The hypothesis that the product of discharge and concentration of nitrogen (N) in river water is equal to the atmospheric deposition was verified in the mountainous basin of the Tedori River in Japan. To verify this relationship, long-term data are required to eliminate the effect of short-term variation of the N components. The basin has very high mountains, including Mount Hakusan (2702 maltitude), which is covered with deep snow in winter. Therefore, limited data were used for the estimation of the deposition of the entire basin by assuming a linear relationship of altitude. As a result, it was found that the estimated N concentration coincided well with observed concentrations at six sites—the Shiramine and Kuwajima (upper stream), Nakajima (lower stream) and Dainichi dam, Tedori dam and Senami sites (middle stream). The seasonal variation of N concentrations was low in the snowmelt period and high in autumn through to winter. This was not due to the larger discharge in snowmelt season as it was also found that N deposition was high in winter and low in spring, which indicated a clear relationship between N concentration and monthly atmospheric deposition including N storage in snow pack.
The total nitrogen concentration (TNC) in rivers is of great concern because it has a close relationship with water use. River water is used municipally for irrigation and industry, which requires an adequate N concentration. The N concentration is greatly affected by agricultural practices, sewage treatment, industrial uses and other environmental components.
This study investigates the relationship between atmospheric (wet and dry) N deposition (Atdep), precipitation (discharge) and the TNC of a river in a mountainous basin. The relationships have not been investigated previously in mountainous river basins [
The N balance of this study is based on the hypothesis that input is Atdep and output is N outflow from a river where there is little artificial disturbance. This study is also based on steady state (the total N in the study area does not change during the investigated period), i.e., it is in a dynamic equilibrium state. This stable state idea is supported by Crocker and Major [
The related components of earlier studies are 1) Atdep or wet deposition was measured worldwide, and the amount of Atdep is 2 - 20 kg·ha−1·year−1 worldwide and ~5 kg·ha−1·year−1 in Japan [
Nitrogen outflow was estimated by the product of TNC and river discharge (Qh). The TNC of rivers is investigated worldwide because it affects water use. Hirose [
There are many studies on individual components of the N balance separately, but there is limited research on the relationships between the three components together. This study was conducted using 16 years of continuous data based on N balance in the mountainous basin of the Tedori river in the Hokuriku region (which receives much precipitation in winter) of Japan. The N balance three components used were Atdep, TNC and Qh. Using these three components, how the N concentration of a river is determined was investigated in a mountainous basin.
The purpose of the research was to verify the validity of the relationships between the components by comparing observed and estimated TNC.
The N cycle has been studied long term in forests by many ecological and hydrological researchers. Study has focused on the internal N cycle of forests, in which there are many N transfers such as those through soil and water, reef and tree trunks and forest animals, all with different life and cycling times. The relationships between the internal components are complicated and have been subject to long term studies from various aspects [
In the external N cycle, the only related component is that of Atdep, TNC and Qh in unit area without nitrification and denitrification by plants. If a long period is chosen, the following relationship will be satisfied by the mass conservation theorem of N:
Rewriting Equation (1), the TNC is defined as:
where TNC is total nitrogen concentration, Atdep is Atmospheric deposition and Qh is Discharge height.
To verify the relationship of Equation (2), long-term data are required to eliminate the short-term variation or “noise”. Here, “noise” means the small changes of N stored inside the forest. A dynamic stable state of the N cycle is assumed because of the following reason: If Atdep falls into a forest, the N will be distributed throughout the soil and water, grass, the trunks and leaves of trees or feed for wild animals. Nitrogen will gradually become saturated in the forest, and the excess nitrogen will then flow out to the river. It is assumed in this study that if this N cycle continues for a very long period, the flow of N in the forest approaches a stable state. Based on the above concept, three continuous sets of available data for Atdep, Qh and TNC were selected for 16 years, divided by yearly intervals.
The research site is located in the southern part of the Ishikawa prefecture, Japan, which receives a lot of precipitation, especially in winter as snowfall. The research river is the Tedori River which has an 809 km2 basin area as shown in
Plant cover was different according to the altitude of the basin. There was a mountainous belt (altitude 400 - 1500 m), a semi-high mountain belt (1500 - 2000 m) and a high mountain belt (>2000 m). The upstream area belongs to the high mountain belt and is dominated by the Hakusan National Park and covered with low height pine trees. In the semi-high mountain belt, high mountain grass is developed and called flower garden. Representative trees Betula Ermanii Chanisso and Abies Mariesii Mast were planted in this area. The former tree was developed in higher areas than the latter. In the mountainous area, high quality beech trees are well developed while Quercus Crispula Blume and Japan Marple are
developed in lowland, and red pine trees on ridges and cedar in valley areas of mountains [10-12].
Here, Hc is altitude with 200 m belt area weighted, Hi is the center altitude of the belt, Ai is the area of the belt, n is the number of belt and A is the total area of the test site.
The Tedori dam basin has the direct and indirect basin from the Senami and Ozo river. In the Dainichi river, discharge was observed at the dam site and TNC was observed at the Kamikawai site located in the lower reaches of the Dainichi dam site. Therefore, these two sites were divided into two columns in
The Atdep was investigated at the Taiyougaoka site (altitude 120 m) weekly over 16 years by the Research Institute of Ishikawa Environment and Health [
River discharge in the Tedori River Basin was investigated at three points—Nakajima (upstream of the Hakusan Head works) the Tedori dam and the Dainichi dam (
Hc: Center of weighted area of altitude belts.
The TNC data of the Tedori River were published as an Annual Report by the Ishikawa Water Supply Office of the Tedori River [
The TNC was measured by the ultraviolet adsorption method and inorganic N concentration (Ninorg) was measured by the ion chromatograph method. Organic N (Norg) concentration was obtained from the difference between TNC and Ninorg. The sampling interval was once a month from 1994 to 2003 and four times a year. after 2004 (May, August, November, February). However, at the Tedori dam site the TNC was measured every month after 2004.
Temporal changes of Ninorg and Norg in yearly averages are shown in
Temporal changes of Ninorg were measured at the six aforementioned sites (
Areal distribution of N in Atdep is a big problem in this study area. The Atdep N may originate mainly from exhaust NOx gas from fossil fuel consumption. Therefore, the Atdep is a function of the distance from the original sites of the NOx production, but the representative distance was not determined. Therefore, the appropriate methods to express these terms in the study area are not available and the areal Atdep N is assumed to be expressed with the function of altitude.
The study area has high mountains, such as Hakusan (2702 m) that are covered with snow in winter. This results in limited observation data. The Atdep was observed at the Sanpoiwa site at an altitude of 1450 m beside the Taiyougaoka site. There is no effective measure without using the data of the Taiyougaoka and the Sanpoiwa sites so the hypothesis of altitude dependence is applied as a second best method.
The relationship of the Atdep between the Taiyougaoka and the Sanpoiwa site in the same observation month (June to October for seven years) is displayed in
The relationship between the two sites of wet deposition is as follows:
Here, Tn is wet deposition at Taiyougaoka. Sn is wet deposition at Sanpoiwa.
It is assumed that the Atdep is directly proportional to altitude at the Taiyougaoka and Sanpoiwa sites. Because the Taiyougaoka site is 15.3 kg·ha−1 at 120 m altitude and the Sanpoiwa site is 8.38 kg·ha−1 at 1450 m altitude, the linear relationship of total Atdep between the two sites is:
Here, Dc is the Atdep. Hc is the center of the basin defined in Equation (3).
According to Equation (5), the annual average Atdep, based on the data from the Taiyougaoka site, is shown in
It has already been clarified that the relationship between average altitude and precipitation in the basin is close [17,18]. Using the data of
Here, Nq is Qh at the Nakajima site and Kq is Qh at the Kanazawa site.
The relationship between Qh at the Kanazawa and the Nakajima sites was assumed to change proportionally with altitude as well as Atdep. The data used here were a Qh at Kanazawa of 1603 mm during 16 years at 7 m altitude and of 3299 mm at Nakajima at 943 m altitude
(
There are small villages in the study area, with a total population of 7330 (2389 households) and farmland of 511 ha, which is mainly paddy fields [
The N cycle in rural areas is expressed in
Fertilizer N use in this area is estimated at 47.15 t·yr−1 (
Nitrogen in sewage effluent flowing to the Tedori River was estimated at 4.30 t·yr−1 based on 11 g·person−1·d−1and a removal efficiency of 85.2% by the oxidation ditch method [25,26]. Besides the above N load, 130 head of cattle are breeding in the study area. The N load from the cattle is estimated at 10.55 t·yr−1 based on 290 g·day−1·head−1 [
Accordingly, the total N load without Atdep in the study area is estimated at 29.36 t·yr−1, which flows to the lower reaches of the Tedori River. This load is equivalent to 0.40 kg·ha−1, which corresponds to 2.4% of the total Atdep of 16.6 kg·ha−1.
First, the accuracy of the altitude correction of the Atdep hypothesis is verified indirectly by comparing the estimated TNC with observed TNC. The next step, using both the altitude correction of Qh based on the Nakajima site and the Atdep based on the Taiyougaoka site, is the estimation of NTC at various sites. After that, the hypothesis of Qh is verified by comparison between observed and estimated TNC. Additionally, the reasonability of Qh estimation based on the Nakajima site was verified by comparing it with observed and estimated Qh at the Tedori dam and the Dainichi dam sites [
There was no need to estimate Qh at the Nakajima, Tedori and Dainichi dam sites because Qh was observed, thus it was only necessary to verify the altitude correction of Atdep. The comparison of observed and estimated TNC using only the altitude correction of Atdep is shown in
The relative error is 0.034 at the Tedori dam site, 0.012 at the Nakajima site and 0.063 at the Kamikawai site, which coincides with observed TNC. This shows that the altitude correction hypothesis of Atdep is reasonable.
As the next step, the TNC was estimated at the six sites aforementioned using relative runoff ratios and Atdep in
Average-N FAR: Average nitrogen fertilizer application rate; Total-N FAR: Total nitrogen fertilizer application rate.
The observed discharge at the Dainichi dam site was converted to the Kamikawai site using the relative runoff ratio (
The observed TNC at the Senami site is larger than the estimated TNC. The reason why is as follows: the sampling point is inside the Tedori reservoir and is not adequate because of the high velocity of the indirect Senami Ozo river basin through tunnels; thus, the sampling boat The relative error is 0.086 at the Dainichi dam site in the
middle reaches of the river, which coincides well with the observed TNC. There are errors of 0.111 at the Nakajima site and 0.142 at the Tedori dam site. Despite the altitude dependence hypothesis of Qh and Atdep by equation 6, the estimated and observed TNC coincide well at various sites, which show the procedure is reasonabledoes not approach the takeoff point of water correctly.
The estimated Qh at the Dainichi dam site was estimated based on the Nakajima Qh (relative discharge coefficient, 0.886). To confirm the validity of the procedure, the observed and estimated Qh were compared (
A comparison was conducted between observed and estimated Qh by applying altitude correction (relative discharge coefficient 1.05). The relationship coincides well (
The monthly change of Ninorg at the Hirose site, every month for nine years during 1995-2003, is shown in
April to June was not explained by the high Qh in snow melting season.
The monthly change of Atdep at the Taiyougaoka site is shown in
The relationship between Atdep and Ninorg of a monthly
average over nine years is shown in
Here, Atdep is the monthly average wet deposition and TNC is the monthly average total nitrogen concentration.
In case of organic nitrogen concentration
Equation (8) and
In winter season, sow pack contain N within water. Beginning of snow falling season (Oct-Jan), TNC in the river keeps relatively low concentration because N in Atdep stored in the accumulating snow pack, while snow melting season (Jan-April), the TNC in the river increase because N in the snow melting water flow out to the river even if Atdep decreased.
For the mountainous Tedori River Basin, research on the relationships between Atdep, Qh and TNC of river water and N balance analysis was conducted. As a fundamental concept of the research, the external N cycle of the mountainous basin was chosen over the internal, because the number of components can be reduced, resulting in a simpler N cycling system despite the long-term data required. Fortunately, Atdep data was measured during 16 years at the Taiyougaoka site, Qh data was measured at the Nakajima, Tedori dam and Dainichi dam sites for 35 years, and TNC was measured at six sites in the basin for 35 years. Therefore, the analysis was conducted during 1995-2010, with the three components of Atdep, Qh and TNC commonly applied.
Expressing the components in a large spatial area is a big problem because it cannot be observed during heavy snowfall. Therefore, the Atdep in the basin was estimated by the altitude dependence hypothesis using the observed data of the Taiyougaoka and Sanpoiwa sites, which were limited to seven years of only the summer season. The Qh in the basin of the observed sites also assumed the altitude dependence hypothesis from the relationship between the Kanazawa and Nakajima sites.
The reliability of this research was confirmed by comparison between observed and estimated TNC. The TNC was estimated by the ratios between Atdep over Qh estimated by Equation (1) at various sites.
The estimated and observed TNC coincide well at six sites distributed over the entire basin; the Hirose site at the lower reaches of the river, the Kuwajima and Shiramine sites at the upper reaches, and the Kamikawai, Senami and Tedori dam sites at the middle reaches. This result shows that the research is reasonable despite the general hypothesis of altitude dependence for Atdep and Qh. In addition, the estimation accuracy of Qh was confirmed by the comparison between estimated and observed Qh at the Tedori dam and Dainichi dam sites.
On the other hand, the monthly change of Ninorg was low in snow melting season and high in the winter season. To analyze this phenomenon, the hypothesis that a lot of Qh during snow melting season caused low the Ninorg was disproved. It was found that the Atdep was low in summer but high in winter meaning that the monthly Atdep and Ninorg in the river had a close relationship. This relationsip recognized between TNC and Atdep. Finally amount of N stored in snow pack expressed using relationship between TNC and Atdep.
The TNC of the Tedori River was low compared with rivers on the Pacific Ocean side of Japan because it has a lot of precipitation and less Atdep owing to the high mountains. In addition, although nitrification and denitrification were not considered, the effect of these components was not great because the reliability of this research was confirmed.
The assumption of stable state of N flow in the basin was important in this study. This assumption was, in other words, called N saturation which was also recognized by Ohrui, Mitchel and Iwatsubo [31-33].
We sincerely thank the Ishikawa Prefecture for providing valuable data. We also express thanks to the co-researchers of a study entitled “Normal hydrologic cycle as a core of irrigation water in the Tedori River basin” at Ishikawa Prefectural University, supported by the Ministry of Agriculture, Forests and Fishery, for many valuable comments.