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

Abstract

Abstract: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 particulate 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.

Suspended sediments – associated water quality elements

“…The maximum contents of Ca, P and Mg were carried contemporary with the mine exploitation period and flood occurrence. This finding agrees with Zhang et al (2007) and Hunter and Walton (2008) who believed in the high effectiveness of hydrological condition on the transportation rates of water pollutants with suspended sediment. However, variation of N did not follow the SS trend in the study area which verified the high potential of N for transport in dissolved form that has been reported for forest areas.…”

“…This is due to accumulation of phosphorus on the soil surface and its dependency on soil erosion and sedimentation. It is confirmed by the results of Zhang et al (2007) in Japan on increase of phosphorus during flood events. It was concluded that the device used in this study could not identify particulate Nitrogen; this result concurs with Esberg et al (2004).…”

Simulating the impact of flooding events on non-point source pollution and the effects of filter strips in an intensive agricultural watershed in China

“…These results imply a close relationship between the export of sediment and organic N. Many previous studies have demonstrated pollution export during flooding events and regarded hydrological processes as the main controlling factors (Zhang et al 2007(Zhang et al , 2008Dai and Lu 2010). It is widely agreed that storm rainfall usually causes a rapid surface flow, which dissolves large amounts of nitrate N. Meanwhile, the flow also involves a large quantity of sediments, to which organic matter from the surface of the land adheres.…”

“…This deficiency has been shown in many previous studies in China (Zhang et al , 2013. However, as nutrient transport depends largely on the runoff and sediment, we are sure that an accurate model based on the methods described above could provide key parameters to capture the inner mechanism and processes of environment behaviours in SWAT to some level (Zhang et al 2007;Chen et al 2014). Additionally, because of the importance of nitrate N and organic N for surface water quality, we estimated their outputs using the validated SWAT model.…”

“…Previous studies reached a consensus that diffuse sources of nutrients and rainfall distribution in catchments are the key factors controlling the nutrient variability during flooding events (Oeurng et al 2010). However, it is not easy to quantify the relations between flooding events and nutrients because it is difficult to predict extreme precipitation and flooding events to conduct field measurements (Zhang et al 2007(Zhang et al , 2008. Additionally, it is often challenging for researchers to collect and monitor frequently the spatio-temporal changes of climate and water quality on the watershed scale.…”