater resources. In this study, the temporal and spatial variability and periodicity of groundwater level were studied in the Xining region of the eastern Qinghai–Tibet Plateau China using traditional statistical method, geographic information system and Morlet wavelet analysis. Results show that the groundwater dynamics in the study area are mainly controlled by three factors, namely, the amount of river water infiltration, discharge from the groundwater, and artificial exploitation. The aforementioned factors can be divided into three dynamic types according to their combination relationship: hydrological, hydrological exploitation, and runoff-discharge types. The groundwater depth showed a trend of first increasing, then stabilizing, and finally decreasing from 1980 to 2020. The analysis in spatial variability demonstrated that the groundwater depth in different periods has a great spatial difference, with a moderate spatial variation intensity. Moreover, the spatial correlation of groundwater in the abundant season is lower than that in the dry season, which is mainly caused by the strengthening of artificial exploitation. The groundwater depth in the Xining region presents a pattern of deep in the south and east and shallow in the north and west by Kriging interpolation of spherical model in geographic information system. Meanwhile, the inter-annual groundwater has continued to decline since the chronic overexploitation between the 1960s and 2000s, with a maximum cumulative depth of 15 m. Then, the amount extraction had been further reduced, and rainfall had significantly increased in recent years. Accordingly, the cone of depression has undergone an evolutionary process from an expansion period to a stable period to a shrinking period. Furthermore, the annual groundwater level of most monitoring wells in study area has the same multi-year scale time variation characteristics with an evident regular periodic variation on the 9-14a and 17-25a time scales by using Morlet wavelet transform analysis. The temporal sequence of groundwater from 1980 to 2020 has the first and second main periods of 12a and 21a. In accordance with the two time-scales, the groundwater level will continue to rise in the short term in the future, which provides a scientific theoretical basis for the long-term sustainable development of groundwater resources and government decision-making.