Takeo Onishi

Distribution of dissolved and particulate radiocesium concentrations along rivers and the relations between radiocesium concentration and deposition after the nuclear power plant accident in Fukushima

This study involved measurement of concentrations of dissolved and particulate radiocesium ((134)Cs and (137)Cs) in river water, and determination of the quantitative relations between the amount of deposited (137)Cs and (137)Cs concentrations in river waters after the Fukushima Daiichi nuclear power plant accident. First, the current concentrations of dissolved and particulate (134)Cs·(137)Cs were determined in a river watershed from 20 sampling locations in four contaminated rivers (Abukuma, Kuchibuto, Shakado, and Ota). Distribution characteristics of different (137)Cs forms varied with rivers. Moreover, a higher dissolved (137)Cs concentration was observed at the sampling location where the (137)Cs deposition occurred much more heavily. In contrast, particulate (137)Cs concentration along the river was quite irregular, because fluctuations in suspended solids concentrations occur easily from disturbance and heavy precipitation. A similar tendency with dissolved (137)Cs distribution was observed for the (137)Cs concentration per unit weight of suspended solids. Regression analysis between deposited (137)Cs and dissolved/particulate (137)Cs concentrations was performed for the four rivers. The results showed a strong correlation between deposited (137)Cs and dissolved (137)Cs, and a relatively weak correlation between deposited (137)Cs and particulate (137)Cs concentration for each river. However, if the particulate (137)Cs concentration was converted to (137)Cs concentration per unit weight of suspended solid, the values showed a strong correlation with deposited (137)Cs.

Improving the performance of temperature index snowmelt model of SWAT by using MODIS land surface temperature data.

Simulation results of the widely used temperature index snowmelt model are greatly influenced by input air temperature data. Spatially sparse air temperature data remain the main factor inducing uncertainties and errors in that model, which limits its applications. Thus, to solve this problem, we created new air temperature data using linear regression relationships that can be formulated based on MODIS land surface temperature data. The Soil Water Assessment Tool model, which includes an improved temperature index snowmelt module, was chosen to test the newly created data. By evaluating simulation performance for daily snowmelt in three test basins of the Amur River, performance of the newly created data was assessed. The coefficient of determination (R 2) and Nash-Sutcliffe efficiency (NSE) were used for evaluation. The results indicate that MODIS land surface temperature data can be used as a new source for air temperature data creation. This will improve snow simulation using the temperature index model in an area with sparse air temperature observations.

Impact of Land-Use Change on Flooding Patterns

Recent unplanned and rapid urbanisation of Dhaka has a possibility to induce serious urban flooding in the near future. To evaluate the impacts of land-use changes on flood propagation patterns, we conducted flood inundation simulations using a two-dimensional finite element method with simplified Saint Venant equations. We used as a study site the mid-eastern part of the city of Dhaka, popularly known as “mid-eastern Dhaka”. Two different land-cover datasets were prepared—one showing land use in 1990 and the other for 2011. In addition, complete land-cover change scenarios were also considered. As for the boundary conditions for flood simulations, we first estimated river discharge by constructing a kind of conceptual hydrologic model called a tank model, since we only have water-level data with daily time resolution at the Balu River mouth. Changes in inundation areas, related to these different land-cover patterns, were then evaluated. The study shows that although no significant difference was detected between the results for land use in 1990 and in 2011, under the complete land-use conversion scenarios, with all wetlands converted to other uses, both flood propagation time and flooding area will significantly change. Thus, the simulation results prove that the presence of wetland in land cover reduces flood risk, as compared with other land use. While further validations of flood simulation results are required, our results may provide data useful for proper flood management in achieving urban sustainability.

Experimental evaluation of irrigation methods for soil desalinization

Soil salinization has provided a serious threat for global agriculture throughout human history. It is becoming ever more prevalent as human land use intensifies in recent years, and the reclamation is one of major challenges in agroecology. Flood irrigation is a typical method for leaching saline soil. Yet the practice needs a large amount of water, and it is difficult to remove salt uniformly throughout soil layers. In this study, an experiment was conducted to evaluate leaching efficiencies of four different methods, namely: flood irrigation, spray irrigation, paper-covered flood irrigation, and puddling irrigation. Flood irrigation was applied at three plots with different infiltration capacities. Spray irrigation, paper-covered flood irrigation, and puddling irrigation were applied at other three plots with medium infiltration capacities. Results showed that salt removal rates of flood irrigation tended to be higher near the surface of soil with smaller infiltration capacity, and that spray irrigation, paper-covered flood irrigation, and puddling irrigation were more efficient in removing salt than flood irrigation. Paper-covered irrigation was the only leaching method that reduced horizontal heterogeneities in salt content, while flood irrigation and puddling irrigation significantly increased the horizontal heterogeneities. The present study indicated that leaching efficiencies were highly affected by irrigation intensity and also by irrigation water volume only when irrigation intensity was considerably low, and that paper-covered irrigation is an efficient method in removing salt homogeneously from soil profile. Further studies need to be conducted to optimize irrigation intensity and water volume for given soil and water environmental conditions.

The Dilemma of Boundaries in Environmental Science and Policy: Moving Beyond the Traditional Watershed Concept

We are repeatedly drawing and redrawing various boundaries around us. The function of these boundaries is not only to divide people but also to bound people. Thus, boundaries are essential for daily life. The articles collected in this book shed light on human made boundaries from the viewpoint of water related problems. Amongst various water related environmental problems, the concept of the watershed has been the central dogma in water management schemes, and it also presents boundary dilemmas. Critically considering the conventional watershed concept, we tried to identify the limitation inherent in the watershed concept. As a result we found that in some cases conventional watershed boundaries work well, but in some cases, they may not work well at all. Since global and local water are inevitably inter-connected and the structure of inter-connectedness is changing, boundaries should change according to the structural change of inter-connectedness. Thus, the ability of our social systems to adapt to newly emerging boundaries is important. The same kind of idea can be found in the field of ecology and information technology. The common theme found in both fields is ‘management’. Though ‘management’ has not been thought of as a true scientific objective, particularly in the natural sciences, these sciences should, with the aid of the social sciences, include ‘management’ as a challenging objective.