Gaku TAKAKI

Study on the Characteristics of Flow Recession and its Seasonal Variation

Minjiao LU


The drawdown characteristics of low runoff in rivers are important for the use of water resources. Recession equation is a method for expressing attenuation characteristics. Ando et al. (1985) calculated the constants of the fractional recession constants for each season (spring: March to May, summer: June to August, fall: September to November, and winter: December to February) and showed the relationship between seasonality and geology. The fractional recession constant is calculated as follows: a higher value indicates a faster recession. Lu (2023) proposed a method of extracting a recession curve from the daily flow rate as an efficient method for water reduction analysis. Lu (2023) defined evapotranspiration excluding canopy evaporation as soil evapotranspiration, and proposed a method for estimating the seasonally varying recession constants by dividing it into the basin-specific recession constants determined by basin characteristics such as geology and soil evapotranspiration with seasonal characteristics.
In this study, we first calculated the recession constants in the Tsunekanebashi watershed of the Doki River, and confirmed that the seasonal nature of the recession constants correlates with the evaporation capacity, as in previous studies. The Tsunekanebashi watershed is covered with forests, and we considered that evapotranspiration is caused by vegetation. Therefore, we conducted an analysis of recession constants for forested watersheds in various regions of Japan. We compared the fractional attenuation constants obtained by our method with those obtained by Ando et al. to see if the method proposed by Lu for extracting recession curves from daily streamflow is applicable to the watersheds in this study. The correlation coefficients were high, suggesting that the method proposed by Lu is applicable to watersheds in various regions of Japan. Therefore, the analysis was conducted using the recession constants obtained by the method of this study. the basin-specific recession constants were obtained from hypothetical soil evapotranspiration, analyzed for trends, and exponential approximation equations were developed to obtain the basin-specific recession constants for each basin. 
Using the approximate equation, we calculated the the basin-specific recession constants using the soil evapotranspiration estimated from the latitude of the observation stations, and examined the relationship with the basin characteristics. The the basin-specific recession constants for a complex geological basin were also estimated based on the reference values of geological classification. In this study, we proposed a method to estimate the basin-specific recession constants from soil evapotranspiration, and examined its relationship with geological classification. The results indicate that it may be useful to use the basin-specific recession constants when discussing the relationship between the recession curve and the geology of the basin. The estimated the basin-specific recession constants for a composite geological basin were estimated, and the validity of the estimated formulas was demonstrated.