This study examines hydropower projects in 3S sub-basins considering the largest branch of the Mekong. The sub-basins are international basins that run through Cambodia, Laos, and Vietnam. To encourage cooperative development in these basins, a decision support tool has been developed. By applying the concept of Robust Decision Making, the tool can calculate benefits quantitatively when these countries develop hydropower cooperatively, considering the uncertainty of data used for numerical simulation. As a form of cooperation, independent development, two country cooperation (three ways), and all countries’ cooperative development are considered. Simulation results show 5,011,875 combinations of values while changing three input values such as an electricity replacement cost, environment cost and the development budget. Of those results, three-country cooperation can produce the greatest advantage in 4,504,231 cases. Results clarify that the three-country tie-up is the robust policy. Results also clarify that, when specific conditions related to the budget are met, Cambodia-Vietnam cooperation can be the most beneficial. Furthermore, even when the conditions were not met, benefits could be maximized by Laos partially sharing the budget. These results emphasize the benefits of three-country cooperative development.

 In September 2015, heavy rainfall in the Kanto-Tohoku region caused a levee breach and overtopping at the lower Kinu River Basin near Joso city, leading to severe flood damage. The total rainfall in the upper basin reached about 500 mm. At the catchment of Yunisikawa dam reservoir, initial precipitation of 140 mm was followed by 19.5 mm/h of rainfall (SD, 3.5 mm/h; min-max, 14.0-27.7 mm/h) for 10 h. The observed inflow at Yunisikawa dam reservoir was equivalent to 11.0 mm/h (SD, 0.5 mm/h; min-max, 10.5-11.6 mm/h) for 6 h in the latter part of the period. The objective of this study is to elucidate the flood runoff process with a distributed rainfall-runoff model coupled with a Time-Space Accounting Scheme (T-SAS). Results suggest that representing infiltration losses from soil layers to bedrock was necessary to reproduce the observed hydrograph patterns. Moreover, the T-SAS analysis showed that the source of runoff was changing, even during nearly constant discharge. For instance, contributions from the catchment ridge increased throughout the period.

 Because of huge landslides in the upper reaches of the Tedori River, highly concentrated turbid water with average concentration of 585 mg/L and maximum concentration of 4,012 mg/L has been discharged continuously into the river since May 2015. This study was conducted to evaluate the high-turbidity water effects on percolation from paddy fields in the Tedori River alluvial fan. Water requirement rates in the alluvial fan paddy fields were investigated in 2014 when huge landslides were not occurring and in 2016 when huge landslides were occurring. Changes in percolation from the paddy fields were then analyzed. Average percolation from all paddy fields was found to be 12.4 mm/day in 2014 and 7.9 mm/day in 2016, indicating a significant decrease in percolation. Moreover, paddy fields from which percolation decreased were distributed in the middle and lower reaches of the alluvial fan. Results suggest that fine soils are supplied to the middle and lower reaches of the alluvial fan. We estimated the groundwater recharge based on the observation results, which revealed that the groundwater recharge from the paddy fields was reduced by 36 %. The total groundwater recharge was reduced by 25 %.