Eighty years have passed since systematic river improvement works in the Ishikari River basin started in 1910.
Laid down by the alluvial soil along the middle reaches and sand clay in the downstream reaches, topographic features caused the meandering of the Ishikari River and a rather small gradient of river bed.
Since 1917 cutoff works were readily adopted as the best method to direct the flood overflow smoothly, to prevent flood overflow and to lower the ground water level of many swamps, based on detailed studies of hydraulics and regional developments in the future.
River realignments by cutoff projects implemented in 29 channel reaches have succeeded in preventing flood damages in the Ishikari River basin.
This paper describes the decision-making processes of cutoff works adopted in the Ishikari River and tremendous effects of large-scale projects on hydraulic characteristics and regional developments.

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With regard to hydrology, water budget, water, quality, and ecology of Lake Biwa-ko, we analyzed their seasonal characteristic within one-year basic cycle and tried to present average aspects of the seasonal changes of the water environment in the form of a calender. As a result, it was proved that four seasons can be set : in hydrological calender in compliance with seasonal change of precipitation; in water budget calender in compliance with the process of formulation and collapse of lake water temperature stratification; in water quality calender in compliance with seasonal change of nutrition salt, organic matters, plant planktons and so on; and in ecological calender in compliance with seasonal change of plant planktons, hydrophytes, benthos, and fishes. Preparation of these water environment calenders is highly effective and helpful in pigeonholing and consolidating existing knowledge and findings regarding water environment of lakes and marshes, in grasping them as a whole, and in establishing supporting system for actual maintenance and protection of the water environment.

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A hydrological simulation model, the Tropical Hydrology Simulation Model 1 (Trophydsim-1), was created in order to simulate the run-off processes in mountainous terrain in a tropical monsoon region. The hydrological cycles in five major land covers, i.e., forest, upland, plantations (normally tobacco areas), paddy fields, and settlements were each analogized by a series of four storages representing interception, depression, upper-zone and lower-zone storages. A set of mathematical equations expressing empirical formula as well as physical laws were employed in the model. An automatic parameter optimization routine was presented.

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