Characteristics and determinants of flow regimes across Japanese rivers

Abstract

Flow regime is recognized as a prominent factor which determines stream biota, and thus the alteration of flow regime by dams or land use may cause serious degradation of river ecosystems. Natural and altered flow regimes have been evaluated using a variety of flow hydrologic metrics. Such analyses can contribute to elucidate the habitat template of stream-dwelling organisms and to support environmental flow management. Yet, flow regimes of Japanese rivers have not been analyzed to the present using hydrologic metrics in a nation-wide scale. We calculated 171 hydrologic metrics from 20 years of flow records of 452 gauging stations in Japanese rivers. These metrics were used to characterize flow regimes by principal component analysis (PCA). PCA showed that the magnitude of flow variability is the most important component of flow regime, which generates the spatial variability in flow regime across Japanese rivers. This result was consistent with those of previous large scale studies, but the importance of the frequency and the timing of flow events were found to be lower in the present study. Flow regimes of Japanese rivers were suggested to have relatively high temporal synchronism owing to small land area. Next, Japanese rivers were classified into eight hydrologic groups by K-means clustering based on PC factors. The groups were spatially overlapped, whereas some of them showed clear geographical differences. Finally, we used generalized linear models (GLM) to assess influence of catchment factors on the obtained PC factors. Climatic condition was a predominant factor in determining the flow regime of Japanese rivers. Flood control by dams and land use seemed to alter flow regime but the influences of those anthropogenic factors were weaker than that of climate, probably because of small sizes of dams, flashy flow, and narrow plains in Japan.