Tadanobu Nakayama

The Oze region in central Japan was largely spared from large‐scale hydropower development; however, a small‐scale water diversion scheme for electricity generation was implemented in the mid‐20th century. This study applies the process‐based NICE‐BGC model to the Lake Ozenuma–Ozegahara Marsh basin to evaluate the long‐term impacts of upstream water diversion on hydrological processes and downstream vegetation dynamics over the past 57 years. Model results indicate that alterations in lake water balance and diversion practices have significantly influenced downstream hydrology, including discharge to the Nushiri River and water level dynamics in Ozegahara Marsh. Following water diversion, lake levels exhibited increased seasonal variability and more rapid recovery during spring (April–June), leading to changes in riverine inflow to the marsh. Despite the marsh's flow‐regulating capacity, simulations suggest that long‐term changes in surface water and groundwater regimes have contributed to vegetation shifts, in addition to climatic influences. These effects are particularly pronounced in the eastern marsh during the earlier part of the simulation period. The results further indicate expansion of high‐moor vegetation ( Oxycocco–Sphagnetea ) towards riparian zones and an increase in intermediate marsh vegetation ( Moliniopsietalia japonicae ), consistent with previously reported observations. The model captures spatially explicit relationships between hydrological change and vegetation succession across major plant communities. These findings provide a basis for assessing basin‐scale hydrological alteration, identifying groundwater‐dependent wetland ecosystems and informing adaptive management strategies for wetland conservation under altered hydrological regimes.

In Mongolia, the traditional pastoral system has changed by the overuse and degradation of water resources. Currently, there is a research gap between the socio-economic transition and ecosystem degradation on the existing knowledge. In the present study, a process-based eco-hydrology model, NICE (National Integrated Catchment-based Eco-hydrology) (Nakayama et al., 2021a, 2021b, 2023), was applied to the total of 29 river basins in the entire country to quantify the heterogeneous distribution of livestock water use and its relation to pasture degradation there (Nakayama, 2025). The authors also evaluated the change of grassland carrying capacity and carrying status index during the last few decades (Yan et al., 2023). The result showed that the livestock water use in entire basins was the same order of magnitude as mining and urban water uses and that the estimated total water use was similar to that on constant assumption in the previous study. In addition, the simulation also clarified heterogeneous distributions of water uses of 5 types of typical livestock and higher water use in the central part of the country. However, the carrying status index showed more serious situation of overgrazing in the transitional zone between grassland and the Gobi desert. This means that the excessive use of water resources is indirectly related to the degradation of natural vegetation in grassland. These results also imply that the excessive use of livestock water intake can lead to groundwater decline, grassland degradation, and ultimately a reduction in the amount of water available to each livestock head. This methodology is effective to evaluate the grassland carrying capacity and its constraint of water resources (Lu et al., 2020), and to propose solutions to unsustainable pastoral land use patterns. ReferencesLu, H., et al. 2020. Environmental Science and Pollution Research, 27, 10328-10341, doi:10.1007/s11356-019-07559-9.Nakayama, T., et al. 2021a. Ecological Modelling, 440, 109404, doi:10.1016/j.ecolmodel.2020.109404.Nakayama, T., et al. 2021b. Ecohydrology & Hydrobiology, 21(3), 490-500, doi:10.1016/j.ecohyd.2021.07.006.Nakayama, T., et al. 2023. Ecohydrology & Hydrobiology, 23(4), 542-553, doi:10.1016/j.ecohyd.2023.04.006.Nakayama, T. 2025. Environmental Science and Pollution Research, 32, 13626-13637, doi:10.1007/s11356-025-36083-2.Yan, N., et al. 2023. Ecological Indicators, 146, 109868, doi:10.1016/j.ecolind.2023.109868.

Plastic contamination has been receiving considerable attention during the last few decades. In the present study, the authors extended process‐based eco‐hydrology models, NICE (National Integrated Catchment‐based Eco‐hydrology) and NICE‐BGC (BioGeochemical Cycle), to link them with a plastic debris model, and applied it to all of the 109 first‐class (Class A) river basins throughout Japan. The model included advection, dispersion, diffusion, settling, dissolution, and deterioration due to light and temperature, interaction with suspended matter (heteroaggregation), resuspension and biofouling. These processes could help to evaluate the effect of mismanaged plastic waste (MPW) and point sources (tyres, personal care products, dust and laundry) on the spatio‐temporal dynamics of macro‐ and micro‐plastics there. The result clarified the finer resolution promoted more settlement during the runoff process to slightly decrease the flux of macro‐plastics, particularly in urban regions. The model also revealed that the amount of micro‐plastic flux calculated by accumulating point information at sewage treatment plants could be replaced by analysis using grid data categorised for treatment methods (sewage, septic tank, untreated) in each grid instead of global data of per capita emission and treatment rates. The result also clarified that the plastic cycle, particularly micro‐plastic, in rivers flowing through urban areas has been significantly altered. Finally, the authors evaluated the method to improve the plastic cycle in urban regions towards understanding the urban plastic cycle with fewer inventory data. These results help to quantify the impacts of plastic waste on the biosphere in urban systems and may aid the development of solutions and measures to reduce plastic input to the ocean.

Plastic contamination has been receiving considerable attention during the last few decades (Siegfried et al., 2017). In the present study, the authors extended process-based eco-hydrology models, NICE (National Integrated Catchment-based Eco-hydrology) and NICE-BGC (BioGeochemical Cycle) (Nakayama, 2017), to link them with plastic debris model (Nakayama and Osako, 2023a, 2023b), and applied to all of the 109 first-class (class A) river basins throughout Japan. The model included advection, dispersion, diffusion, settling, dissolution and deterioration due to light and temperature, interaction with suspended matter (heteroaggregation), resuspension, and biofouling. These processes could help to evaluate effect of mismanaged plastic waste (MPW) and point sources (tyres, personal care products, dust, and laundry) on spatio-temporal dynamics of macro- and micro-plastics there. The result clarified the finer resolution slightly decreased the flux of macro-plastics, particularly in the urban regions. The model also revealed that the amount of micro-plastic flux calculated by accumulating point information at sewage treatment plants could be replaced by analysis using grid data categorized for treatment methods (sewage, septic tank, untreated) in each grid instead of global data of per capita emission and treatment rates (Jones et al., 2021). The result also clarified that the plastic cycle, particularly micro-plastic, in rivers flowing through urban areas has been significantly altered (Wagner et al., 2019). Finaly, the author evaluated the method to improve plastic cycle in urban regions towards understanding of urban plastic cycle with fewer inventory data (Strokal et al., 2021). These results help to quantify impacts of plastic waste on biosphere in urban systems, and may aid development of solutions and measures to reduce plastic input to the ocean. ReferencesJones, E.R., et al. 2021. Earth System Science Data, 13, 237-254, doi:10.5194/essd-13-237-2021.Nakayama, T. 2017. Journal of Geophysical Research: Biogeosciences, 122, 966-988, doi:10.1002/2016JG003743.Nakayama, T., Osako, M. 2023a. Ecological Modelling, 476, 110243, doi:10.1016/j.ecolmodel.2022.110243.Nakayama, T., Osako, M. 2023b. Global and Planetary Change, 221, 104037, doi:10.1016/j.gloplacha.2023.104037.Siegfried, M., et al. 2017. Water Research, 127, 249-257, doi:10.1016/j.watres.2017.10.011.Strokal, M., et al. 2021. Urban Sustainability, 1, 24, doi:10.1038/s42949-021-00026-w.Wagner, S., et al. 2019. Environmental Science & Technology, 53, 10082-10091, doi:10.1021/acs.est.9b03048.

In Mongolia, the traditional pastoral system has changed by the overuse and degradation of water resources. However, there is a research gap between the socio-economic transition and ecosystem degradation on the existing knowledge. In the present study, a process-based eco-hydrology model, National Integrated Catchment-based Eco-hydrology (NICE), was coupled with inverse method (NICE-INVERSE) and applied to the total of 29 river basins in the entire country to quantify the heterogeneous distribution of livestock water use and its relation to pasture degradation there. The result showed that the livestock water use in entire basins (100–120 million m³/year) was the same order of magnitude as mining and urban water uses and that the total water use estimated by this new model was almost similar to that by constant assumption in the previous study. Nevertheless, the model showed that unit water use of sheep has regional variations between 3.11 and 9.80 l/day in each river basin and that the assumption of a constant value as in existing studies does not hold. In addition, the model estimated that goats’ water intake was 10% lower than previous assumptions in comparison to other livestock, reflecting an overpopulation of goats over the past 40 years (about tenfold increase) for high demand for cashmere wool and meat products. Furthermore, the simulation clarified heterogeneous distributions of water uses of five types of typical livestock (sheep, horses, cattle, goats, and camels) are indirectly related to the degradation of natural vegetation in grassland. So, the present study uniquely contributes to filling the gap beyond previous studies. The author also detected hot spots of groundwater degradation by human activity including grazing impact in the national scale. These results imply that the excessive use of livestock water intake can lead to groundwater decline, grassland degradation, and ultimately, a reduction in the amount of water available to each livestock head. This methodology is effective to quantify the spatio-temporal variations of livestock water use, to quantify its relation to pasture degradation, and to propose solutions to unsustainable pastoral land use patterns.

Environmental contamination by plastics has been receiving considerable attention from scientists, policymakers and the public over the last few decades. Though some of the models have succeeded in simulating the transport and fate of plastic debris in freshwater systems, a complete model is now being developed to clarify the dynamic characteristics of the plastic budget on a continental scale. Recently, the author linked two process‐based eco‐hydrology models, NICE (National Integrated Catchment‐based Eco‐hydrology) and NICE‐BGC (BioGeochemical Cycle), to a plastic debris model that accounts for both the transport and fate of plastic debris (advection, dispersion, diffusion, settling, dissolution and biochemical degradation by light and temperature), and applied this new model on a regional scale and also for global major rivers. The present study newly incorporated resuspension and bedload transport by extending the author's previous investigations. The simulated results showed that large‐sized micro‐plastics were distributed more in riverbeds than in river water. Although small‐sized micro‐plastics are suspended in the water and large‐sized micro‐plastics settle in the riverbed under normal flow, floods disturb this equilibrium completely and resuspend large‐sized micro‐plastics in the water. Because the percentage of exported micro‐plastic load stored in the riverbed during flood periods is relatively high in some global major rivers, the amount of plastic deposited in riverbeds might be smaller than in lakes and dams. The riverine plastic transport to the ocean revised in the present study was 1.218 ± 0.393 Tg/yr, with macro‐plastic flux 0.793 ± 0.305 Tg/yr and micro‐plastic flux 0.426 ± 0.248 Tg/yr, being within the range of previous values, that is, 0.41–4.0 Tg/yr. These results aid the development of solutions and measures for the reduction of plastic input to the ocean, and help to quantify the magnitude of plastic transport under climate change.

Plastic pollution has been receiving considerable attention from scientists, policy makers and the public during the last few decades. Though some models succeeded to simulate transport and fate of plastic debris in freshwater systems (Meijer et al., 2021), a complete model is under development to elucidate the whole picture of plastic dynamics in continental scale. Previously, process-based eco-hydrology models, NICE (National Integrated Catchment-based Eco-hydrology)-BGC (BioGeochemical Cycle) (Nakayama, 2017), was applied to evaluate biogeochemical cycling in river basins ranging from local/regional to continental/global scales. Recently, the author linked NICE-BGC to plastic debris model that accounts for transport and fate of plastic debris (advection, dispersion, diffusion, settling, dissolution and biochemical degradation by light and temperature), and applied this new model to regional scale (Nakayama and Osako, 2023a) and global major rivers (Nakayama and Osako, 2023b). NICE-BGC was also improved to include the mass budget in water and bed sediment in order to show the seasonal variations of plastic fluxes. In this study, NICE-BGC was further extended to incorporate biofouling (with algae and phytoplankton) and heteroaggregation (with suspended particulate matter) to improve the accuracy of global plastic dynamics in global river basins and the amount of plastic flows from land into rivers and finally into the ocean. Model simulated size distribution of plastics in water and riverbed sediment of global major rivers and showed the difference of effect of biofouling and heteroaggregation in each river. In addition, the simulated result showed that flood events have a great impact on plastic mobilization and its high intra-annual variability mainly caused by settling, resuspension, and bedload transport (Hurley et al., 2018; van Emmerik et al., 2019). These results aid development of solutions and measures to reduce plastic input to the ocean, and help to quantify magnitude of plastic transport under climate change. ReferencesHurley, E., et al. 2018. Nature Geoscience, 11, 251-257, doi:10.1038/s41561-018-0080-1.Nakayama, T. 2017. Journal of Geophysical Research: Biogeosciences, 122, 966-988, doi:10.1002/2016JG003743.Nakayama, T., Osako, M. 2023a. Ecological Modelling, 476, 110243, doi:10.1016/j.ecolmodel.2022.110243.Nakayama, T., Osako, M. 2023b. Global and Planetary Change, 221, 104037, doi:10.1016/j.gloplacha.2023.104037.van Emmerik, T., et al. 2019. Scientific Reports, 9, 13549, doi:10.1038/s41598-019-50096-1.

Environmental contamination by plastics has been receiving considerable attention from scientists, policy makers and the public during the last few decades. Though some of the models have succeeded in simulating the transport and fate of plastic debris in freshwater systems, a complete model is now being developed to clarify the dynamic characteristics of the plastic budget on a continental scale. Recently, the author linked two process‐based eco‐hydrology models, NICE (National Integrated Catchment‐based Eco‐hydrology) and NICE‐BGC (BioGeochemical Cycle), to a plastic debris model that accounts for both the transport and fate of plastic debris (advection, dispersion, diffusion, settling, dissolution and biochemical degradation by light and temperature) and applied these models on a regional scale and also for global major rivers. The present study was newly modified to incorporate the plastic dynamics in estuaries by extending the previous studies. The model was employed to conduct a 2‐year global simulation aimed at evaluating changes in plastic dynamics in major rivers including 130 tidal estuaries. The model simulated the impact of estuaries on plastic budget and its seasonal variability caused by settling, resuspension and bedload transport during 2014–2015. The model showed that plastics with smaller particle sizes account for more in the water of estuaries than that of rivers, and plastics with larger particle sizes accumulate more on the riverbed. The simulated result also showed that estuaries trap more plastic than lakes and riverbeds (0.218 ± 0.053 Tg/year) although not as much as reservoirs (0.386 ± 0.103 Tg/year). More than 40% of plastics were retained by lakes, reservoirs, riverbeds and estuaries and the riverine plastic transport to the ocean was revised from 1.749 ± 0.371 Tg/year in the author's previous study to 1.000 ± 0.397 Tg/year in the present study. These results aid the development of solutions and measures for the reduction of plastic input to the ocean and help quantify the magnitude of plastic transport under climate change.