Relation between Land Use and River Water Quality of the Tama River Watershed

To optimize the reactive nitrogen cycle in an ecosystem, technologies to increase nitrogen use efficiency and reduce emissions of nitrogen must be developed. In a watershed-ecosystem, land uses with purification abilities can be a powerful tool to mitigate nitrogen loads from nonpoint sources. This study analyzes the influence of land use on nitrate concentration in the watershed of the Tama River, a typical urban river in Japan. The upstream area is occupied by forest, while the downstream area is dominated by urban land use. In the Tama River watershed, 59% of the total land use is forest ; 23% is urban area ; and, only 5% is occupied by agricultural land. Urban areas are distributed downstream from the middle reaches. The average nitrate nitrogen (NO3--N) concentration in 2004 increased from upstream to downstream: it was 0.7 mg N L-1 in the upstream area, while it rose to 6.0 mg NO3--N L-1 in the downstream area. The river water NO3--N concentration showed a positive correlation with the proportion of urban land use, while it showed a negative correlation with the proportion of forest. However, some small sub-watersheds have low values for NO3--N concentration despite highly urbanized land use. These sub-watersheds are characterized by higher proportions of paddy rice fields to the total area, ranging from 0.3 to 3.0%, and higher proportions of water body areas, ranging from 8.3 to 30.6%, compared to other sub-watersheds. This might indicate the purification ability of the water bodies and paddy rice fields. Land use within 0 to 50 m from the river water surface influenced water quality and forest and other water bodies reduced nitrate concentration. Construction of artificial wetlands or riparian forests would decrease the N load into the river.