Runoff characteristics of major ionic species during rain events in forested watershed

The runoff characteristics of major ionic species from a stream in a forested watershed were investigated during two rain events. The values of EC and the concentrations of alkalinity, anions and cations, except for NO₃⁻, decreased according to the increase of discharge, and showed a sharp lower peak. On the other hand, the concentrations of NO₃⁻ and K⁺ indicated an opposite change. The amount of output of anions and cations was also larger than those of the input, especially in a storm event. During a storm event, the NO₃⁻ concentrations in soil water 20 cm deep taken by a tension lysimeter were not detected, even though the surface soil of 0-5 cm deep included 20 to 50 mg/kg of NO₃⁻. The direct contribution for NO₃⁻ concentrations by suspended solids in water was estimated through three percentages of the stream water output. Surface runoff was also not observed. These results suggest that the prompt subsurface runoff of the direct runoff from surface layer of soil may be predominant during rainfall in the forested area, and the increase of NO₃⁻ concentrations in the stream may be caused through the process.