The objective of this study is to clarify the mechanism of long-term variation for nutrient discharge in the Seto Inland Sea. Especially, we 1)evaluated the effect of Submarine Groundwater Discharge(SGD)on nutrient discharge from land to the sea using newly developed model, and 2)confirmed the effect of climate change and human activity on the long-term nutrient discharge in watershed, and 3)evaluated the nutrient retention efficiency by small weirs on the stream using hydrodynamic ecological model, and 4)discussed the mechanism of nutrient discharge from land to the sea and suggested the reuse of phosphorus. The results are summarized as follows:
1)The estimated SGD by the new model corresponded with the water balance method. Therefore, this method made possible to evaluate SGDs spatially and quantitatively. Amount of nutrient discharge via SGD based on the new method was equal to 10% of total amount of river discharge. On the other hands, phosphorus discharge of SGD was 3.6 times larger than that of river discharge. Therefore, it was confirmed that SGD was one of the important nutrient sources to the sea.
2)The decrease trend of precipitation and discharge were confirmed from long-term record of the Asahi River watershed. Result of analysis for nutrient discharge in long-term using a quasi-distributed hydrological model(Soil Water Assessment Tool; SWAT)indicates there is a relationship between precipitation and magnitude of phosphorus discharge. This result indicates that phosphorus could be delivered to the sea mainly in the wet year. Based on the long-term analysis, land use change from farmland to residence has confirmed due to increasing of human population in the Takaya River watershed. The variation of nutrient concentration indicated two significant trends. One is that nitrate has been increasing while particulate phosphorus has been decreasing. This result suggested that nitrate and particulate phosphorus were influenced by human population and total area of farmland in the watershed, respectively. Another is that phosphate and ammonium have decreased since 1998 by construction of large weirs on the stream.
3)A result of simulation for nutrient dynamics in a small reservoir using a hydrodynamic ecological model indicates that phosphorus overflows from water column to downstream in flood event, while it circulates with sedimentation - resuspension prosess inside the reservoir in ordinary condition. The net flux of phosphorus was estimated that 23% of total inflow is trapped in the reservoir and the residue is overflowed toward downstream. On the other hands, the net flux of nitrogen was estimated that 15% of total inflow is trapped as sedimentation, 26% is attenuated by denitrification and the residue is overflowed. These results suggest that the small reservoir has a capability of nutrient retention.
4)A result of trace for source of phosphorus in the Takaya River watershed indicates that the phosphorus discharge from middle stream was higher than that from upper stream because there were not anthropogenic sources in upper stream. In middle stream, soluble phosphorus discharged from domestic wastewater while particle phosphorus discharged from agricultural wastewater. Source of phosphorus in the Takaya watershed was consisted of domestic wastewater and agricultural wastewater with 1:1. The total sediment in the reservoir was estimated to be 62352 ton include 42.4 ton of phosphorus. This result was equal to an accumulation of 15 years of the estimated annual sediment load. It was confirmed that phosphorus in the pore water was actually higher concentration than that in the water column. And also, simulation result indicated phosphorus, which is equal to approximately 4.4 times of the river flux, circulates in a short time in the reservoir. Hence, abundant phosphorus in the sediment was confirmed by field survey and simulation. Consequently, these results suggest the small reservoir could be a sink of phosphorus and its possibility for reuse.