In recent years, citizen groups have attracted attention as public service providers as they collaborate with river administrators for river management. However, as time has passed since their establishment, some citizen groups are facing various difficulties in continuing their activities. For this study, we administered a questionnaire survey about the active mass, which evaluates their activities quantitatively, and factors for activity continuation, which were extracted from earlier studies of citizen groups related to rivers. Stepwise multiple regression analysis and correlation analysis were applied to the responses of 64 citizen groups to estimate the factors affecting the active mass variation. Partial correlation network analysis was conducted to estimate the relations among the factors, based on the assumption that a certain relation exists between activity continuation and the variation of the active mass of citizen groups. The factors were discussed using results of these analyses.

　For Japanese forest hillslopes, the water volume of a rain bottle gauge is measured in many cases using a handheld graduated cylinder or digital hanging scale. No reported study has verified the error produced when using these measuring instruments. For this study, to verify the measurement accuracy of water amounts in bottles on forest hill slopes, we conducted a measurement experiment using a handheld graduated cylinder and a digital hanging scale. Results of measurement experiments conducted with a handheld graduated cylinder demonstrated tendencies to tilt the upper part forward by 2° to 3°, with reading of more than the actual amount of water. The horizontal state must be maintained to reduce these errors. Results of a measurement experiment using a digital hanging scale indicated that the generated error is less than that of a handheld graduated cylinder for weight of 5 kg or more. However, the water surface must be kept stable. A suitable instrument must be devised depending on conditions such as the measurer and the water contained in the bottle.

　We investigated changes in the water quality of springs and rivers in Aso caldera related to the 2016 Kumamoto earthquake. We used data available for the three or more years before and after the earthquake, respectively. Using these data, we preformed statistical analyses to assess differences. Results show that the pH of all spring and river waters was higher throughout the caldera after the earthquake. At springs located in the southern part of the caldera, electric conductivity, RpH−pH, and the concentration of SiO₂ were all greater after the earthquake. Specifically, it is considered that the groundwater residence time was longer after the earthquake. Analysis of the pH of rivers in the northern part of the caldera suggests that the discharge point of high SO₄^2- groundwater changed, which is consistent with the results obtained from earlier studies.

　Eagerness to contribute to flood-related research issues.