Study on the systematic manegement of plantation forests for water resources.

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The research program consists of the following four subjects to accomplish the research objective. The research results are summerized as follows.<I> ) Effect of soil characteristics and slope topography on runoff process on hillslopes: Vertical infiltration was analysed by using hydraulic head profile maps. Suction field maps were drawn to analyse the behavior of saturated throughflow. A simulating model on the effect of transpiration on discharge from a hillslope by using Richard's equation showed that transpiration on the lower half slope gives a distinct diurnal fluctuations on hillslope discharge.<II> ) Effect of forest treatments on water yield: All small watershed experiment data available are grouped with the net radiation of every 10 kcal/ <cm^2> .year. A predicting model of annual water yield increase after forest cutting is presented in these 10 kcal regions. Interception of a matured tree is analysed by a weighing method. The effect of prunning and thinning on interception of Sugi forests is evaluated with plot experiments catching all stemflow, throughflow and canopy drops.<III> ) Effect of tending works on nutrient cycle and water quality in a experimental watershed: Total discharge of mineral compounds from a forested watershed is highly correlated with water discharge amount, except a few years after forest cutting when cutting affects greatly compound discharge. In paleozoic soils, absorption of magnesium by Sugi trees is slow, while kalium is a quickly rotating compound in a nutrient cycle.<IV> ) Forest manegement system favourable for water resources: A consistent explanation is attempted for so-called "forest function of uniforming streamflow" basing on the soil-vegetation-runoff relationship. Forest manegement in Hiizumo district was investigated as one of the representative cases of multi-storied forest manegment which is most favourable for water resources and soil erosion.