New strategy for evaluating the spatiotemporal distribution of groundwater resource quantity under seasonal freeze/thaw in mountainous areas

Mountainous areas that experience seasonal freeze/thaw are distributed worldwide, and the impact of freeze/ thaw processes on the groundwater cycle is substantial. For these mountainous areas, the rational division of two key parameters, namely, the groundwater resource quantity calculation unit (GCU) and groundwater resource quantity calculation period (GCP), is a prerequisite for the scientific evaluation of groundwater resource quantity. This is especially important for the Changbai Mountains in Jilin Province, China, which have a distinct geology formed by volcanic processes and have experienced soil seasonal freeze/thaw cycle, implying drastic impacts on the spatiotemporal distribution of groundwater resources. Based on the groundwater flow system theory and considering the influences of freeze/thaw cycles on the groundwater resource quantity, we propose a new strategy to deal with the division of GCUs and GCPs. Three first-level GCUs, eight second-level GCUs, and fifty third-level GCUs were delineated within the Changbai Mountains. Five GCPs were then determined, and the spatiotemporal distribution of groundwater resource quantity was calculated for the different GCUs and GCPs. The groundwater resource quantity in the five GCPs (no freezing, unstable freezing, stable freezing, unstable melting, and stable melting periods) accounted for 62.25%, 11.51%, 11.58%, 5.05%, and 9.61% of the total, respectively. The results provide a scientific basis for the exploitation of groundwater resource in different re-gions and periods of the study area, and the proposed method offers a new reference strategy for the evaluation of groundwater resource in similar regions.