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The Yodo River, which has two big tributaries and an annual mean flow of 268m

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The Yodo River, which has a large discharge area, is a source of tap water for fourteen million citizens in the Kinki area. It is also an infl ow river supplying the largest pollutant load into Osaka Bay. The fl ow of the Yodo River, which has the Lake Biwa basin and seven dams in its upper reaches, its fl ow is usually stable and large water quality changes occur only during fl ooding stages. During our highly frequent routine observation once every three days , eleven events of the ordinary fl ooding in [2005][2006] and one of the superfl ooding due to the 1318 typhoon were observed in our past observation period. For superfl ooding the highest concentrations of T-COD, TOC, and SS, and the largest loadings of all pollutants were observed during the observation period of 1995-2013. The results may be useful reference data for automation operation systems in water supply plants and for the estimation of the impact of infl ow pollutants on Osaka Bay. The shares of loads, discharged during the superfl ooding stage to the annual total load in 2013 were estimated. The differences between the regression equations of total loads of pollutant for the six largest fl ooding events, included the superfl ooding event, and all twelve events were clarifi ed. Annual total loads of pollutants were estimated using L-Q equations on the basis of the highly frequent observations in 2013 and ΣL-ΣQ equations for the six fl ooding events. The total loads in the Yodo River could be estimated with high precision using by the regression equations.

Yodo River is an important source of tap water for 12 million people in Kinki area. The statistical characteristics in pesticide concentrations for R. Yodo and three major tributaries were made clear by high-frequency observations. The relationships between total pesticide loads during observation periods and the areas of river watershed and its paddy field became almost linear on a log-log graph. The mass balances of pesticide loads between the three major tributaries and R. Yodo during the downstream reach were calculated during the observation period. As the sums of total pesticide loads of the three major tributaries accorded to those of R. Yodo at Yodo River New Bridge during observation period in 2011, the decreases of pesticide loads by degradation during the downstream reach were considered to be very small. The frequency of routine observation was evaluated in order to precisely estimate the total runoff pesticide loads mainly during an irrigation period. The estimated total loads for three pesticides detected widely in long term during the monthly observation distributed widely in the range of 10 -472% of those by every three days of observations. The weights of the three pesticides in DI value (detecting index value) were calculated as 27 -88% of all pesticide species in the big rivers during the irrigation period.