1:45 PM - 3:15 PM
[AHW21-P01] Evaluation of important areas and factors for the spring water discharge using a surface and subsurface integrated model in Kumamoto area, southern Japan
★Invited Papers
Spring waters have been used for domestic and agricultural purposes as an important water resource in the local region. In addition, the beautiful landscapes formed by spring waters have provided various values such as recreation and tourism. However, the spring water discharge has been declining or even drying up in some cases. This phenomenon is thought to be caused by changes in precipitation, excessive use of groundwater, which is the origin of spring water, and reduced recharge due to urban development. The distributed hydrological model, which integrates surface water and groundwater flow processes, can be a very powerful tool for quantitatively evaluating the factors that control the increase or decrease of spring water discharge as a response of the hydrological system.
Kumamoto area has abundant groundwater resources and is dependent on groundwater. The spring waters that originate from the groundwater make up the region's good water environment. In addition, the past decline in the discharge of spring water has triggered efforts to recharge rice paddies in areas that are considered to be the main recharging areas. However, the factors that cause changes in the discharge of spring water are still unknown, although several hypotheses have been proposed, and as a result, it is not clear what actions are important.
In this presentation, we try to clarify the factors that cause changes in the discharge of spring water in this area using a distributed hydrological model, which integrates surface water and groundwater flow processes. Four models with confirmed performance in simulating groundwater levels and river flow rates were used. The spring water to be evaluated was Lake Ezu, which is a representative spring in the Kumamoto area.
First, the performance of the four models with observed discharge of spring water was checked, and the average observed value was 435,000 m^3/day, while the simulated value ranged from 222,000 m^3/day to 315,000 m^3/day. Since the results were slightly underestimated but did not differ by order of magnitude, factors were analyzed using these four models. To clarify the important factors, the modeling area was first divided into eight areas based on topography and flow characteristics, and sensitivity analysis was used to identify the areas that were important for the discharge of spring water in Lake Ezu. Next, it was confirmed that the combination of precipitation, land use change, and water use change would not result in extreme changes in the discharge of spring water. Then, an alternative model with high calculation speed was constructed, and a what-if simulation was conducted to try to predict the impact using this model.
The results showed that the most important factor affecting the discharge of spring water in Lake Ezu was the change in pumping in the area near Lake Ezu, followed by the effective precipitation in the same area. On the other hand, the physical properties of the ground surface (hydraulic conductivity and Manning’s roughness coefficient) were more influential in the middle reaches of the Shirakawa River, which is considered to be a recharge area, than in the area near Lake Ezu. These results indicate the importance of water balance values directly related to the discharge of spring water as well as the importance of land use in the recharge area, and are considered to be findings that provide important suggestions for future spring water conservation activities in the Kumamoto region.
Kumamoto area has abundant groundwater resources and is dependent on groundwater. The spring waters that originate from the groundwater make up the region's good water environment. In addition, the past decline in the discharge of spring water has triggered efforts to recharge rice paddies in areas that are considered to be the main recharging areas. However, the factors that cause changes in the discharge of spring water are still unknown, although several hypotheses have been proposed, and as a result, it is not clear what actions are important.
In this presentation, we try to clarify the factors that cause changes in the discharge of spring water in this area using a distributed hydrological model, which integrates surface water and groundwater flow processes. Four models with confirmed performance in simulating groundwater levels and river flow rates were used. The spring water to be evaluated was Lake Ezu, which is a representative spring in the Kumamoto area.
First, the performance of the four models with observed discharge of spring water was checked, and the average observed value was 435,000 m^3/day, while the simulated value ranged from 222,000 m^3/day to 315,000 m^3/day. Since the results were slightly underestimated but did not differ by order of magnitude, factors were analyzed using these four models. To clarify the important factors, the modeling area was first divided into eight areas based on topography and flow characteristics, and sensitivity analysis was used to identify the areas that were important for the discharge of spring water in Lake Ezu. Next, it was confirmed that the combination of precipitation, land use change, and water use change would not result in extreme changes in the discharge of spring water. Then, an alternative model with high calculation speed was constructed, and a what-if simulation was conducted to try to predict the impact using this model.
The results showed that the most important factor affecting the discharge of spring water in Lake Ezu was the change in pumping in the area near Lake Ezu, followed by the effective precipitation in the same area. On the other hand, the physical properties of the ground surface (hydraulic conductivity and Manning’s roughness coefficient) were more influential in the middle reaches of the Shirakawa River, which is considered to be a recharge area, than in the area near Lake Ezu. These results indicate the importance of water balance values directly related to the discharge of spring water as well as the importance of land use in the recharge area, and are considered to be findings that provide important suggestions for future spring water conservation activities in the Kumamoto region.