Investigating rainfall-runoff and turbidly behavior during the rainy season in west of Japan

Abstract Understanding the recent variability in massive rainfall patterns is essential to provide a clear image about underlying hydrological processes that occur over a watershed scale. The main objective of this study is to investigate the variability of rainfall events in Western Japan as a result of climate change and examine the related runoff-turbidity dynamics during rainy season. To fulfill the research objectives, we investigated the changeability of the precipitation records in the Gōno River watershed for the last two decades and the related runoff-turbidity behavior during floods using the turbidity-discharge (T-Q) loops and quantified using an improved hysteresis index. The findings revealed a kind of intense rainfall periodicity of 3 ~ 4 years. In addition, spatial pentads analysis exhibited various intensities of accumulated precipitation suggesting that there is no specific spatial zone of extreme rainfall. Regarding the turbidity-discharge behavior, it was found that clockwise hysteresis patterns were induced by sediment sources from near channel areas, alternatively, anticlockwise pattens were produced due to soil erodibility from the nearby areas. Another interesting finding was a notable behavior of turbidity during floods such as “bursts” or “local peaks” may represent an earlier (or later) arrival of turbid waters from nearby distant sources at an upstream section as a result of an intense precipitation. One of the fundamental challenges in the quantification of hysteresis patterns is that there is no accepted definition on how to determine the start and the end of a flood event which may led to a bias in the quantification of these patterns.