5:15 PM - 6:45 PM
[HTT16-P05] Reducing the risk of salinization in the Saijo Plain through hydrological analysis using geological tracers
Keywords:salinization, antimony, tracer, irrigation, drought, groundwater level
1. Introduction
The Saijo Plain in Ehime Prefecture is rich in groundwater, with surface water supplied from a number of sources: irrigation water from the Kamo River, shallow groundwater discharged from the middle reaches of the plain, and pressurized deep groundwater. Groundwater is used for irrigation, and in times of drought, large amounts are pumped up, causing groundwater levels to fall. In July 1994, severe drought caused a significant fall in the groundwater level, which resulted in extensive groundwater salinization and agricultural damage in coastal areas. In this study, the hydrological cycle was analyzed using tracers and models, and possible ways to reduce the fall in groundwater level during drought were considered.
2. Materials and Methods
On August 1 and 2, 2021, when shallow groundwater had ceased discharging due to drought, surface water and groundwater were collected and water flow rates were measured at 43 points in the Saijo Plain (Kamo River flow rate: 2.7 m3 s−1). We focused on the difference in the content of Sb and Si in each water source, and used them as tracers for hydrological analysis. The ratio of the two tracers in the water source at each point was determined from simultaneous equations using the concentrations of Sb and Si. A lumped parametric model was used to simulate the groundwater level (Takase and Tokumasu, 2019).
3. Results and Discussion
In 1994, there was no rainfall during the irrigation period, especially from July 1st to 23rd. According to Takase and Tokumasu (2019), the amount of irrigation water pumped up was 4.3 m3 s−1, and the measured groundwater level was −2.52 m, the lowest in the past 40 years. To quantify the effect of groundwater pumping on the fall in groundwater level in July 1994, the simulation was conducted using the above model. The period was 23 days without precipitation, the flow rate of the Kamo River (3.27 m3 s−1) and the weather conditions were adjusted to reflect the conditions in July of 1994, and the groundwater pumping rates were varied to 3, 4, and 5 m3 s−1. When the pumping rate was 3 m3 s−1, the groundwater level hardly fell, but when the pumping rate was 4 m3 s−1, the level fell by about 1 m. Assuming the Kamo River flow rate was 4 m3 s−1, the decrease would be 0.7 m. The contribution of each water source at the end of the system in 2021, when the hydrological environment was the same as in 1994, was calculated using Sb as a tracer. The results showed that irrigation water from the Kamo River accounted for 73% in the eastern area, deep groundwater 65% in the central area, and shallow and deep groundwater 99% in the western area. In terms of flow, irrigation water from the Kamo River was 0.46 m3 s−1, 0.43 m3 s−1 from shallow groundwater, and 0.45 m3 s−1 from deep groundwater, for a total of 1.34 m3 s−1 to the sea. The natural source of water for rivers in the Saijo Plain is shallow groundwater, which does not flow during drought periods. The results show that even during a drought season, more than 1 m3 s−1 of water is supplied. Adjusting oversupply can reduce groundwater pumping from 4 to 3 m3 s−1 during droughts. It has been found that during times of drought, which occur only once every few decades, the risk of salinization can be significantly reduced by efficient allocation of irrigation water from the Kamo River and proper management of groundwater pumping.
The Saijo Plain in Ehime Prefecture is rich in groundwater, with surface water supplied from a number of sources: irrigation water from the Kamo River, shallow groundwater discharged from the middle reaches of the plain, and pressurized deep groundwater. Groundwater is used for irrigation, and in times of drought, large amounts are pumped up, causing groundwater levels to fall. In July 1994, severe drought caused a significant fall in the groundwater level, which resulted in extensive groundwater salinization and agricultural damage in coastal areas. In this study, the hydrological cycle was analyzed using tracers and models, and possible ways to reduce the fall in groundwater level during drought were considered.
2. Materials and Methods
On August 1 and 2, 2021, when shallow groundwater had ceased discharging due to drought, surface water and groundwater were collected and water flow rates were measured at 43 points in the Saijo Plain (Kamo River flow rate: 2.7 m3 s−1). We focused on the difference in the content of Sb and Si in each water source, and used them as tracers for hydrological analysis. The ratio of the two tracers in the water source at each point was determined from simultaneous equations using the concentrations of Sb and Si. A lumped parametric model was used to simulate the groundwater level (Takase and Tokumasu, 2019).
3. Results and Discussion
In 1994, there was no rainfall during the irrigation period, especially from July 1st to 23rd. According to Takase and Tokumasu (2019), the amount of irrigation water pumped up was 4.3 m3 s−1, and the measured groundwater level was −2.52 m, the lowest in the past 40 years. To quantify the effect of groundwater pumping on the fall in groundwater level in July 1994, the simulation was conducted using the above model. The period was 23 days without precipitation, the flow rate of the Kamo River (3.27 m3 s−1) and the weather conditions were adjusted to reflect the conditions in July of 1994, and the groundwater pumping rates were varied to 3, 4, and 5 m3 s−1. When the pumping rate was 3 m3 s−1, the groundwater level hardly fell, but when the pumping rate was 4 m3 s−1, the level fell by about 1 m. Assuming the Kamo River flow rate was 4 m3 s−1, the decrease would be 0.7 m. The contribution of each water source at the end of the system in 2021, when the hydrological environment was the same as in 1994, was calculated using Sb as a tracer. The results showed that irrigation water from the Kamo River accounted for 73% in the eastern area, deep groundwater 65% in the central area, and shallow and deep groundwater 99% in the western area. In terms of flow, irrigation water from the Kamo River was 0.46 m3 s−1, 0.43 m3 s−1 from shallow groundwater, and 0.45 m3 s−1 from deep groundwater, for a total of 1.34 m3 s−1 to the sea. The natural source of water for rivers in the Saijo Plain is shallow groundwater, which does not flow during drought periods. The results show that even during a drought season, more than 1 m3 s−1 of water is supplied. Adjusting oversupply can reduce groundwater pumping from 4 to 3 m3 s−1 during droughts. It has been found that during times of drought, which occur only once every few decades, the risk of salinization can be significantly reduced by efficient allocation of irrigation water from the Kamo River and proper management of groundwater pumping.