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Hydrological Effects on Relationships Between δ 15 N of River Nitrate and Land Use in a Rural River Basin, Western Japan
Author(s) -
Ide J.,
Somura H.,
Nakamura T.,
Mori Y.,
Takeda I.,
Nishida K.
Publication year - 2015
Publication title -
river research and applications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.679
H-Index - 94
eISSN - 1535-1467
pISSN - 1535-1459
DOI - 10.1002/rra.2756
Subject(s) - hydrology (agriculture) , drainage basin , environmental science , nitrate , structural basin , land use , discharge , geography , ecology , geology , paleontology , geotechnical engineering , cartography , biology
Abstract This study aimed to examine how the relationship between δ 15 N of nitrate ( δ 15 N NO3 ) in rivers and land use within a river basin changes with varying hydrological conditions. This information would aid in identifying the dominant source contributing to increased nitrate concentrations in rural rivers. For this, δ 15 N NO3 in river water was investigated monthly in the five subbasins of the Hii River basin (area: 911 km 2 ), western Japan, for 1 year and 3 months. There were significant correlations ( p < 0.05) between δ 15 N NO3 and the land‐use ratio (i.e. ratios of forested, agricultural and residential areas in a subbasin) for the majority of the observation days, indicating that δ 15 N NO3 reflected land use within the basin. δ 15 N NO3 ranged from +1.4‰ to +8.5‰ and was lower in a subbasin with a higher forested area ratio. We found that the absolute value of the regression slope of the relationship between δ 15 N NO3 and the land‐use ratio decreased with increasing river discharge. This finding demonstrates that differences in δ 15 N NO3 among subbasins with different land‐use compositions became smaller under higher flow conditions. Because δ 15 N NO3 decreased with increasing river discharge, the small absolute value of the regression slope under high flow conditions indicates that forested areas could be the dominant source of river nitrate during high flows in all subbasins investigated regardless of land‐use composition. The results suggest that forested areas make a large contribution to the increase in nitrate concentration in downstream rivers during high flows, because the nitrate concentration increased with increasing river discharge. Copyright © 2014 John Wiley & Sons, Ltd.