Characteristics of nonpoint source pollution from an agricultural watershed during rainfall events

Rainstorms can flush large amounts of nonpoint source (NPS) pollution from land sources into water bodies, threatening water supply. On the basis of rainfall events monitoring data, the runoff NPS pollution dynamics were assessed and NPS pollution loads were quantified in the Shaojiagou Stream, a tributary of Shitoukoumen Reservoir. Based on two complete rainfall events data, the average NPS pollution loads were 76.573 kg/ha(suspended sediment, SS), 0.13 kg/ha(total nitrogen, TN), 0.016 kg/ha(dissolved nitrogen, DN), 0.155 kg/ha(particulate nitrogen, PN), 0.021 kg/ha(total phosphorus, TP), 0.042 kg/ha(particulate phosphorus, PP). In addition, the major species of TN and TP before rainfall events were DN and PP, respectively. In contrast, they turned into PN and PP during rainfall events, respectively, indicating the dominated nutrient species were different on rainy and non-rainy days. According to the cumulative load curves, temporal concentration levels of SS, total nutrients (TN, TP) and particulate nutrients (PN, PP) were different from those of dissolved nutrients (DN, DP), suggesting transport pathways for these groups were different and soil erosion was the main driving force for nutrients movement. The results suggested that management of soil erosion in the source watersheds of the Shitoukoumen Reservoir during particularly sensitive day (e.g. intensive rainfall day) would be necessary for protection of drinking water quality.