Streamflow reflects variable land conditions in the drainage area, such as landform, subsurface geology. But there are many indefinite matters about its mechanism. So the purpose of this paper is to clarify the relation between runoff and land conditions, using the "tank model",which is one of the runoff analysis models. The study areas are the catchment areas of Koyagawa-Dam, Sabagawa-Dam and Mukunashi-Dam. The author compares the hydrographs with the outputs from the tank models, mainly to clarify seasonal influences of land conditions, and the parameters of the tank models with the scores of the land conditions given in proportion to the importance for runoff in each study area. The results are summarized as follows; 1) The influence of irrigation water is remarkable in Koyagawa and Mukunashi, which have many paddy field. In these area the discharge lowers about 1 mm from April to September. The influence of irrigation ponds is evident in Mukunashi. In a dry year the discharge continues to lower for a short period even after the paddy cultivation in this area. 2) The influence of snowfall is predominant in Sabagawa. During winter the hydrograph does not coincide with the output from the tank model in this area. The maximum difference is about 15-20 mm. 3) The "time lag" caused by the local precipitation, Is related to the relief, the shape and the size of the catchment area. 4) Highly vegetation stabilizes the fluctuation of the direct runoff, and gentle inclination decreases the direct runoff. Humic soil and deeply weathered crust restrains the intermediate runoff. Pored subsurface geology and faults check the fluctuation of the base runoff.
