Abstract
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.
