Abstract
The loading process of pollutants and inorganic ions from a paddy field was observed on nine occasions caused by rainfalls of 14-38mm in depth during a non-planting period from 14 September, 1991 to 14 May, 1992 (243 days).The water samples were collected in a small weir which was placed at the outlet of the paddy field (2, 070m2, soil infiltrability: 1.3×10-1mm·h-1) to measure the volume of discharges.Precipitation and dry fallout were deposited and then collected once a month to estimate the amounts of the external supply of the materials.
The first drop from the weir occured, on the average, after a precipitation of 3.5mm.Before the surface soil of the paddy was plowed, the highest concentrations of SS, TCODMn., TN and TP containing particulate constituents were found in the first outflow of the discharge, followed by a gradual decrease.After the plowing, the profiles of the pollutographs showed a similar tendency, but the concentrations became several times higher than those shown during the non-plowing period and reached at the maximum around the peak of the hydrograph.Correlations between the surface loading of the materials Lsi and the amount of the corresponding rainfall Ri were found to give the following Eq., Lsi=a (Ri-r) b, where a, b were coefficients calculated by the least squares method.γ(=3.5 in this case) was the depth of the rainfall which did not cause surface discharge because of percolation.Values of a and b for TCODMn, TN, TP and NO3-N calculated from the data obtained before the plowing differed from those after.From the equations and the data of the rainfalls during the non-planting period, the runoff loadings of TCODMn, TN, TP and K+ for the period were calculated as 62.6, 14.4, 3.37 and 37.4 kg·ha-1, respectively.These values were as large as 2.38, 1.29, 7.12 and 17.0 times, respectively, of the external supply from the precipitation and the dry fallout during the same period.
