Nutrient runoff from forested watersheds in central Japan during typhoon storms: implications for understanding runoff mechanisms during storm events

摘要

We investigated the characteristics of nutrients (N and P) and dissolved organic carbon (DOC) concentrations during high, intense typhoon storm and base flow periods from three watersheds (W1, W2 and W3) in central Japan. Concentrations in base flow were low and relatively constant. In contrast, concentrations of N, P, and DOC in runoff during typhoon storms became highly dynamic. Particulate nitrogen (PN) and paniculate phosphorus (PP) concentrations in runoff were 4-50 times greater than those in base flow for all storm events. Because PN and PP concentrations were significantly correlated with suspended sediment concentrations, particulate nutrient transport may be associated with overland flow in forested hillslopes. Dissolved organic nitrogen (DON), NO_3-N, DOC and dissolved phosphorus (DP) concentrations in runoff were 1-10 times greater than those in base flow. All nutrients showed a clockwise, discrete hysteresis pattern, which implied that the sources of nutrients were from forested hillslopes. Additionally, the time constant (t_(1/e)) calculated from a decreasing function suggested that particulate nutrients were mainly sourced from surface flow, whereas dissolved nutrients were provided from both surface flow and subsurface flow. The ratios of DOC : nitrate and UV: DOC suggested that higher values occurred in the early period of a storm event and was potentially related to the occurrence of overland flow. Our findings suggested that investigation of the parameters (e.g. DOC: nitrate and UV: DOC) were potentially useful for identifying sources of nutrients and potential applications of hydrological separation. Our results also contribute towards understanding the nutrient dynamics during intense storm events when various hydrological processes such as surface and subsurface runoffs govern the nutrient concentrations in headwater streams.