Sub-theme
4. Supporting Aquatic Ecosystem Health and Functions
Topic
4-7. Groundwater and ecosystem
Body
According to the groundwater flow system theory, the pattern of groundwater circulation is organized into a hierarchical set of flow systems, i.e., local, intermediate, and regional flow systems could develop. Identifying the groundwater flow patterns is essential for understanding the hydrological cycle and sustainable management of groundwater resources. This however still remains difficult especially for a three-dimensional (3D) groundwater nested structure due to the multiple scales and complexity of the 3D flow field. The main objective of this study was to develop a quantitative method to partition the nested groundwater flow system into different hierarchies in three dimensions. Groundwater residence time, which is the total time required by water particles to move from recharge to discharge areas, contains a lot of hydrogeological information about groundwater circulation, and it can be used as a comprehensive indicator for groundwater flow and transport. A regional-scale groundwater flow numerical model was developed to study the characterization of residence time distribution in a 3D groundwater basin. The simulation results were used to examine the relationship between the residence time distribution and the hierarchically nested structure in space, attempting to partition the 3D groundwater flow system into different subsurface compartments. The model was subsequently used to investigate how the characteristics of groundwater flow system were affected by a variety of climatic and hydrogeological settings including recharge rate, anisotropic permeability and hydrothermal effect. The simulated groundwater residence time distribution showed a favorable consistency with the spatial distribution of flow fields. Probability density function was used to quantitatively describe the groundwater residence time statistical distribution. Each segment in the function distribution was associated to the specific groundwater flow hierarchy. A critical residence time can be recognized from the discrete feature of residence time distribution, and it was used to partition the nested groundwater flow system into different hierarchies. Furthermore, a series of characteristic parameters was acquired to quantitatively indicate the changes in the groundwater flow system under the impact of those climatic and hydrogeological factors. The results of the present study showed that the residence time distribution described by the probability density function was a useful tool to identify the nested structure of groundwater flow system. This study provided new insights and an efficient way to analyze groundwater circulation and evolution in three dimensions from the perspective of time domain.