IJERD – International Journal of Environmental and Rural Development (2016) 7-1

Research articleerd

Proposed the Model for Estimation of Nitrogen Load in theAgro-Forestry Watershed
YURI YAMAZAKIThe United Graduate School of Agricultural Sciences, Iwate University, Iwate, JapanTOSHIMI MUNEOKA*
Obihiro University of Agriculture and Veterinary Medicine, Hokkaido, Japan
Email: muneoka@obihiro.ac.jp

HIROMU OKAZAWA
Faculty of Regional Environmental Science, Tokyo University of Agriculture, Tokyo
Received 15 November 2015

Accepted 11 April 2016(*Corresponding Author)

Abstract Increasing nitrogen concentration in the river water caused by agricultural activities
was reported in the Tokachi River basin, Hokkaido, Japan. The water quality conservation isrequired for the compatibility between the agricultural production and the environmentalprotection. It is important to analyze the nitrogen load in the watershed for water qualityrestoration. Nitrogen load is estimated by the nitrogen concentration in the river water and thewater discharge. However, this model needs frequent samples of the nitrogen concentrations
and water discharges. Also, many observations at multipoint have been required to figure out
where and how much the nitrogen load occurs in the watershed. Here, we proposed a model toestimate the nitrogen load by land use in a watershed. The land use data such as watershed areaand land use classification could be taken easily from a satellite image. Also, it can estimate thenitrogen load at any investigation point by using land use data for the estimation model. In theTokachi River basin, the nitrate-nitrogen concentration in the river water had a positive
correlation with the proportion of agricultural land in the watershed. Further, the water
discharge was proportional to the watershed area. Thus, the estimation model of nitrogen load
could be substituted the nitrogen concentration with the proportion of agricultural land, and thewatershed discharge with the watershed area. From this, there is a high possibility to estimate
the nitrogen load in the watershed by the agricultural land area in the Tokachi River basin.
Future subject of this model is how to correct for the variations of nitrogen concentration andriver water discharge at different investigation periods.
Keywords nitrogen load, estimation model, land use, nitrogen concentration, water discharge

INTRODUCTION
The Tokachi River basin is a large scale agricultural land located in the eastern part of Hokkaido, Japan.
Although Japan’s food supply is heavily dependent on imports from overseas because of the low selfsufficiency in food production, the Tokachi River basin plays an important role as a major food base ofJapan and has been expected to help develop a sustainable food supply. However, pollution of the riverwater by nitrogen caused by agricultural activities has been reported in this basin (Okazawa et al.,2011; Muneoka et al., 2013; Yamazaki et al., 2013, 2014). From the investigation in 2007 carried outby Okazawa et al. (2011), total nitrogen (T-N) concentrations in the river water were in the range of
0.30-7.00 mg/L (mean: 1.81 mg/L), and positive correlations were observed between the T-Nconcentration in the river water and the proportion of agricultural land in the watershed in the TokachiRiver basin. Methods to mitigate the nitrogen pollution of the river water in the Tokachi River basinⒸ ISERD148

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include changing the method and amount of nitrogen fertiliser, proper management of livestock
manure and modifying the land use in the watershed. The development of adequate and effective
measures to determine the source and amount of the nitrogen component runoff into the river isessential for reducing the amounts of the nitrogen load from agricultural areas.
Estimation of Nitrogen Loads
Different methods exist for estimating loads in rivers. For example, one method is to estimate theamount of emerging toxic substances by a basic unit and then multiplying by a runoff ratio. Anothermethod is to calculate the load based on the water discharge and the concentration of substances in theriver water. For the first method, we need to validate the accuracy and time variation of the basic unit
and the runoff ratio. The second method, Walling and Webb (1985) introduced a typical model formulafor toxic substances. The basic model formula to estimate the river loads was computed by multiplying
the river discharge and the concentration of substances in the river water (Eq. 1).

N loadC Q(1)where Nload is the base flow of nitrate load in the river water (g/s), C is the base flow of the T-Nconcentration in the river (mg/L), and Q is the river discharge rate (m3/s).
Walling and Webb (1985) provided some comments regarding the reliability of load estimatesderived using instantaneous water sampling data. They reported that high frequency (i.e. continuouslysampled) and multipoint observation data was required to obtain correct river loads. However,extensive work is required to obtain the water sample, investigate the discharge rate and analyse thewater quality at high frequency and accuracy in large-scale watersheds.
The nitrogen concentration present in one of the parameters of the model formula to estimate thenitrogen loads of the river has been correlated with the land use in the watershed (Eq. 2). Muneoka et al.
(2012) reported that there is a significant and positive correlation between the nitrate-nitrogen (NO3-N)
concentration in the river water and the proportion of agricultural land in the Tokachi River basin. Also,
the river discharge has a proportional relationship with the associated watershed area in general (Eq. 3).

AcropCDQJ A

ArE(2)(3)

where Acrop is the agricultural land area in the watershed (km2), A is the watershed area (km2) and α, βand γ are the coefficient factors.
Substituting Eq. 2 and Eq. 3 into the Eq. 1 provides a method to estimate the nitrogen loads (Eq.
4):

Nload

D c  Acrop r D cc  A(4)where α' and α" are the coefficient factors. The agricultural area of the watershed can be calculated
easily from the public land use information or satellite images.
In this study, we examined the correlations between the T-N concentration and proportion ofagricultural land, river discharge and watershed area using a decade of investigation data. Additionally,
we considered the construction of a model formula to estimate the nitrogen loads from the size of theagricultural area in the Tokachi River basin.

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METHODOLOGYThe Tokachi River basin is located in the eastern part of Hokkaido, Japan (142.68–144.02°N, 42.55–43.65°E, 0–2,077 m altitude), with a stream length of 156 km and a drainage area of 9,010 km2 (Fig. 1).
The annual mean air temperature is 6.8°C, with the highest and lowest recorded temperatures of 25.2°C(in August) and −13.6°C (in January), respectively, as measured in Obihiro City from 1981 to 2010.
The annual precipitation is 887.8 mm/year. The soil types in the Tokachi River basin are volcanic,lowland, upland and peat soils. The upper part of the Tokachi River and the tributaries are forested
areas covered with mixed forests of conifers and broad-leaved trees. The low-lying areas are used for
agricultural purposes, such as cropland, grass and pasture. The crop cultivation period of this basin isfrom May to November, and the cultivated crops are rotated through wheat, potato, sugar beet andbeans. Both chemical fertilisers and livestock manure are applied to the agricultural land to provide therequired nutrients for crop growth.
The daily river discharge rates and the total nitrogen concentration of the river water wereobtained from the Water Information System provided by the Ministry of Land, Infrastructure,Transport and Tourism, Japan to determine the trends and fluctuations of the water information on theTokachi River. Five investigation points on the Tokachi River were selected in the decade between
2004 and 2013. However, we selected the water information during April to November to exclude theimpact of snow cover in the winter season and subsequent melting.

Fig. 1 Watershed and land use information in the Tokachi River basinⒸ ISERD

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We analysed the land use in the watershed with GIS software to utilise the watershed information
as an influencing factor on the river discharge and water quality. The National Land Numerical
Information provided by the Ministry of Land, Infrastructure, Transport and Tourism, Japan was usedfor basic watershed information. The watershed area of each investigation point was calculated basedon the ‘river’ and ‘watershed boundaries’ of the National Numerical Information with GIS. We also
calculated the area and proportion of agricultural land in the watershed based on the ‘Land use
fragmented mesh’ released in 2009 because there was almost no variation in the cultivation area in theTokachi River basin from 2000 to 2015 according to the Census of Agriculture and Forestry provided
by the Ministry of Agriculture, Forestry and Fisheries, Japan.
RESULTS AND DISCUSSION
Correlations Between the T-N Concentration and the Proportion of Agricultural Land,
River Discharge and Watershed Area
Figure 2 (a) shows the correlation between the T-N concentration of the river (decade mean values)
and the proportion of agricultural land, and Figure 2 (b) shows the relationship between the riverdischarge (decade mean values) and the watershed area. In the Tokachi River basin, the T-Nconcentration had a positive correlation with the proportion of agricultural land (y = 0.043x + 0.26, r =0.96), and the river discharge also had a positive correlation with the watershed area (y = 0.036x, r =0.95) over the investigation points. Based on these results, the T-N concentration could be replaced by
the proportion of agricultural land and the river discharge could be replaced by the watershed area todevelop an estimation model of the nitrogen load of the river. Thus, the nitrogen load of the TokachiRiver basin could be estimated by the size of the agricultural land area in the watershed and theassociated coefficients.

Fig. 2 (a) and (b) Relationships between the T-N concentration and the proportion ofagricultural land (a); the river discharge and the watershed area (b)However, there were variations in the correlational equations between the T-N concentration andthe proportion of agricultural land and the river discharge and the watershed area by annual meanvalues (Table 1). Muneoka et al. (2013) reported variations in the correlational equations between theNO3-N concentrations and the proportion of agricultural land from their water quality investigation inthe Tokachi River basin from 2007 to 2009. Yamazaki et al. (2013) also reported similar variations intheir investigation of the northwestern region of the Tokachi River basin from 1992 to 2012. The
nitrogen concentrations of the river were considered to have been affected by the agricultural scheduleⒸ ISERD151

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and the meteorological environment, and the river discharges fluctuated due to daily and seasonalchanges in the precipitation and evaporation in the Tokachi River basin. In other words, we had to set
some parameters that considered the impact of the fluctuation of the T-N concentrations and the riverdischarges to estimate the nitrogen loads accurately by the agricultural land area in the watershed.
Relationships Between the Nitrogen Load and the T-N Concentrations or the River Discharges
Table 1 Regression expression and coefficient correlation (r) between the T-N concentrationand the proportion of agricultural land and the river discharge and the watershedarea by annual mean values

Fig. 3 (a) and (b) Relationships between the nitrogen load and the T-N concentration (a)and the river discharge (b)We calculated the nitrogen load of the river by the T-N concentrations and the river discharges at 5
investigation points in the Tokachi River basin. The T-N concentrations and the river discharges used
for calculation of nitrogen load were investigated once a month for a year from 2004 to 2013. Figures 3(a) and (b) show the correlations between the nitrogen load of the river and the T-N concentrations (Fig.
3 (a)) or the river discharges (Fig. 3 (b)), respectively. From this figure, the nitrogen load had a strong
positive correlation with the river discharge (y = 1.73x, r = 0.89); i.e. the fluctuation of the nitrogenⒸ ISERD

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load was dependent on the fluctuation of the river discharge more than the T-N concentrations. This
result suggested that the parameters for calculating the river discharge fluctuation were required toestimate the nitrogen load in the river water.
CONCLUSION
In this paper, we examined the correlations between the T-N concentrations and the proportion ofagricultural land, as well as the river discharges and the watershed area based on a decadal
investigation in the Tokachi River basin located in the eastern part of Hokkaido, Japan. The decadalmean T-N concentration had a positive correlation with the proportion of agricultural land, and thedecadal mean river discharge also had a positive correlation with the watershed area. Also, the nitrogenload of the river, calculated by multiplying the T-N concentration and the river discharge, had a strong
correlation with the river discharge. From the basic model formula of nitrogen load and the results as
discussed above, the nitrogen load of the Tokachi River basin could be estimated by the agriculturalland area of the watershed. However, the parameter could also be set by considering the impact of theriver discharge fluctuation because the nitrogen load strongly depends on the river discharge. Also, theresults could potentially be used to verify the nitrogen runoff behaviour during flood conditions as afuture subject.
ACKNOWLEDGEMENTS
This research was supported by JSPS KAKENHI, Grant Number 15J04743, 2015.We would like toexpress my gratitude for the research assistance provided by the students, the Obihiro University ofAgriculture and Veterinary Medicine.
REFERENCES
Muneoka, T., Yamazaki, Y., Wakou, S,. Shimura, M., Yoshino, K., Tsuji, O. and Tabuchi, T. 2013. The nitrogen
runoff characteristics in agricultural watersheds after enforcement of animal waste regulation. IJERD, 4 (2),56-61.
Okazawa, H., Muneoka, T., Kudo, Y. and Takeuchi, Y. 2011. Effect of land use agglomeration on nitrogenconcentrations in river water in the Tokachi River. IJERD, 2 (1), 37-42.
Yamazaki, Y., Muneoka, T., Wakou, S,. Shimura, M., Yoshino, K., Tsuji, O. and Tabuchi, T. 2013. The
difference of agricultural land use in watersheds and long term fluctuation on the river water quality. IJERD,4 (1), 152-157.
Yamazaki, Y., Muneoka, T., Wakou, S,. Kimura, M. and Tsuji, O. 2014. Characteristics of ion components inriver water with multivariate analysis and piper diagram in agricultural area. IJERD, 5 (1), 160-164.
Walling, D.E. and Webb, B.W. 1985. Estimating the discharge of contaminants to coastal waters by rivers: somecautionary comments. Marine Pollution Bulletin, 16 (12), 488-492.

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