Abstract
The concentrations and loadings of herbicides were investigated in paddy fields (40.5ha) located around Lake Biwa from May to October in 2005 and 2006. The herbicide losses from the basin determined from the cumulative load from May to July and the application amount determined by the questionnaire method by Japan Agricultural Cooperatives (JA) were estimated in 2005 and 2006, respectively, to be 26.3% and 36.5% for Daimuron, 24.5% and 30.8% for Bomobutide, 21.5% and 18.6% for Simetryn, 15.7% and 9.30% for Benfresate, 9.14% and 11.3% for Pretilachlor, 5.49% and 5.33% for Mefenacet, and 0.32% and 0.84% for Esprocarb, and in 2005 to be 5.52% for Thenylchlor and 4.74% for Cafenstrol. A Simple model was developed to estimate herbicide losses by multiple regression analysis. The specific loss, which is calculated from the total loss and the average discharge on nonrainy days, was used as the criterion variable. The water solubility, Henry's law constant and soil adsorption constant were used as explanatory variables. The most important contributor to herbicide losses is the adsorption constant of soil. The specific losses calculated from the result of multiple regression analysis were consistent with the observed values. The errors between the observed and calculated values were within ± 0.5% for four herbicides and almost ± 1.0% for five herbicides.
