Nutrients Runoff from a Forested Catchment during an Extreme Storm Event: A Case Study of the Headwaters of the Shimanto River in the Western Part of Kochi Prefecture, Japan in July, 2011

We investigated the loads of suspended solids （ SS ） , total （ TN ） and dissolved （ DN ） nitrogen during an extreme storm event （ total rainfall, 742 mm ） on 18 and 19 July 2011 in a forested mountain headwater catchment of the Shimanto River in the western part of Kochi Prefecture, Japan. We compared the extreme storm event with the normal event （ total rainfall, below 100 mm ） and the storm event （ total rainfall, 100 – 300 mm ） . The experimental catchment （ 73 ha ） has an underlayer of sandstone and is mainly covered with a secondary forest that comprises evergreen trees. Samples of streamwater were collected every two hours using an automatic water sampler during six events （ total rainfall, 44 – 742 mm ） . The load of TN during the extreme storm event was about 5 kg ha -1 event -1 , which is similar to the domestic average annual TN load （ 4.9 kg ha -1 yr -1 ） . The proportion of DN to TN in the specific cumulative load decreased sharply from 97% – 99% in the normal event and 77% – 97% in the storm event to 5% in the extreme storm event phase. The results clearly show that the load of particulate nitrogen was markedly greater than the load of DN during the extreme storm event.