Climate change and the future of freshwater resources of the island: a case study on the Rishiri Island, Japan

摘要

A comparison of the d-excess values of pre-cipitation and of spring water, streams, groundwater wells and submarine groundwater discharge indicated that the precipitation that occurred during winter season was an important source of groundwater recharge. Due to the steep slope of the island, most of the short duration and high intensity precipitation is lost through direct surface runoff. The comparison indicated that snowmelt is an important resource of groundwater recharge on Rishiri Island. Future climate change will continue to diminish the snowpack, and therefore, reduce groundwater recharge. It may cause the decline of the groundwater level in the coastal area and possibly shift the saline-freshwater boundary on the island. Chloride data indicated that saltwater intrusion is begin-ning to occur on the western flank of the island. A Piper diagram shows that the water samples are characterized by the dominance of the Ca-HC0_3 and Na-Cl type. Their chemistry probably results from sea salt spray and the dissolution of minerals. These results support the need for the effective management of groundwater resources.