During the transition from chemical oxygen demand (COD) to total organic carbon (TOC) in a system, the relationship between the two can be elucidated. The aims of this study were to obtain the regression between TOC and COD in the irrigation water of paddy fields and to elucidate the effects of farming patterns on the regression using analysis of covariance. A regression, TOC＝0.62×COD＋0.40, was obtained regardless of irrigation/drainage or farming stages. On the other hand, some changes in the water quality or the differences in the irrigationsystemscanaffectthe regression between TOC and COD in the irrigation and drainage system as well as by the farming style.

The aim of this study was to investigate the impact of volcanic ash from the 2016 eruption of Mount Aso on the water quality of rivers in Oita. The explosive eruption occurred on October 8, 2016, and the volcanic ash spread in the basins of both Oita and Ohno rivers. The river water quality measurements revealed that F^-, SO₄^2-, and H⁺ concentrations were higher after the eruption. The influence of volcanic ash on the water quality was negligible near the intake point for drinking because of dilution or deposit due to a reservoir. Therefore, it is necessary to monitor the water quality near the intake point for drinking when the deposited volcanic ash is more than 100 g/㎡. However, volcanic ash hardly affects the water quality further downstream. Likewise, if there is a reservoir, the effect of volcanic ash on the water quality is less significant downstream of a river. The effects of volcanic ash on the water quality of rivers become less significant and almost disappear after 1 month.