Abstract
Change of landuse from forest to grassland somehow affects soil properties and runoff characteristics of a watershed. Hence, in order to evaluate the effects of the change, two sets of study watersheds, major landuse of which was forest and grassland respectively, were set up for comparison in the Kitakami mountainous area； namely the Nawasirozawa and the Yamaya watersheds, and the Sotoyama—daiiti and the Sotoyama—daini watersheds. Following results were obtained from the research.
(1) The surface layer of grassland soil was shown to have different porosity from that of forest soil in respect to macropores.
(2) Based on the measurements at the watersheds, infiltration capacity was evaluated as medium on grassland soil, while it was large on forest soil.
(3) Regression analysis of direct runoff, lost rainfall and peak discharge vs. rainfall showed that there were statistically significant differences in the regressions between the Sotoyama— daiiti watershed and the Sotoyama—daini watershed. It was proved that these were caused by difference in proportion of grassland and forest in the watersheds.
(4) Based on the measurements, it was analyzed that baseflow possibly decreased by the conversion in the Nawasirozawa and the Yamaya watersheds, while it remained unchanged in the Sotoyama—daiiti and the Sotoyama—daini watersheds.
(5) Distribution factors of the unit hydrograph for each watershed were presented. Runoff hydrographs were also presented based on the typical storms on the watersheds.
(6) Runoff characteristics and their probable changes by the conversion were summarized based on the original soil properties, and the historical, the present and the future landuse.
