Is snowmelt runoff timing in the Japanese Alps region shifting toward earlier in the year?

Inter-annual variations of snowmelt runoff timing in 15 basins across central Japan were analyzed across 30 years, from 1980-2009, to determine if mountain hydrology has been affected by global warming. Observed daily river discharge was utilized to calculate center time (CT) of mass of flow. CT was found to be occurring significantly earlier in the year at two northern basins, with a rate of change of around five days per decade. While decreasing trends in CT in the other basins were not significant, negative correlations between CT and winter temperature was significant except for the central to northeastern basins. The effect of winter warming on snowmelt runoff was more significant in northern basins on the Sea of Japan side, where CT also correlated with the flowering date of cherry trees. Positive correlations between precipitation and discharge were stronger in southern basins, disturbing winter warming effect on spring discharge. These findings support the notion that winter warming accelerates snowmelt runoff, although year-to-year fluctuations were more pronounced than progressive warming over the three decades. Our results highlight inter-basin differences in hydrological response to climatic change, serving to validate down-scaling of climate simulations over the Japanese Alps region.ArticleHydrological Research Letters. 6:87-91 (2012)journal articl

Is snowmelt runoff timing in the Japanese Alps region shifting toward earlier in the year?

Inter-annual variations of snowmelt runoff timing in 15 basins across central Japan were analyzed across 30 years, from 1980-2009, to determine if mountain hydrology has been affected by global warming. Observed daily river discharge was utilized to calculate center time (CT) of mass of flow. CT was found to be occurring significantly earlier in the year at two northern basins, with a rate of change of around five days per decade. While decreasing trends in CT in the other basins were not significant, negative correlations between CT and winter temperature was significant except for the central to northeastern basins. The effect of winter warming on snowmelt runoff was more significant in northern basins on the Sea of Japan side, where CT also correlated with the flowering date of cherry trees. Positive correlations between precipitation and discharge were stronger in southern basins, disturbing winter warming effect on spring discharge. These findings support the notion that winter warming accelerates snowmelt runoff, although year-to-year fluctuations were more pronounced than progressive warming over the three decades. Our results highlight inter-basin differences in hydrological response to climatic change, serving to validate down-scaling of climate simulations over the Japanese Alps region

Impact of Bottom-Sediment Removal on ¹³⁷Cs Contamination in an Urban Pond

Many irrigation ponds in Fukushima Prefecture were decontaminated due to the contamination of radiocesium released from Fukushima Daiichi Nuclear Power Plant. To evaluate the impact of decontamination on 137Cs dynamics in an urban pond in Koriyama City, Fukushima Prefecture, Japan, temporal changes in 137Cs concentrations in bottom sediments and pond water were investigated before and after bottom-sediment removal. Post-decontamination, 137Cs inventories in bottom sediments decreased by 46–89%. 137Cs inventories in bottom sediments were relatively high in fine sediments before decontamination, and were also high at points near the water inlet after decontamination. Following decontamination, the mean 137Cs concentration in suspended solids (SS) and the mean dissolved 137Cs concentration in pond water decreased by 52% and 5%, respectively. Even after decontamination, the normalized 137Cs concentrations in SS and in water, which were calculated by dividing the 137Cs concentrations by the mean 137Cs inventories in each area, were higher than those in rivers, dam reservoirs, and ponds elsewhere in Fukushima. The positive correlations between δ15N values, an indicator of the source contribution to bottom sediments, and 137Cs concentrations in the upper 5 cm of bottom sediments after decontamination implied that SS from urban areas gradually increased the 137Cs inventories in the pond. The results underline the importance of secondary inputs of 137Cs from highly urbanized catchments

Modelling the dilution of radioactive contamination in sediment transiting Fukushima coastal rivers (2011-2015)

International audienc

Concurrent datasets on land cover and river monitoring in Fukushima decontaminated catchment during 2013–2018

Abstract After the Fukushima nuclear accident, the Japanese government implemented extensive decontamination work in 137Cs contaminated catchments for residents’ health and local revitalization. Whether dramatic land use changes in the upstream decontaminated regions affected river suspended sediment (SS) and particulate 137Cs discharge downstream remain unknown because of the poor quantification on land cover changes and long-term river SS dynamics. We here introduce a 6-year concurrent database of the Niida River Basin, a decontaminated catchment, including the first available vector decontamination maps, satellite images in decontaminated regions with a spatial resolution of 10 m, and long-term river monitoring datasets spanning decontamination (2013–2016) and subsequent natural restoration stages (2017–2018). These datasets allow us, for the first time, to directly link the transport dynamics of river SS (particulate 137Cs) to land use changes caused by humans in real-time, which provide fundamental data for better understanding the river response of sediment to land use change. Moreover, the data obtained by interdisciplinary methods offer a template for land use change impact assessment in other river basins

Quantifying the dilution of the radiocesium contamination in Fukushima coastal river sediment (2011–2015)

International audienceFallout from the Fukushima Dai-ichi nuclear power plant accident resulted in a 3000-km 2 radioactive contamination plume. Here, we model the progressive dilution of the radiocesium contamination in 327 sediment samples from two neighboring catchments with different timing of soil decontamination. Overall, we demonstrate that there has been a ~90% decrease of the contribution of upstream contaminated soils to sediment transiting the coastal plains between 2012 (median-M-contribution of 73%, mean absolute deviation-MAD-of 27%) and 2015 (M 9%, MAD 6%). The occurrence of typhoons and the progress of decontamination in different tributaries of the Niida River resulted in temporary increases in local contamination. However, the much lower contribution of upstream contaminated soils to coastal plain sediment in November 2015 demonstrates that the source of the easily erodible, contaminated material has potentially been removed by decontamination, diluted by subsoils, or eroded and transported to the Pacific Ocean

Radionuclides from the Fukushima Daiichi Nuclear Power Plant in terrestrial systems

Author Correction: Radionuclides from the Fukushima Daiichi Nuclear Power Plant in terrestrial systems. Nat Rev Earth Environ 1, 694 (2020). https://doi.org/10.1038/s43017-020-00119-7Author Correction: Radionuclides from the Fukushima Daiichi Nuclear Power Plant in terrestrial systems. Nat Rev Earth Environ 2, 584 (2021). https://doi.org/10.1038/s43017-021-00198-0International audienceThe 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, Japan, released the largest quantity of radionuclides into the terrestrial environment since the 1986 accident at Chernobyl. This accident resulted in 2.7 PBq of radiocaesium (137Cs) contaminated forests, agricultural lands, grasslands and urban areas, which subsequently migrated through soil and waterways in the Fukushima Prefecture. In this Review, we synthesize knowledge regarding the deposition, distribution and transport of fallout radionuclides, especially 137Cs, in the terrestrial environment after the FDNPP accident, which were revealed by extensive and continuous environmental monitoring. Anthropogenic activities, high run-off and steep topography led to a rapid decline in the activity concentration of 137Cs in soils and rivers, especially in the first year after the accident. The decline in exposed radioactivity was notably faster than that seen after the Chernobyl Nuclear Power Plant accident, likely related to differences in geography and climate, and the intensive remediation activities in Fukushima. However, forests in Fukushima have retained a notable amount of 137Cs in the upper centimetres of soil and could persist as a source of 137Cs into rivers. For continued understanding of both natural and fallout radionuclide behaviour in the environment, there must be long-term accessibility of the data collected in response to the FDNPP accident