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[HCG23-P02] Impact of social change on the groundwater environment in the Kofu Basin
Keywords:Land use change, Groundwater level, Kofu basin, MODFLOW
The purpose of this study is to clarify the impact of land use change on the groundwater environment in the Kofu Basin. The Kofu Basin in Yamanashi Prefecture is surrounded by steep mountains and is a compound fan-shaped area between the Kamanashi River flowing from the northwest and the Fuefuki River from the east. Trend tests of groundwater, river water, and precipitation in the Kofu Basin showed statistically significant increasing trends in river discharge of the Kamanashi River, precipitation in the upper reaches of the Kamanashi River, and groundwater levels throughout the Kofu Basin. Both major recharge sources of groundwater in the Kofu Basin are increasing, and groundwater levels are likely to continue to rise. In addition, social changes are expected to affect the groundwater environment. In this study, we examined the effects of land use change on groundwater level fluctuations using eight time periods from 1976 to 2021.
In this study, we used MODFLOW, which was developed by the U.S. Geological Survey (USGS) and whose source code is publicly available. The analysis area for this study was the basin area between the Kamanashi and Fuefuki-gawa Rivers, and was discretized horizontally in a 100 m × 100 m grid. Elevation data was used a 10 m DEM. The vertical structure was divided into five layers based on 500 m borehole core data near Chiaki Bridge in the Kofu Basin, and the analysis area was uniformly given. Boundary conditions were set as a constant water head boundary condition and a river boundary condition. In the mountainous area to the north, groundwater is self-plumbing in some areas, so the height of the ground surface was used as the constant water head boundary condition. The river boundary conditions were set for the Kamanashi River, which flows from the northwest to the south and the Fuefuki River, which flows from the east to the southwest. Observation data from Funayama Bridge and Kamekabashi Bridge, which are under the jurisdiction of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), were used, respectively. The evaluation period was 2007-2009, when there were no missing data on flow and groundwater levels.
The results of the validation showed that the correlation coefficient between the analytical and observed groundwater levels was 0.86. The groundwater level contours in 1977 were compared with the analytical contours, and the results were in good agreement, reproducing the groundwater environment in the Kofu Basin. Using this model, we examined the impact of social changes on the groundwater environment using land use data for eight time periods (1976, 1991, 1997, 2006, 2009, 2014, 2016, and 2021).
In this study, we used MODFLOW, which was developed by the U.S. Geological Survey (USGS) and whose source code is publicly available. The analysis area for this study was the basin area between the Kamanashi and Fuefuki-gawa Rivers, and was discretized horizontally in a 100 m × 100 m grid. Elevation data was used a 10 m DEM. The vertical structure was divided into five layers based on 500 m borehole core data near Chiaki Bridge in the Kofu Basin, and the analysis area was uniformly given. Boundary conditions were set as a constant water head boundary condition and a river boundary condition. In the mountainous area to the north, groundwater is self-plumbing in some areas, so the height of the ground surface was used as the constant water head boundary condition. The river boundary conditions were set for the Kamanashi River, which flows from the northwest to the south and the Fuefuki River, which flows from the east to the southwest. Observation data from Funayama Bridge and Kamekabashi Bridge, which are under the jurisdiction of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), were used, respectively. The evaluation period was 2007-2009, when there were no missing data on flow and groundwater levels.
The results of the validation showed that the correlation coefficient between the analytical and observed groundwater levels was 0.86. The groundwater level contours in 1977 were compared with the analytical contours, and the results were in good agreement, reproducing the groundwater environment in the Kofu Basin. Using this model, we examined the impact of social changes on the groundwater environment using land use data for eight time periods (1976, 1991, 1997, 2006, 2009, 2014, 2016, and 2021).