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[AHW28-14] Estimation of spring recharge area around Suizenji and Lake Ezu, and comparison of recharge effect by land use
Keywords:Lake Ezu, groundwater discharge rate, recharge area, land use
Kumamoto Prefecture is blessed with abundant water resources. In particular, the area around Suizenji and Lake Ezu in Kumamoto City has become an area where a large amount of groundwater gushes out, and groundwater discharge was calculated monthly from river flow data. The study area was divided into three zones, and only the positive values of each groundwater discharge were summed, resulting in an average of approximately 160,000,000 m³/year from January 2007 to December 2015. The objective of this study is to estimate the area of the recharge area necessary to support the groundwater discharge rate recharged by precipitation out of the total groundwater discharge. I also aim to consider the difference in recharge efficiency based on the land use conditions within the recharge area.
In this study, groundwater basins were determined by checking the groundwater flow paths determined from topographic maps with the GETFLOWS groundwater flow trace maps. Recharge areas were calculated using discharge coefficients, precipitation, and evapotranspiration. The discharge coefficients were based on the values provided by the Ministry of Land, Infrastructure, Transport and Tourism. For precipitation and evapotranspiration, data presented by the Kumamoto Meteorological Office were used. Evapotranspiration was calculated using the Advection-Aridity model proposed by Brutsaert and Stricker(1979). Since the groundwater basin consists of building sites, rice paddies, fields, and forests, the recharge area was calculated in this study using discharge coefficients and evapotranspiration rates by land use. The groundwater in the study area is a mixture of groundwater recharged by precipitation and the Shirakawa River water. In this study, groundwater discharge originating from river water was calculated for each zone using the proportion of Shirakawa River water calculated from the ionic component ratios in the spring water sampled in each zone. The groundwater discharge originating from precipitation was then calculated by subtracting the calculated values from the total groundwater discharge. The groundwater discharge was calculated to be 120,649,738 m³/year by subtracting the groundwater discharge originating from river water from the total groundwater discharge.
The calculated groundwater discharge was used to calculate the recharge area, which was 182.33 km². This area includes the middle reaches of the Shirakawa River and the Takuma Plateau at the western foot of Mt. Aso, which is the origin of groundwater recharge identified in previous studies. The highest percentage of land use within this range was land for buildings (54.3%), while the percentage of agricultural land such as rice paddies and fields were 38.5%. The percentage of recharge by land use within the recharge area was 25.8% for building land, which is small relative to the area of the recharge area. On the other hand, agricultural land accounted for 64.5% of the recharge volume. These results suggest that the groundwater gushing from Suizenji and Lake Ezu is recharged in the area including the middle reaches of the Shirakawa River and the Takuma Plateau, and that the amount of recharge from agricultural lands such as rice paddies and fields has a particular impact on the groundwater gushing from these areas.
In this study, groundwater basins were determined by checking the groundwater flow paths determined from topographic maps with the GETFLOWS groundwater flow trace maps. Recharge areas were calculated using discharge coefficients, precipitation, and evapotranspiration. The discharge coefficients were based on the values provided by the Ministry of Land, Infrastructure, Transport and Tourism. For precipitation and evapotranspiration, data presented by the Kumamoto Meteorological Office were used. Evapotranspiration was calculated using the Advection-Aridity model proposed by Brutsaert and Stricker(1979). Since the groundwater basin consists of building sites, rice paddies, fields, and forests, the recharge area was calculated in this study using discharge coefficients and evapotranspiration rates by land use. The groundwater in the study area is a mixture of groundwater recharged by precipitation and the Shirakawa River water. In this study, groundwater discharge originating from river water was calculated for each zone using the proportion of Shirakawa River water calculated from the ionic component ratios in the spring water sampled in each zone. The groundwater discharge originating from precipitation was then calculated by subtracting the calculated values from the total groundwater discharge. The groundwater discharge was calculated to be 120,649,738 m³/year by subtracting the groundwater discharge originating from river water from the total groundwater discharge.
The calculated groundwater discharge was used to calculate the recharge area, which was 182.33 km². This area includes the middle reaches of the Shirakawa River and the Takuma Plateau at the western foot of Mt. Aso, which is the origin of groundwater recharge identified in previous studies. The highest percentage of land use within this range was land for buildings (54.3%), while the percentage of agricultural land such as rice paddies and fields were 38.5%. The percentage of recharge by land use within the recharge area was 25.8% for building land, which is small relative to the area of the recharge area. On the other hand, agricultural land accounted for 64.5% of the recharge volume. These results suggest that the groundwater gushing from Suizenji and Lake Ezu is recharged in the area including the middle reaches of the Shirakawa River and the Takuma Plateau, and that the amount of recharge from agricultural lands such as rice paddies and fields has a particular impact on the groundwater gushing from these areas.