5:15 PM - 7:15 PM
[ACG48-P04] Relationship between rainfall and river water level fluctuations in rice paddies in Satoyama – Field observation at Fugenji River in Kyotanabe City: Preliminary report
Keywords:Kyotanabe City, Fugenji River, Rainfall, Water level, Field observation, Environment in Satoyama
Mountain rivers have steep gradients and experience significant water level fluctuations due to rainfall, making them vulnerable to sudden flooding. The Fugenji River, a tributary of the Kizu River, originates in the northern Ikoma Mountains in southwestern Kyotanabe City, Kyoto Prefecture. Flowing north-northeast along Prefectural Road 65, it eventually joins the Kizu River. This river is essential for local agriculture, particularly rice cultivation, but its rapid water level fluctuations necessitate disaster prevention measures (Ministry of Land, Infrastructure, Transport and Tourism, 2025). Kyoto Prefecture has published flood hazard maps and urged residents to take safety precautions.
Despite the importance of water management for agriculture, data on mountain river flow conditions remain insufficient. This study aims to improve agricultural productivity and regional disaster prevention by analyzing rainfall and water level fluctuations in mountain rivers and creating a comprehensive database.
To clarify the rainfall-water level relationship, a water level gauge was installed upstream in the Fugenji River, with rainfall estimated using data from five nearby locations: four in Kyoto Prefecture (Takafune, Hishida, Tanabe, and Kannabi) and one in Nara Prefecture (Takayama). The inverse distance weighting (IDW) method was applied to estimate rainfall at the study site. Water level data were collected from: (1) Downstream Fugenji River (Miyamaki) - data provided by Kyoto Prefecture. (2) Upstream Fugenji River (Utta) - newly installed HOBO U20L Water Level Logger, operational since January 2025.
The measurement reference point was set at 0:00 on January 1, with a 5.1 km straight-line distance and 75-meter elevation difference between the sites. Using these data, we analyzed the relationship between precipitation and water levels in detail. The Fugenji River originates from a reservoir above terraced rice fields, and its flow path is mostly lined with concrete embankments.
For June 2024, we compared rainfall at Takafune with water levels at Miyamaki. The sites are 5.091 km apart with a 204-meter elevation difference. Rainfall at Takafune peaked at 20 mm by 9:00 AM on June 18, followed by a gradual rise in the Miyamaki water level about an hour later. Post-peak, the water level declined slowly.
A similar trend was observed in January 2024, showing a correlation between Takafune rainfall and Miyamaki water levels. Rainfall exceeding 2 mm led to sharp water level rises with a time lag of 1-3 hours. When rainfall was below 1 mm, water levels remained stable, suggesting a threshold for significant impact. Slight fluctuations on rain-free days were likely due to groundwater inflow, upstream contributions, and evaporation.
Additionally, we examined water level changes at Utta and Miyamaki during a rainfall event on January 6, 2025 (7:00 AM-6:00 PM). Between 7:00 AM and 11:00 AM, rainfall had little impact on Utta's water level, while Miyamaki's rose by only 0.01 m, confirming that a threshold is necessary for water levels to increase. Utta's water level continuously declined after 7:00 AM, while Miyamaki's began rising after 11:00 AM, peaking between 2:00 PM and 4:00 PM, approximately 1.2 hours after rainfall increased.
The differing water level trends between Utta and Miyamaki suggest that upstream inflow alone does not determine water level variations. Factors such as watershed topography, infiltration characteristics, and flow velocity likely contribute. This study further examines the influence of land use, topography, and geology on rainfall-water level fluctuations.
Rainfall data were obtained from four locations and water level data from one location, provided by the Yamashiro North Civil Engineering Office, Kyoto Prefecture, and rainfall data from one location by the River Management Division, Nara Prefecture Land Management Department. We express our gratitude for their generous data provision.
Despite the importance of water management for agriculture, data on mountain river flow conditions remain insufficient. This study aims to improve agricultural productivity and regional disaster prevention by analyzing rainfall and water level fluctuations in mountain rivers and creating a comprehensive database.
To clarify the rainfall-water level relationship, a water level gauge was installed upstream in the Fugenji River, with rainfall estimated using data from five nearby locations: four in Kyoto Prefecture (Takafune, Hishida, Tanabe, and Kannabi) and one in Nara Prefecture (Takayama). The inverse distance weighting (IDW) method was applied to estimate rainfall at the study site. Water level data were collected from: (1) Downstream Fugenji River (Miyamaki) - data provided by Kyoto Prefecture. (2) Upstream Fugenji River (Utta) - newly installed HOBO U20L Water Level Logger, operational since January 2025.
The measurement reference point was set at 0:00 on January 1, with a 5.1 km straight-line distance and 75-meter elevation difference between the sites. Using these data, we analyzed the relationship between precipitation and water levels in detail. The Fugenji River originates from a reservoir above terraced rice fields, and its flow path is mostly lined with concrete embankments.
For June 2024, we compared rainfall at Takafune with water levels at Miyamaki. The sites are 5.091 km apart with a 204-meter elevation difference. Rainfall at Takafune peaked at 20 mm by 9:00 AM on June 18, followed by a gradual rise in the Miyamaki water level about an hour later. Post-peak, the water level declined slowly.
A similar trend was observed in January 2024, showing a correlation between Takafune rainfall and Miyamaki water levels. Rainfall exceeding 2 mm led to sharp water level rises with a time lag of 1-3 hours. When rainfall was below 1 mm, water levels remained stable, suggesting a threshold for significant impact. Slight fluctuations on rain-free days were likely due to groundwater inflow, upstream contributions, and evaporation.
Additionally, we examined water level changes at Utta and Miyamaki during a rainfall event on January 6, 2025 (7:00 AM-6:00 PM). Between 7:00 AM and 11:00 AM, rainfall had little impact on Utta's water level, while Miyamaki's rose by only 0.01 m, confirming that a threshold is necessary for water levels to increase. Utta's water level continuously declined after 7:00 AM, while Miyamaki's began rising after 11:00 AM, peaking between 2:00 PM and 4:00 PM, approximately 1.2 hours after rainfall increased.
The differing water level trends between Utta and Miyamaki suggest that upstream inflow alone does not determine water level variations. Factors such as watershed topography, infiltration characteristics, and flow velocity likely contribute. This study further examines the influence of land use, topography, and geology on rainfall-water level fluctuations.
Rainfall data were obtained from four locations and water level data from one location, provided by the Yamashiro North Civil Engineering Office, Kyoto Prefecture, and rainfall data from one location by the River Management Division, Nara Prefecture Land Management Department. We express our gratitude for their generous data provision.