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A more objective assessment of the freshwater scarcity risk at a watershed basis scale contributes to the achievement of the Sustainable Development Goals (SDGs), in which water plays a key role in issues like safe and affordable drinking water, water-related disasters, forest management, biodiversity and ecosystems. Within the SDGs, water scarcity is assessed primarily by using the ratio of withdrawal to availability (WTA). This index has been developed to evaluate the average annual conditions on a broad scale, such as country or large basin. However, there are insufficient cases for assessing specific and detailed risk conditions by using this original WTA concept, due to seasonal and interannual variations of the river discharge and withdrawal volumes. For example, infrastructure (e.g. large dams) and its operation, can drastically change the available water and demand volumes at temporal scales finer than annual. Because recently many daily and monthly records of river discharge and water use have become publicly available, it is possible to conduct water scarcity risk assessments on a monthly basis. By employing these datasets, in this study, we propose to compare volumes of available water and demand by using a Sectoral and Statistical Demand to Availability (SS-DTA) index. The novelty of this index is marked not only by the use of monthly volumes of available and required water but also by how environmental water requirements are included and the statistical relevance of the evaluation. Usually, to determine the available water resources, existing methods deduct from the river discharge the volumes that are required to preserve the surrounding ecosystems. Our proposed method considers the environmental flow requirements as one of the water-demand sectors allowing a simultaneous evaluation of water use that is not possible with existing methods. We applied this method to three representative river basins, two located in Japan (the Tone and the Arakawa River basins) and the Brantas River basin in Indonesia. The results showed the variations of water scarcity risk level through the river operation in each river basin. In the Tone River basin, it was succeeded in a drought year to decrease its risk level from 1 to 0, and to transport sufficient volume of water to the Arakawa River basin through an artificial channel to meet the demands at the same time. In the Brantas River basin, the river operation changed the SS-DTA score from 3 to 2, which means the water scarcity risk level was alleviated, while the main impact of the operation was found in high-water season to decrease the flood risk. Because the number, type, and priority of water demands can be adjusted based on the local situation and all the timeseries data of each demand sector are visualized in parallel, the result includes not only the whole risk level of the river basin but also the practical and specific degree of the risk with demand sector information.