Surface failures induced by rainfall cause most sediment disasters. Many lead to extensive damage. Assessment of the risk of such disasters is crucially important for the safety and security of society. Such risk assessment is becoming increasingly important in this era of global warming, when torrential rains and localized heavy rains are proliferating. Instead of an index such as accumulated rainfall amount, we attempted to conduct risk assessment using the soil moisture deficit, i.e., the remaining capacity of the soil column to accept rainwater. After a numerical model was developed to calculate the soil moisture deficit in any or all MESH3 (about 1 km resolution) grid cells with existing routine data, the model was applied to 446 MESH3 grid cells in Niigata prefecture, where failures occurred during 2010-2011. The model showed that the soil moisture deficit near the failure occurrence became smaller, i.e., the soil column was closer to complete saturation. This finding suggests that the soil moisture deficit can be a good indicator of risk. Furthermore, a probability distribution model was constructed from minimum soil moisture deficit data near the failure occurrence. Using this probability model, the threshold of the soil moisture deficit to put an area on alert, corresponding to the disaster coverage ratio, can be calculated easily. The possibility of early warning is also demonstrated by combining the soil moisture deficit at a certain time with a short-time rainfall forecast for several hours in the future.

　Hydrograph recession analysis, which is a commonly used method for elucidating the dynamics of water (especially groundwater) in a watershed after rainfall, is extremely important for the separation of runoff components, modeling and parameter estimation of all runoff components, and long-term low-water management. However, various factors are involved in hydrograph recession. The high degree of uncertainty in the recession curves or equations is attributable to hydrograph recession analysis. For this study, we derived new recession equations while considering the effects of evapotranspiration, which have long been pointed out. We also elucidated the effects of evapotranspiration on the recession constants and their seasonal variation. The seasonal variation of the constants of recession equations is determined by the basin-specific recession constants, which are determined by basin characteristics such as geology and evapotranspiration, which have strong seasonality.

　Worsening human and social damage caused by flooding and drought, presumably of the result of climate change, have highlighted the improvement and upgrading of reservoir operations as urgent issues. Nevertheless, the actual status of reservoir operations and planning remains unknown in many countries and regions. Moreover, few reservoirs can be subjected to intensive research. Therefore, to contribute to the improvement of reservoir operation techniques, to promote research, and to improve current operation practices, the authors have catalogued and published the operational plans and actual operational statuses of 33 reservoirs in Thailand. This paper presents details of data and information related to reservoirs, with published datasets used for analyses of the actual operational statuses of the reservoirs. Results indicate that many reservoirs have different discharge operations than those planned, despite the fact that planned discharges are specified on a weekly basis. However, the total amounts of monthly and annual releases were generally consistent with the plan. Furthermore, more reservoirs were operated according to the plan during the dry season than during the wet season. Findings indicate that the inflow rate during the wet season and the storage rate during the dry season might influence reservoir operations.

　In inland areas of the Kanto Plain, where the groundwater level has recovered from its lowest depths because of pumping restrictions, the distribution of hydraulic heads was examined during the temporary low level of the annual cyclic change. Long-term groundwater level data from observation wells in Saitama prefecture accumulated during 25 years since 1992 were used to confirm changes to hydraulic heads of two types, with long-term and annual cyclic patterns. Before confirming the hydraulic head of shallow groundwater, we selected a representative observation well to assess the hydraulic head of shallow groundwater using classification with similar groundwater level change patterns, seasonality of precipitation, and the relation between hydraulic head and elevation. Results shown on a contour map from the hydraulic head distribution in the observation wells indicate a marked temporary change in June in the central part of the plain between the Tone River and the Arakawa River. The findings suggest the influence of decreased groundwater inflow from the periphery of the plain attributable to the pumping rate distribution.