09:00 〜 09:15
[AHW28-01] Use of hydrogeochemistry to assess water resource stability of Nanmoku watershed, Central Japan
キーワード：Baseflow, end-member, hydrogeochemical parameters, Nanmoku watershed
Depopulation and aging in various hilly and mountainous rural areas of the globe, including certain areas of Japan, have been deteriorating agricultural production, infrastructure, and the local environment [1]. Being a consistent and reliable water supply, smart utilization (e.g., electricity production) of stable water resources is vital in fostering sustainable development of such areas. Baseflow contributes to maintaining streamflow in a watershed and helps stabilize stream discharge even during dry spells. By using the case of the Nanmoku watershed (where Nanmoku village is suffering from aging and depopulation), this research aims to find hydrogeochemical methods to find the baseflow contribution, such methods can be applied in other hilly and mountainous areas globally.
Baseflow fractions were calculated by using the hydrogeochemical parameters in the mass balance equation [2]. pH, electrical conductivity (EC), dissolved oxygen (DO), oxidation-reduction potential (ORP), temperature, major ions (Na+, K+, Ca2+, Mg2+, NH4+, SO42-, Cl-, HCO3-, NO3-), radon (222Rn), oxygen-18 (δ18O), and deuterium (δD) of the water samples at the stream outlet of sub-basins of Nanmoku watershed were analyzed in November-2023, and January, March, May, July, and September of 2024. Additionally, the discharges of streams were measured using an electromagnetic flow meter in March, May, July, and September.
The instantaneous discharge of the stream at the outlet of a sub-basin located in the northern part (sub-basin J) in March, May, July, and September was found to be 0.072 m3/s, 0.030 m3/s, 0.122 m3/s, and 0.158 m3/s respectively, and for a sub-basin located in the southern part (sub-basin G) 0.158 m3/s, 0.084 m3/s, 0.117 m3/s, and 0.148 m3/s respectively. In these sub-basins, we conceptualize total streamflow at the outlet as a combination of two primary end-member components: flash flow (encompassing both surface and subsurface flow) and baseflow. As Cl- is chemically stable, resistant to degradation or transformation, exhibits minimal involvement in chemical reactions or biological processes, and has a low tendency to adsorb onto the soil or rock surfaces, its concentration in rainfall was used as a representative of its concentration in flash flow. For the baseflow, the Cl- concentration of the water sample in the January survey was used because, in January, the influence of precipitation was very low compared to the other survey months. The relation between discharge and Cl- concentration in the streams of both sub-basins was found to be inversely proportional, indicating Cl- concentration can represent the discharge and function as a reliable tracer. Based on the Cl-concentration on mass balance equation, the baseflow contribution was determined in the streams of the two sub-basins. The baseflow contribution to the total discharge of the stream of sub-basin J in March, May, July, and September was found to be 0.018 m3/s (25.08%), 0.012 m3/s (38.39%), 0.025 m3/s (20.72%), 0.021 m3/s (13.11%) respectively. For the stream of sub-basin G, the baseflow contribution in the same month was found to be 0.016 m3/s (10.14%), 0.015 m3/s (17.92%), 0.020 m3/s (16.75%), 0.019 m3/s (12.69%) respectively.
Keywords: Baseflow, end-member, hydrogeochemical parameters, Nanmoku watershed
References
[1] Ochiai, M. (2022). Sustainable Development Disciplines for Society (Urata et al., Eds.), pp. 33-44.
[2] Pinder, G. F., & Jones, J. F. (1969). Water Resources Research, 5(2), 438-445.
[3] Geological Survey of Japan. https://gbank.gsj.jp/geonavi/geonavi.php
Baseflow fractions were calculated by using the hydrogeochemical parameters in the mass balance equation [2]. pH, electrical conductivity (EC), dissolved oxygen (DO), oxidation-reduction potential (ORP), temperature, major ions (Na+, K+, Ca2+, Mg2+, NH4+, SO42-, Cl-, HCO3-, NO3-), radon (222Rn), oxygen-18 (δ18O), and deuterium (δD) of the water samples at the stream outlet of sub-basins of Nanmoku watershed were analyzed in November-2023, and January, March, May, July, and September of 2024. Additionally, the discharges of streams were measured using an electromagnetic flow meter in March, May, July, and September.
The instantaneous discharge of the stream at the outlet of a sub-basin located in the northern part (sub-basin J) in March, May, July, and September was found to be 0.072 m3/s, 0.030 m3/s, 0.122 m3/s, and 0.158 m3/s respectively, and for a sub-basin located in the southern part (sub-basin G) 0.158 m3/s, 0.084 m3/s, 0.117 m3/s, and 0.148 m3/s respectively. In these sub-basins, we conceptualize total streamflow at the outlet as a combination of two primary end-member components: flash flow (encompassing both surface and subsurface flow) and baseflow. As Cl- is chemically stable, resistant to degradation or transformation, exhibits minimal involvement in chemical reactions or biological processes, and has a low tendency to adsorb onto the soil or rock surfaces, its concentration in rainfall was used as a representative of its concentration in flash flow. For the baseflow, the Cl- concentration of the water sample in the January survey was used because, in January, the influence of precipitation was very low compared to the other survey months. The relation between discharge and Cl- concentration in the streams of both sub-basins was found to be inversely proportional, indicating Cl- concentration can represent the discharge and function as a reliable tracer. Based on the Cl-concentration on mass balance equation, the baseflow contribution was determined in the streams of the two sub-basins. The baseflow contribution to the total discharge of the stream of sub-basin J in March, May, July, and September was found to be 0.018 m3/s (25.08%), 0.012 m3/s (38.39%), 0.025 m3/s (20.72%), 0.021 m3/s (13.11%) respectively. For the stream of sub-basin G, the baseflow contribution in the same month was found to be 0.016 m3/s (10.14%), 0.015 m3/s (17.92%), 0.020 m3/s (16.75%), 0.019 m3/s (12.69%) respectively.
Keywords: Baseflow, end-member, hydrogeochemical parameters, Nanmoku watershed
References
[1] Ochiai, M. (2022). Sustainable Development Disciplines for Society (Urata et al., Eds.), pp. 33-44.
[2] Pinder, G. F., & Jones, J. F. (1969). Water Resources Research, 5(2), 438-445.
[3] Geological Survey of Japan. https://gbank.gsj.jp/geonavi/geonavi.php