Abstract
We investigated the annual nitrate nitrogen (NO3--N) load for seven years, taking stormy flood events into consideration, in a forested catchment in the headwaters of the Shimanto River, Shikoku, Japan. This area frequently receives heavy rain exceeding 100 mm. The vegetation of the experimental catchment (18.7 ha), underlying sedimentary rock, is a natural forest comprising predominant conifers (Abies firma and Tsuga sieboldii), evergreen and deciduous trees. Streamwater was collected twice a month, adding streamwater of 17 flood events (total rainfall of 18 - 289 mm). It was revealed that NO3- concentration before rainfall was an effective predictor except runoff for evaluating the cumulative specific nitrate load. Based on the modified ΣL - ΣQ equation added this predictor, the annual nitrate load was evaluated 3 to 5 kg·ha-1·yr-1 and did not significantly correlate to the annual rainfall or annual runoff. This is due to the remarkable decline in NO3- concentration during stormy flood events, suggesting that cumulative specific nitrate load tends to plateau. The remarkable decrease in NO3- concentration would be due to that subsurface water on hillslope rather than near stream, which has an extremely low NO3- concentration, contributes predominantly to streamwater during stormy flood events, excluding the early stage.
