IJERD – International Journal of Environmental and Rural Development (2012) 3-2

Research articleerd

The Nitrate Nitrogen Concentration in River Water andthe Proportion of Cropland in the Tokachi River
Watershed
TOSHIMI 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, JapanOSAMU TSUJI
Obihiro University of Agriculture and Veterinary Medicine, Hokkaido, Japan

MASATO KIMURA
Obihiro University of Agriculture and Veterinary Medicine, Hokkaido, Japan
Received 14 December 2011

Accepted 18 April 2012(*: Corresponding Author)

Abstract It is widely known that nitrate pollution of river water and groundwater inagricultural regions has become severe in Japan and in other agricultural countries aroundthe world. On the other hand, in the Tokachi River Watershed, where large-scale uplandfield cultivation with dairy farming is the main form of agriculture, few surveys and little
research have addressed the impact of agriculture on water quality in the area. This study
examined the relationship between the nitrogen concentration in river water at normal
water level and the proportion of cropland in the Tokachi River Watershed with taking
notice of both maintaining sustainable agriculture and conserving water quality
environment. At 37 locations on the Tokachi River main stream and its tributaries (theirdownstream ends), the NO3-N concentration in river water and the proportion of croplandin the watersheds were surveyed for three years. Nitrate nitrogen as a percent of total
nitrogen in river water ranged from 84% to 91%. The maximum T-N in water from thetributaries was approx. 8.8 mg/L. There was a correlation (significance of 1%) between theNO3-N concentration in river water and the proportion of cropland in the watersheds. The
correlation coefficients ranged from 0.80** to 0.95** in the water from the main stream. It
was confirmed that the impact factor, or the slope of the regression curve that represents
the relationship between the two, ranged from 0.037 to 0.075, which was greater than thevalues reported for various other locations in Hokkaido. Future studies on conserving thewater environment by reducing the nitrogen loading into the river system while
maintaining the cropland in the Tokachi River Watershed in favorable condition will needto explore the feasibility of land use assessment methods based on the new viewpoint.
Keywords Tokachi River, NO3-N concentration, proportion of cropland, impact factor

INTRODUCTION
It is widely known that nitrate pollution of river water and groundwater in agricultural regions has
become severe in Japan and in other agricultural countries around the world. The close relationshipbetween agricultural land use, which causes nitrogen loading, of the watershed in particular and thenitrate nitrogen concentration in river water has been pointed out (Carpenter et al., 1998; Shimuraand Tabuchi, 2000).
In Hokkaido, where large-scale agriculture predominates, surveys and research on agriculturalland use and river water quality have been conducted in many areas since the  1990’s.  In  Eastern  ⒸISERD193

IJERD – International Journal of Environmental and Rural Development (2012) 3-2

Hokkaido, where the scale of dairy farming is particularly large, it has been reported in numerous
studies that the high density of pasture and cattle population in the watershed tends to result in ahigh nitrate nitrogen concentration in river water. (Tabuchi et al., 1995; Nagasawa et al., 1995;
Inoue et al., 1999).
In the Tokachi River Watershed, where large-scale upland field cultivation with dairy farmingis the main forms of agriculture, few surveys and little research have addressed the impact ofagriculture on water quality in the area. Tabuchi et al. (1995) and Anbumozhi and Yamaji (2001),
for example, attempted to assess the impact of land use on the water quality of the northwesternarea of the Tokachi River Watershed, by using the nitrate nitrogen concentration of river water as
an index of land use. In each of these cases, however, investigation was based on only one water
quality survey or on only a short period of such survey. Accurate understanding of secular or
annual fluctuations in river water quality (N concentration in particular) has not been attempted in
the Tokachi River Watershed, which possesses local characteristics that are distinct among regionswith an Asian monsoon climate.
This study examined the relationship between the nitrogen concentration in river water at
normal water level and the proportion of cropland in the Tokachi River Watershed based on athree-year observation. To ensure sustainable agriculture by maintaining the water quality, thisstudy also investigates a new land use assessment method for the region, taking the current
proportion of cropland in the Tokachi River Watershed into account.
METHODOLOGYThe Tokachi River Watershed measures 9,010 km2, making it the second largest watershed in
Hokkaido and the sixth largest in Japan. A map of the Tokachi River Watershed and basic data of
the survey sites are shown in Fig. 1 and Table 1. In the watershed, where survey site No. 17(Tokachikakouhashi) is located, 30% of the land is used for cropland and 62% is forests. The major
upland crops in this area are wheat, sugar beets, legumes and potatoes. Feed crops are also grown.
Watershed boundary

Pacific Ocean
Watershed boundary■Cropland ■Forest □■Sampling point

Fig. 1 Location and sampling points ofthe Tokachi River WatershedⒸISERD194

IJERD – International Journal of Environmental and Rural Development (2012) 3-2

In the Tokachi River Watershed we conducted river water sampling at ordinary water level,water quality analysis and land use evaluation. The survey sites numbered 37: 17 on the mainstream of the Tokachi River (No. 1 to No. 17), and 20 on tributaries that flow into the TokachiRiver’s  main  stream.  The  tributary  sites  were  all  at  the  downstream  end  of  the tributaries (A to T;
Fig. 1, Table 1). The samples were kept in airtight containers at low temperature and carried to thelaboratory for analysis of total nitrogen (T-N) and nitrate nitrogen (NO3-N) according to the Japan
Industrial Standards (JIS). The period for water sampling was the three years from 2007 to 2009,nine times in total. Samplings for each year were done in late June, late August (or earlySeptember) and late October.
The GIS software application ArcGIS Desktop (Ver9.3, ESRI) was used for analyzing the land
use of the Tokachi River Watershed. The data on land use and watershed boundaries were extracted
from digital national information (National and Regional Planning Bureau, Ministry of Land,Infrastructure, Transport and Tourism). The data on administrative boundaries, rivers andelevations were extracted from 1/25000 national digital cartographic data (Geographical SurveyInstitute). Land was categorized as forest or agricultural land based on the digital national land data
(1997). Vector data were created for the land use in each watershed. These created data were thenconverted into raster data with 100-m grids.
Table 1 Land use of the Tokachi River Watershed
Sampling point
No.

Bridge nameLand use (%)

River nameArea(km2)

Cropland

ForestOthersTokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River
Tokachi River632
658
801806
840
858
1,289
1,531
1,777
1,798
2,669
2,683
2,686
4,479
5,098
8,224
8,982

1
12
35715
1921
21
31
32
32
32
3630
3088
88
88
87
85
84
77
727070
60
5959
58
5461
62

1111101010108899999101088

Penkenai River
Penkenikorobetsu River
Penkenikorochin River
Sahoro River
Memuro River
Piuka River
Bibaushi River
Bisei River
Shinobihiro River
Shibusarabibaushi River
Shikaribetsu River
Otofuke River
Satsunai River
Obihiro River
Shihoro River
Tobetsu River
Sarubetsu River
Toshibetsu River
Ushisyubetsu River
Rebunnai River

2348
72337
2102635180
164
33667
693
704
197
316
127
449
2,850
173
66

0
1
1
26
38
71
79
1674
53
48
19
3066
64
79
69
20
24
4398
98
98
68
56
121974
16
4345
69
61
14
29
13
2675
61
51

2
1
12
6
17
3
8
10
3
8
12
9
20
78
5
5
3
6

Main stream
12
3
4
5
6
7
8
9
10
11
12
13
14
15
1617

Oojikahashi
Iwamatsuhashi
Shinseihashi
Kyoueihashi
Kamikawahashi
Shimizuoohashi
Tokachihashi
Shoueihashi
Memurooohashi
Nakajimahashi
Heigenoohashi
Suzuranoohashi
Tokachioohashi
Tokachichuuouoohashi
Chiyodaoohashi
Moiwaoohashi
Tokachikakouhashi

Tributaries (downstream ends)
ABC
DEFGHIJKLMNOPQRST

Pennaihashi
Penkenikorobetsuhashi
Minamiiwamatsuhashi
Sahorohashi
Kenenakajimahashi
Motomachihashi
Bimanhashi
Biseihashi
Kitanisenhashi
Shinshibusarahashi
Kunimihashi
Houraihashi
Satsunaihashi(Downstream end)
Asahihashi
Senjyuuhashi
Yamuwakkahashi
Kawaioohashi
Noyaushihashi
HoueihashiⒸISERD195

IJERD – International Journal of Environmental and Rural Development (2012) 3-2

For the main stream and tributaries (the lowest reaches), tests for noncorrelation for theregression coefficients (significance of 1% to 5%) were conducted to clarify the relationshipbetween the NO3-N concentration in river water and the proportion of cropland in each watershed.
RESULTS AND DISCUSSION
When the data from the main stream and tributaries of the Tokachi River were examined, a highcorrelation was found between the T-N concentration and NO3-N concentration of the river water(significance of 1%). The coefficients of determination r2 were 0.93** and 0.97**, respectively(Fig. 2 (a), (b); Table 2 (a), (b)). NO3-N as a percent of T-N in the river water ranged from 84% to91%, which is extremely high.
The relationship between the NO3-N concentration in river water when the water level isnormal and the proportion of cropland in the watershed will be examined.
First, there was a positive correlation (significance of 1%) between the NO 3-N concentrationin the river water and the proportion of cropland in the watershed of the Tokachi River main stream.
The coefficients of correlation r showed extremely high values, ranging from 0.80** to 0.95** (Fig.
3 (a); Table 3(a)). The same relationship for the tributaries was also positively correlated(significance of 1%) (Fig. 3 (b); Table 3 (b)). The coefficients of correlation r, however, were notvery high, ranging from 0.66** to 0.82**.
As discussed above, regardless of the survey year or season, for the watershed with a highproportion of cropland, the NO3-N concentration in the river water at the ordinary water level ishigh. The correlation between the two is remarkably high in the Tokachi River Watershed as awhole. The difference between the data for the main stream and its watershed and that for thetributaries and their watersheds seems to be because the size of the watershed and the distances ofcroplands from the river greatly varied according to the history of development of each area. These
differences seemed to be reflected in the difference between the coefficients of correlation of thetwo.
The slopes of the regression curves that represent the relationship between the NO3-N
4.0

10.0

(a) Main stream

8.0

T-N (mg/L)

3.0

T-N (mg/L)(b) Tributaries2.0
1.0

6.0
4.0
2.0

r2 = 0.93※※
0.0

r2 = 0.97※※

0.0
0.0

1.0

2.0

3.0

4.0

0.0

NO3-N (mg/L)

2.0

4.0

6.0

8.0

NO3 -N (mg/L)

Fig. 2 The relationship between T-N and NO3-N (2007-2009)
Table 2 The regression curve between T-N and NO3-N (2007-2009)
SamplingpointRegression cuｒve(a) Main stream T-N = 1.2 NO3-N + 0.28(b) Tributaries T-N = 1.1 NO3-N + 0.34※※** significance of 1%

significance of 1%ⒸISERD

196

Coefｆicient ofdetermination rNumberof data**0.93※※
**※※
0.97

147
180

210.0

IJERD – International Journal of Environmental and Rural Development (2012) 3-2

concentration in the river water and the proportion of cropland in the watershed (Table 3 (a), (b))
were examined. The values were evaluated as the impact factors (IF values). The IF values reported
in the studies on the watersheds in the agricultural areas of Hokkaido ranged from 0.0052 to 0.040.
(Tabuchi et al. 1995; Woli et al. 2002; Woli et al. 2004; Okazawa et al. 2009).
Our survey shows that the IF values for the Tokachi River main stream were between 0.037
and 0.075, and those for the tributaries were between 0.042 and 0.071. The highest IF value (0.075and 0.071) was observed for the main stream and tributaries in 2009 respectively. One thing to notehere is the high IF values in the Tokachi River Watershed. Nitrogen components in the chemicalfertilizers that are applied to cropland in large amounts and nitrogen components in cattle manurein the watershed ultimately turn into NO3-N and seep into the ground. Because of these agriculturalactivities, NO3-N constantly flows into rivers in relatively high concentrations at the ordinary waterlevel in the Tokachi River Watershed.
10.0(a) Main Stream 2009.8.26(b) Tributaries 2009.10.24-25

8.0

IF value = 0.075

3.0

NO 3 -N (mg/L)

NO 3 -N (mg/L)

4.0

2.0
1.0

IF value = 0.071

6.0
4.0
2.0

r = 0.95※※
0.0r = 0.79※※

0.0
0102030400

Proportion of cropland (%)

20406080

Proportion of cropland (%)

Fig. 3 The relationship between NO3-N and proportion of cropland
Table 3 The impact factor between NO3-N and proportion of croplandIF value※(Impact factor)

Coefｆicientof corelation r

Numberof data

2220
23-24
28
928-2930
2624-25

0.039
0.048
0.044
0.037
0.045
0.044
0.062
0.075
0.068

0.87※※
※※
0.90
※※
0.87
※※
0.81
※※
0.80
0.80※※
0.92※※※※
0.95
0.91※※

1717
17161616161616

22
20
23-24
28
928-29
30
26
24-25

0.055
0.042
0.048
0.049
0.049
0.044
0.058
0.064
0.071

0.77※※
0.70※※
※※
0.82
※※
0.74
※※
0.74
※※
0.66
※※
0.71※※
0.770.79※※

202020202020202020

Sampling date

N.B.

(a) Main stream
2007 June
AugustOctober
2008 June
September
October
2009 June
AugustOctoberFig. 3 (a)(b) Tributaries
2007 June
AugustOctober
2008 June
September
October
2009 June
AugustOctober※

IF value = NO3-N concentration / Proportion of croplandⒸISERD197※※

Fig. 3 (b)significance of 1%

100

IJERD – International Journal of Environmental and Rural Development (2012) 3-2

Generally, the greater is the proportion of forested area in the watershed, the lower is the NO 3N concentration in the river water, because the water from the forest dilutes the NO3-N-polluted
water from the cropland. To reduce inflow into rivers of nitrogen components that are not used or
fixed in cropland, it is worth considering changing the uses of land adjacent to the rivers. Suchchanges would include the establishment of buffer zones such as riparian forests (Okazawa et al.,2010). In recent years, some basic research has addressed the influence of land agglomeration inwatersheds on the nitrogen concentration of the river water (Okazawa et al., 2009, 2011).
Incorporating the new viewpoint of the positional relationship between cropland and forests inwatersheds into assessment of the impact of land use, we will investigate desirable land use in theTokachi River Watershed from the standpoint of conserving the water environment, which will
make it possible for us to provide ideas for better land uses.
CONCLUSION
The relationship between the NO3-N concentration in the river water and the proportion of croplandin the watershed was investigated for 37 locations on the Tokachi River main stream and its tributaries (their lowest reaches). The maximum slope of the regression curve (IF value) that represents
the relationship between the NO3-N concentration and the land use was found to be 0.075. It wasclarified that the IF values for the investigation locations were far greater than those for other areasin Hokkaido. In future study, it will be necessary to explore the feasibility of land use assessment
methods that are based on the new viewpoint, in examining water environment conservation byreducing the nitrogen loading into the river system while maintaining the cropland in the TokachiRiver Watershed in favorable condition.
ACKNOWLEDGEMENTS
We would like to express my gratitude to Prof. Dr. Yasushi Takeuchi, Faculty of Regional
Environment Science, Tokyo University of Agriculture. The authors are also grateful for theresearch assistance provided by the students of the Laboratory of Hydro-structural Engineering,Tokyo University of Agriculture, and the Environmental and Rural Engineering Unit, Obihiro
University of Agriculture and Veterinary Medicine.
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