In the case of an extremely large freshet, the proportion of nitrogen runoff in particulate components becomes tremendously high

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

Yoshiki Shinomiya (a), Yuichi Yokoyama (b)

(a) Center for Forest Restoration and Radioecology, FFPRI, Tsukuba, Ibaraki, Japan.

(b) Yonden Cunsultants Co., Inc., Kochi branch, Kochi, Japan.

Journal of Japan Society on Water Environment, 42(5):219-229, September 2019 DOI： 10.2965/jswe.42.219( External link )

Over the recent years, record-breaking storms have been hitting various regions, causing concerns that large amounts of turbid water and nutrients may runoff into rivers and result in the deterioration of river water quality. To elucidate the characteristics of mountain stream water during an extreme storm event (hereinafter referred to as an “extreme freshet”), the form of total nitrogen runoff was investigated during the extreme freshet caused by a record-breaking storm with a total rainfall of 742 mm in a forest catchment in Kochi prefecture. The results confirm that the total nitrogen, equivalent to that of the annual total nitrogen load in forests throughout the country (approximately 5 kg-N/ha/yr), flowed out in the single extreme freshet event and that in this extreme freshet, the amount of nitrogen runoff in the form of suspended solids (particles of 2 mm or less in diameter that are insoluble in water) was predominantly larger than that in the dissolved form (soluble in water). Hence, it was revealed that the occurrence of an extreme freshet not only increases the volume of nitrogen runoff from forests into rivers but also qualitatively alters it.

While research on river water quality has been conducted primarily on the dissolved form of nitrogen until now, it is now apparent that information on nitrogen contained in suspended solids during extreme freshets is important. Furthermore, at present, when extreme meteorological phenomena frequently occur, highly turbid raw water has begun to threaten safe and stable water supplies. As shown in this study, the maximum concentration of suspended solids and the changes over time with respect to concentrations of suspended solids during the occurrence of an extreme freshet are expected to become critically important information sources for water suppliers to determine countermeasures, including the water intake restriction, for highly turbid raw water.

Photo 1. Flow rate observation facility.

Photo 2. Rain gauge (left) and automatic water sampler (right). The automatic water sampler starts operating during an occurrence of heavy rains and collects mountain stream water at constant time intervals.

Figure 1. Correlation between the cumulative specific nitrogen and the percentage of particulate nitrogen to the cumulative specific loads of total nitrogen. This ratio was 1%–3% in a freshet with a total rainfall of less than 100 mm; 5%–22% in a freshet with a total rainfall of 100 mm to 300 mm; and 95% in the case of an extreme freshet. This reveals that nitrogen runoff during an extreme freshet is tremendously large in the particulate form.