3:45 PM - 4:00 PM
[MIS09-07] Interaction of land use and hydrologic controls on nitrogen export from catchment
Keywords:nitrogen compounds, watershed, isotope ratio
In order to clarify the spatial characteristics of nitrogen export from catchments, measurements of nitrogen compound concentrations and stable isotope ratios of nitrogen compounds exported from multiple catchments with different land use have been widely conducted and have provided us with many useful findings. However, these studies often focus on multiple catchments at once in order to take in a lot of spatial variation, and hence most of the data is available only for baseflow conditions.
On the other hand, many studies have shown that the concentration and origin of nitrogen exported from a catchment are strongly influenced by hydrologic conditions. In addition, during storm flow, the amount of nitrogen exported from the catchment to the downstream area is also large due to the increased flow. In other words, in terms of nitrogen supply to downstream areas, information on the influence of land use on nitrogen export from catchments by measuring the distribution of nitrogen concentrations during normal water conditions is highly uncertain. On the other hand, studies that have clarified hydrological factors affecting nitrogen export from a catchment often focus on a small number of catchments or small catchments, which lead high spatial uncertainties. In other words, focusing on nitrogen supply to downstream areas, there are not many representative measurements and findings on nitrogen export from catchments, both temporally and spatially. This makes it necessary to make various assumptions when calculating nitrogen export from large catchments, such as in calculation models.
Therefore, the purpose of this study was to analyze the relationship between hydrological conditions and land use effects on nitrogen export from catchments in order to clarify the above uncertainties regarding nitrogen export from catchments. For this purpose, nitrogen concentration was investigated near the mouths of 20 catchments (10.3-369.7 km2) with different land use patterns located in Shiga Prefecture, Japan, under a total of 29 different hydrological conditions. The nitrogen compounds investigated were total nitrogen (TN), dissolved total nitrogen (DN), particulate nitrogen (PN), dissolved organic nitrogen (DON), nitrate (NO3-), and ammonium (NH4+), and the stable isotope ratios of nitrogen and oxygen for nitrate (d15N-NO3-, d18O-NO3-) of nitrate were also measured, and the origin of nitrate was also discussed.
Multiple regression equations with interactions were used to analyze the nitrogen export concentration from the catchment as the objective variable and the river water level and the percentage of forested area in the catchment as explanatory variables. As a result, the effects of hydrological conditions on the relationship between the percentage of forested area in the catchment and the concentration of nitrogen compounds discharged from the catchment could be classified into the following six patterns. The relationship between the percentage of forest area in the catchment and the nitrogen compound concentration as hydrological conditions increase from low to high water table can be divided into the following six patterns: (1) the slope increases from negative to positive and the intercept also increases: PN; (2) the slope increases from negative to positive and the intercept decreases: NO3-; (3) the slope increases from negative but remains negative and the intercept decreases: DTN; and (4) the slope increases from negative to 0 but the intercept decreases and the intercept is constant: TN, (5) the negative slope increases further and the intercept is constant: d15N-NO3-, (6) both slope and intercept are constant: DON, NH4+, d18O-NO3-. These indicate that the influence of hydrologic conditions on the relationship between land use and nitrogen compound concentrations in the catchment differs for each nitrogen compound, and that TN, the total nitrogen export in large rainfall events, is little affected by land use. Factors contributing to variation in individual nitrogen compounds will also be presented at the time of the presentation.
On the other hand, many studies have shown that the concentration and origin of nitrogen exported from a catchment are strongly influenced by hydrologic conditions. In addition, during storm flow, the amount of nitrogen exported from the catchment to the downstream area is also large due to the increased flow. In other words, in terms of nitrogen supply to downstream areas, information on the influence of land use on nitrogen export from catchments by measuring the distribution of nitrogen concentrations during normal water conditions is highly uncertain. On the other hand, studies that have clarified hydrological factors affecting nitrogen export from a catchment often focus on a small number of catchments or small catchments, which lead high spatial uncertainties. In other words, focusing on nitrogen supply to downstream areas, there are not many representative measurements and findings on nitrogen export from catchments, both temporally and spatially. This makes it necessary to make various assumptions when calculating nitrogen export from large catchments, such as in calculation models.
Therefore, the purpose of this study was to analyze the relationship between hydrological conditions and land use effects on nitrogen export from catchments in order to clarify the above uncertainties regarding nitrogen export from catchments. For this purpose, nitrogen concentration was investigated near the mouths of 20 catchments (10.3-369.7 km2) with different land use patterns located in Shiga Prefecture, Japan, under a total of 29 different hydrological conditions. The nitrogen compounds investigated were total nitrogen (TN), dissolved total nitrogen (DN), particulate nitrogen (PN), dissolved organic nitrogen (DON), nitrate (NO3-), and ammonium (NH4+), and the stable isotope ratios of nitrogen and oxygen for nitrate (d15N-NO3-, d18O-NO3-) of nitrate were also measured, and the origin of nitrate was also discussed.
Multiple regression equations with interactions were used to analyze the nitrogen export concentration from the catchment as the objective variable and the river water level and the percentage of forested area in the catchment as explanatory variables. As a result, the effects of hydrological conditions on the relationship between the percentage of forested area in the catchment and the concentration of nitrogen compounds discharged from the catchment could be classified into the following six patterns. The relationship between the percentage of forest area in the catchment and the nitrogen compound concentration as hydrological conditions increase from low to high water table can be divided into the following six patterns: (1) the slope increases from negative to positive and the intercept also increases: PN; (2) the slope increases from negative to positive and the intercept decreases: NO3-; (3) the slope increases from negative but remains negative and the intercept decreases: DTN; and (4) the slope increases from negative to 0 but the intercept decreases and the intercept is constant: TN, (5) the negative slope increases further and the intercept is constant: d15N-NO3-, (6) both slope and intercept are constant: DON, NH4+, d18O-NO3-. These indicate that the influence of hydrologic conditions on the relationship between land use and nitrogen compound concentrations in the catchment differs for each nitrogen compound, and that TN, the total nitrogen export in large rainfall events, is little affected by land use. Factors contributing to variation in individual nitrogen compounds will also be presented at the time of the presentation.