To elucidate changes in water quality and current issues in the Asakawa River in the Tamagawa River system, which runs through Hachioji and Hino cities of Tokyo, we conducted several field surveys and compared population, land use, sewage treatment situation, and electrical conductivity (EC) data to those described in reports of earlier studies of this river. Results indicated that the EC of the main tributaries of this river was lower than it was in the past, indicating that the water environment of this river basin has improved because of the installation of sewage lines in this watershed. However, the degree of EC reduction varied because of the high building site ratio in septic tank installation areas in some catchments. In the upper reaches, concerns related to nitrate ions persist, such as pollution from septic tank effluents and nitrogen saturation in forest ecosystems. Therefore, although this river’s overall water environment has improved, local pollution persists in certain areas.

　Hydrologic recession analysis is a traditionally used method to elucidate and model the runoff processes, separate runoff components, and estimate hydrological parameters through analyzing recession parts of hydrograph. Hydrologic recession analysis is also extremely important for long-term low water management. The influence of evapotranspiration on recession has long been reported, but it has rarely been considered explicitly. This study defines evapotranspiration from soil water, i.e., evapotranspiration excluding canopy evaporation, as soil evapotranspiration. We conducted recession analysis based on recession equations taking into account soil evapotranspiration proposed by this researcher, and attempted to estimate the basin-specific recession constant found from basin characteristics such as geology, and seasonal patterns of soil evapotranspiration. Results demonstrate the possibility of estimating basin-specific recession constants and seasonal patterns of soil evapotranspiration from recession curves.

　This technical note introduces the new datasets of the Strahler stream order and flood plain mask covering the entirety of Japan. To obtain the continuous river channel dataset, we first filled the discontinuities in the river channels, which were found in the Digital National Land Information dataset. This filling enabled us to compute the Strahler stream order for all river channels. Because the Strahler stream order is fundamentally limited to the channel merging points in mountainous regions, we developed a new stream order that covers other regions such as urban areas or plains for practical uses. For all river channels of fourth order or higher, a floodplain mask dataset was created by drawing polygons manually on satellite images. The horizontal spatial error of the proposed data is generally within 10 meters when the flood plain boundaries are clear, but it might be larger when the flood plain boundaries are unclear. Our dataset presents benefits for river channel and floodplain identification in greater detail than when using earlier datasets. The developed data are available at http://kaigan.civil.tohoku.ac.jp/kaigan/papers/opendataj.html.