Spatiotemporal variation and periodic evolution characteristics of groundwater in the Xining area of China, eastern Qinghai–Tibet Plateau

Abstract

As an important water source, groundwater profoundly influences global human survival and production. The spatiotemporal characteristics of groundwater and the periodic variation law of different time scales must be analyzed to understand the dynamic situation of this resource and to provide scientific guidance for its rational utilization and management. In this study, the temporal and spatial variability and periodicity of groundwater level in the Xining region of eastern Qinghai–Tibet Plateau in China were studied using the 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 groundwater, and artificial exploitation. These factors can be divided into three dynamic types, namely, hydrological, hydrological exploitation, and runoff discharge, according to their combination relationship. From 1980 to 2020, the groundwater depth in Xining initially increased before stabilizing and eventually decreasing. As revealed in the spatial variability analysis, groundwater depth demonstrates a huge spatial difference with a moderate spatial variation intensity across different periods. Moreover, the spatial correlation of groundwater in the abundant season is lower than that in the dry season mainly due to increasing artificial exploitation. Meanwhile, the Kriging interpolation of the spherical model in the geographic information system reveals that the groundwater in Xining is deep in the south and east and shallow in the north and west. From the 1960s to 2000s, the inter-annual groundwater in the region continuously declined due to chronic overexploitation, with a maximum cumulative depth of 15 m. Afterward, the amount groundwater extraction was further reduced, and the amount of rainfall significantly increased in recent years. Accordingly, the cone of depression underwent an evolutionary process involving periods of expansion, stability, and shrinkage. The annual groundwater level in most monitoring wells within the study area demonstrated the same multi-year scale time-variation characteristics with an evident regular periodic variation on the 9-14a and 17-25a time scales according to the results of the Morlet wavelet transform analysis. The temporal sequence of groundwater from 1980 to 2020 had two main periods, namely, 12a and 21a. According to these time scales, the groundwater level in Xining will continue to increase in the short term, which provides a scientific theoretical basis for the long-term sustainable development of groundwater resources and government decision-making.