Air pollutants in Tokyo Metropolis has been decreasing since 1990s because of emission control regulations imposed by the Tokyo Metropolitan Government. Still, the concentration of NO2, a representative acidification substance, is high in the central part of Tokyo than in the suburbs. As a result of air purification, the pH of water in many springs in Tokyo has been increasing. Many springs for which pH shows a significant increasing trend are distributed in the suburbs, which is consistent with the distribution of atmospheric NO2 concentrations. The minimum value of pH in the springs appeared mainly in around 1995 and 2008 during October (rainy season), although it appeared in 2008-2009 during February (dry season). The minimum value around 1995 corresponded to long-term variation of NO2 concentration in the atmosphere, whereas that in 2008-2009 corresponded to the beginning of the decrease of cross-border air pollutants.
In the Seto Inland Sea, eutrophic and oligotrophic conditions attributable to excess or deficiency of nutrients (nitrogen and phosphorus) have been observed. The Harima-Nada, located in the eastern part of the Seto Inland Sea, is strongly affected by land-derived nutrients. Therefore, accurate estimation of rivers' nutrients load during rain events is desirable for appropriate nutrient management. For this study, changes in nutrients load in the Kako and the Ibo rivers, which flow into the Harima Nada, were investigated for rain events during 2015 and 2017. Total nitrogen (TN), dissolved total nitrogen (DTN), ammonium nitrogen (NH4-N), nitrite nitrogen (NO2-N), nitrate nitrogen (NO3-N), total phosphorus (TP), dissolved total phosphorus (DTP), and phosphate phosphorus (PO4-P) concentrations in river water samples were measured. The maximum observed increments in comparison to the baseflow conditions for TN and TP loads during a single rainfall event were, respectively, 28 and 76 times. Although TN and TP concentrations showed an increment when the river discharge increased, some other studied species showed no similar behavior. Differences between the rivers were also found. The relation between the river flow rates and the load of NO3-N and PO4-P in the Kako River showed a counterclockwise hysteresis effect for each of these nutrient forms. Results of this study suggest that a numerical hydrological model that incorporates effects of rain events is an effective tool for nutrients management in the Seto Inland Sea.
After experiencing devastating flood damage in major river basins in consecutive years, the Japanese government launched a new flood management policy of basin-wide flood control in 2020: “ryuiki chisui.” For this policy, the Ministry of Land, Infrastructure, Transport and Tourism requests collaboration among all stakeholders in different sectors to reduce flood magnitude and social vulnerability to flooding, together with traditional river training work. The author analyzed the background of this historic policy change, demonstrating that basin development, including the expansion of urban areas and the drainage improvement for agricultural land, impose overly large burdens on river training. To resolve this difficulty, farmland areas in the basin, especially paddy fields (“suiden”), must play an essential role in flood control, reducing flood peaks using drainage facilities and receiving flood excess from rivers in dangerous circumstances. That might be technically possible with minimum damage to farming. However, because the agricultural sector might not readily accept such a new role, the community has no alternative but to ask the farming sector for support with gratitude. Acceptance of the policy might then represent a rational policy alternative for the sector.
I lived and worked in Japan as a Japan Society for the Promotion of Science (JSPS) research fellow during the period 2017-2019. In this essay, I briefly present why I decided to visit Japan on this fellowship, and what I experienced as a foreigner. I believe that this essay will motivate many young researchers to pursue their postdoctoral studies in Japan on a fellowship from the JSPS.