Abstract
For most of water quality indicators in a river an annual total loading for the year with more rainfall became larger than that with less rainfall on the basis of the weekly data for two years. Especially, the discharge-weighted mean of nitrate concentration for one year was larger than the arithmetic mean. The frequency distribution of nitrate loading is not normal but skewed logarithmic-normal. These characteristics were distinct in agricultural-land and mountainous rivers and weren't seen for chloride. When the surface runoff occurs during a storm runoff, the concentration and loading of nitrate becomes peaks behind that of discharge, and the concentration keeps up higher level at the recession stage. Because the concentration of rain-water is low, that of river-water becomes a sudden drop by the dilution of surface runoff at the stage of a sharp increase of discharge. Consequently, most of the nitrate load increased by a storm runoff are drained out from the surface soil layer of river basins and carried by subsurface runoff. A specific total nitrate loading during the direct runoff of a storm could be estimated by the log-log regression equation of specific total discharges for many storm runoff events.
