IJERD – International Journal of Environmental and Rural Development (2013) 4-1

Research article
erd

Water Quality Preservation Effect of Riparian Forests inWatersheds with Dairy Farming Areas in Eastern
Hokkaido
OKAZAWA HIROMU*
Faculty of Regional Environmental Science, Tokyo University of Agriculture, Tokyo, Japan
Email: h1okazaw@nodai.ac.jp

YAMAMOTO TADAO
Research Faculty of Agriculture, Hokkaido University, Hokkaido, Japan.

INOUE TAKASHI
Research Faculty of Agriculture, Hokkaido University, Hokkaido, Japan.

NAGASAWA TETUAKIEmeritus professor, Hokkaido University, Hokkaido, Japan.
Received 14 December 2012

Accepted 27 March 2013(*Corresponding Author)

Abstract The effects of riparian buffer zones consisting of forest and wetland on water
quality functions were investigated by comparing the river water quality in two watershedswith dairy farming grassland in a large-scale dairy farming area of Eastern Hokkaido. The
nitrogen, phosphorus and suspended solids (SS) components from the two watersheds, one
without riparian buffer zones (MB) and the other with riparian buffer zones (HY), werecompared at normal flow and at rainfall runoff periods. Total nitrogen (T-N) and NO3−N
concentrations were lower in the water from the river with riparian buffer zones. For thewater sampled during the rainfall runoff period, all the water quality indicators showed
lower concentrations for the river with riparian buffer zones than for the river without
riparian buffer zones. The amount of nitrogen that flowed out of land into the river(nitrogen load) in the watershed with riparian buffer zones was 70% of that of thewatershed without riparian buffer zones. The differences in water quality between the twowatersheds are assumed to have resulted from water purification by the riparian bufferzones on one of the rivers. It is assumed that the riparian trees adsorbed nitrogen and thatthe riparian wetlands removed nitrogen and captured suspended solids, both of which
contributed to purifying the runoff water. Based on the above findings, it can be
determined that the conservation of riparian buffer zones is effective in preserving theriver water quality on a watershed-wide scale in watersheds consisting of dairy farming
grassland.
Keywords riparian, forest, river, nitrogen, phosphorus

INTRODUCTION
In Japan, measures against river water contamination caused by pollutants including nitrogen (N),phosphorus (P) and suspended solid (SS) from upland fields and dairy farming grassland has beencalled for since the 1980s. The authors have been examining water quality conservation measures
based on land use by conducting studies on the relationship between river water quality and land
use in watersheds with upland fields and grassland in Hokkaido, Japan (Okazawa et al, 2010, 2011
and 2012). We have reported that a major factor in river water contamination is the expansion of
farmland in agricultural watersheds, that the location and scale of farmlands and forests influence
the nitrogen concentration of river water, and that forests and wetlands along rivers, which arecalled riparian buffer zones, are effective in conserving the quality of river water.
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The ability of riparian buffer zones to conserve the water environment has attracted attention
in the US and Europe since the 1980s (e.g. Lowrance et al., 1984; Hill et al., 2000). It has beenclarified that the nitrogen concentration of runoff from farmlands in the shallow groundwater
decreases when it passes through riparian buffer zones. In Japan, Hayakawa et al. (2001) andKanazawa and Hayakawa (2001) examined the relationship between the topographic linkage andthe nitrate nitrogen concentration in groundwater, and clarified that the nitrate nitrogenconcentration in runoff water from upland fields decreases when that water passes though
grasslands. Nakamura and Ishida (2001) examined the effectiveness of riparian buffer zones inconserving the water quality on a watershed-wide scale in a small watershed with dairy farming
grassland in Eastern Hokkaido, and they clarified the relationship between the changes in the water
environment by ground freezing and the water quality conservation of buffer zones. In Japan andother regions that experience Asian monsoons, few studies have addressed the functions of riparianbuffer zones, and proposals for measures for conservation and restoration of riparian buffer zones
are not easily accepted. Even though riparian buffer zones have been recognized as conserving thewater quality, few studies have evaluated the phenomenon on a watershed-wide scale.
OBJECTIVE
This study investigated the influence of riparian buffer zones on the water environment of awatershed by examining a dairy farming area with grassland in Eastern Hokkaido, where relatively
natural riparian buffer zones consisting of riparian forests and wetlands are found. The river waterat the normal flow and rainfall runoff periods was investigated in two watersheds with different
conditions of rivers and surrounding areas: The river in one watershed was artificially modified andthe areas along it was mainly grassland, and the river in the other watershed was natural, with
forests and wetlands along it. By comparing the river water quality of the two watersheds using
statistical method, the effects of riparian buffer zones on water quality functions were clarified on awatershed-wide scale.
METHODOLOGY
The subject area
The  survey  was  done  in  Hamanaka  Town  (E145°13′,  N43°07′),  which  is  in  a  dairy  farming  area  of  Eastern Hokkaido. Hamanaka is located in hills at Konsendaichi in the Nemuro-Kushiro Tableland,
which has a geological basement of pyroclastic deposits and a thick surface layer of black volcanic
ash soil. The daily mean temperature in Hamanaka averages 4.9 °C; it is -12.3 °C in February and19.8 °C in August 2003. The annual precipitation in 2003 was 1,178 mm, and rain frequently falls
in the period from August to October. The snowfall season is from November to April. On theKonsen Plain (Nemuro-Kushiro Plain), where Hamanaka Town is located, large-scale pilot farms
with pasture that are mainly for dairy farming were established during the period from 1950 to2000. The main agriculture in this area is livestock and dairy farming, with upland cultivation alsodone in some parts.
The surveys were done on the two tributaries of the Furen River, which flows throughHamanaka Town (Fig. 1, Table 1). The watersheds are indicated as MB and HY. The watersheds
are similar in area and in proportions of land uses. The locations of grassland and forest relative tothe rivers and the degree of river modification differed between the watersheds. In MB, thegrassland comes very close to the riverbanks and 71% of the river has been modified, which isrelatively high compared to HY. The riverbed and riverbanks are covered with concrete blocks. In
HY, riparian forests of Japanese ash (Fraxinus mandshurica) and alder trees (Alnus japonica) andwetlands are found along the river. This river remains in its natural, unmodified state. The riparian
forests are 30 - 100 m wide. There are livestock barns scattered in both watersheds. The cattle
densities are 91 head/km2 for MB and 85 head/km for HY. The livestock manure is spread on thegrasslands after being composted or processed as slurry.
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Cattle shedRiver
WetlandForestland
Grassland●
Monitoring
point

Fig.1

Landuse map of the MB watershed (Left) and the HY watershed (Right)
Table 1 General description of the two watersheds
Name of the watershed
Watershed area (km2)Land use percentage in the watershed
Grassland (%)
Forestland (%)
Wetland (%)
Others (%)
Length of river (km)River density (km･km-2)
Percentage of improvement length (%)
Stocking density of cattle (Head･km-2)

MB
10.9
72
23
0
5
8.26
0.76
71
91

HY
8.9
70
23
6
1
6.23
0.70
0
85

Survey technique
The survey periods were from September 5 to November 16 2002 and from June 6 to November15
2003. Continuous monitoring was done for precipitation, river water level and water quality
indicators by installing a rain gauge, a water-level gauge and an automatic water sampler at eachmonitoring point.
River discharges were determined from an H-Q equation obtained from the relationshipbetween the measured discharge (Q) and the water level (H). The continuous values for waterlevels collected at 10-minute intervals were converted into discharges. The survey periods wereclassified into that with normal flow (during clear weather) and that with rainfall runoff.
River water was collected using automatic water samplers. Two samples per week werecollected from June 6 to August 4 in 2003, and one sample per day (at 12:00 noon) was collected in
the survey period in 2002 and from August 5 to November 15 (a period with frequent rainfalls) in2003. In addition to the regular water sampling, 24-hour continuous sampling at 1-hour intervals
was done whenever the rainfall reached 5 mm/h or more. The number of samples collected during
the survey periods were 119 (MB) and 109 (HY) for the normal flow period; and 290 (MB),including 10 for flooding, and 340 (HY), including 12 for flooding, for the rainfall runoff period.
The water quality indicators analyzed were T-N (total nitrogen), TON (total organic nitrogen),NO3-N, NO2-N, NH4-N, total phosphorus (T-P), PO4-P and SS (suspended solids). Ion components
were analyzed by ion chromatography, and SS was analyzed by suction filtration. TON was
determined by subtracting NO3-N, NO2-N, and NH4-N from T-N.

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Calculation method of nitrogen load
The area-specific load of nitrogen flowing out into the rivers in the two watersheds during theperiod from June 2 to November 14 in 2003 was determined for the normal flow period and therainfall runoff period (Fig.2). The area-specific load is expressed by Eq. (1).
L = ( Q･C ) / A

(1)
-1

-2

3

-1

where L is the Area-specific load (g ･ s ･ km ), Q is the river discharge (m ･ s ), C is theconcentration (mg/L) and A is the area of watershed (km2). The area-specific load of nitrogen
expresses the flux of nitrogen that flows out of a watershed.
As the changes in nitrogen concentration were smaller during the normal flow period than therainfall runoff period (see Table 2 and 3), the area-specific load for the normal flow period was
obtained by multiplying the average nitrogen concentration of river water at the normal flow periodby the discharge. The area-specific load during the rainfall runoff period was obtained based on thewater quality data collected every hour and the discharge data collected during the rainfall runoff
period. For rainfall events for which water sampling using the automatic water sampler was notdone, the load was estimated by obtaining an equation for the relationship between the riverdischarge (Q) and the load (L) (L-Q formula) from existing data (Fig.2).

Area-specific load of T-N (g･s-1･km-2)

100

10

MB
HY

1

L-Q formula
0.1

MB: y = 6.14x1.35
R² = 0.95**
HY: y = 3.44x1.34
R² = 0.94

0.01

0.001
0.001

0.01
0.1
1
River discharge (m3･s-1･km-2)

10

Fig.2 Relationship between specific load of T-N (L) and river discharge (Q) during rainfall
runoff period
RESULTS AND DISCUSSION
Characteristics of river water quality at normal flow and rainfall runoff periods
The concentrations of water quality indicators (discharge-weighted average) for the two watersheds
during the normal flow period and those during the rainfall runoff period are shown in Tables 2 and3. During the normal flow period, statistically significant by using t-test statistics differences
between the watersheds were found in the concentrations of T-N and NO3-N, and theseconcentrations were lower in MB than in HY. For other water quality indicators, the differences intheir concentrations between the two watersheds were small, and no significant differences
between the two watersheds were found.
Based on the above findings, it can be thought that NO3-N, which readily eluviates from thegrassland soil at normal water level, was purified in the riparian forests, and made theconcentrations of water quality components in MB low.
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During the rainfall runoff period, the concentrations of all the water quality indicators werehigher in MB than in HY. The difference in concentrations of TON, NH4-N, T-P and SS, whichreadily flow with the suspended components, were great between the watersheds. During therainfall runoff period, soil erosion occurs in grassland, and contaminants are adsorbed by sediments
and flow out into rivers. That the concentrations of water quality indicators were lower in HY thanin MB was attributed to the catching, by the riparian forests and wetlands, of contaminants
generated during the rainfall runoff period, which thus kept them from flowing into the river.
Table 2 Mean concentration in normal flow period
MB
T-N
TON
NO3-N
NO2-N
NH4-N
T-P
PO4-P
watershed
M.C.
1.14
0.30
0.80
0.02
0.02
0.08
0.03
S.D.
0.35
0.30
0.16
0.02
0.03
0.05
0.02
C.V.
31
99
20
122
165
66
52
HY
T-N
TON
NO3-N
NO2-N
NH4-N
T-P
PO4-P
watershed
M.C.
0.79
0.30
0.46
0.01
0.02
0.09
0.04
S.D.
0.41
0.24
0.30
0.02
0.04
0.06
0.03
C.V.
52
80
65
120
181
74
75
M.C.:Mean concentration (mg/L), S.D.: Standard division (mg/L), C.V.: Coefficient of variation (%)

SS
12
12
99
SS
10
11
109

Table 3 Mean concentration in rainfall runoff period
MB
T-N
TON
NO3-N
NO2-N
NH4-N
T-P
PO4-P
watershed
M.C.
4.66
3.63
0.63
0.04
0.35
0.61
0.30
S.D.
3.05
2.88
0.36
0.06
0.41
1.23
0.20
C.V.
66
79
57
139
117
200
68
HY
T-N
TON
NO3-N
NO2-N
NH4-N
T-P
PO4-P
watershed
M.C.
2.52
1.89
0.47
0.03
0.14
0.58
0.22
S.D.
1.50
1.45
0.15
0.04
0.16
0.53
0.15
C.V.
59
77
31
159
118
91
70
M.C.:Mean concentration (mg/L), S.D.: Standard division (mg/L), C.V.: Coefficient of variation (%)
1600

SS
107
110
103

Rainfall runoff period
Normal flow period

1400
Area-specific load of T-N (kg･km-2)

SS
204
274
135

1200
1000
800

1298

600

907

400
200
0

170

125

MB

HY

Fig.3 Specific load of T-N during 3 June to 14 November 2003
Nitrogen loading reduction by riparian buffer zones
Figure 3 shows the relationship between area-specific load of T-N in the MB watershed and the HYwatershed. The area-specific load of T-N showed lower values in HY than in MB for both thenormal flow period or the rainfall runoff period. The two watersheds differed in whether they hadⒸ ISERD

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riparian forests and wetlands or did not have these. Therefore, the difference in the area-specific TN between the two watersheds can be attributed to reduction of nitrogen by the riparian forests andwetlands. It can be assumed that the low levels of nitrogen concentration were the result of
purification by the riparian forests and wetlands. Because, many researchers reported that thesurface water, which contained nitrogen components flowing out from grasslands into the riparian
forests and wetlands, was purified when the trees adsorbed nitrogen components and the wetlands
removed nitrogen components (e.g. Takahashi et al., 2003). The area-specific load of T-N in HY
for the normal flow period was 125 kg･km-2, which is 74% that in MB, and for the rainfall runoff
period it was 907 kg･km-2 which is equal to 70% that in MB. This result clarifies that the riparian
forests and wetlands reduced the nitrogen load by 26% during the normal flow period and 30%
during the rainfall runoff period.
CONCLUSION
The water conservation effects of riparian buffer zones (riparian forests and wetlands) were
examined by comparing the water qualities of rivers in two similar dairy farming grassland
watersheds that had different degrees of artificial modification of the rivers and different riparian
environments. Data suggest the concentrations of river water quality indicators, including nitrogen,phosphorus and SS, in the water of the river with such zones to be lower than in the water of theriver without such zones. The zones were found to significantly reduce such load from grasslands,
particularly during the rainfall runoff period. It was clarified that about 30% of the nitrogen load in
runoff was reduced by the riparian buffer zones.
This study was able to clarify the water quality conservation effect of riparian buffer zones ona watershed-wide scale. It also clarified that the water quality functions of riparian buffer zones
differed for every water quality component by flow regime (normal vs. rainfall runoff).
In recent years degradation of the water environment in agricultural watersheds has been a concern
in Hokkaido. It can be said that including the conservation of riparian forests and wetlands inenvironment improvement plans is effective in conserving the water environment of rivers.
ACKNOWLEDGEMENTS
This research was supported by Grant-in-Aid for Young Scientists (B) from The Ministry of
Education, Culture, Sports, Science and Technology (MEXT), Japan and by Important Point
Project of Graduate School of Agriculture by Tokyo University of Agriculture. We would like toexpress our gratitude to Prof. Dr. Yasushi TAKEUCHI (Tokyo University of Agriculture) and Dr.
Keiji UNOKI (Civil Engineering Research Institute for Cold Region). The authors are also gratefulfor the research assistance provided by the students of the Laboratory of Land Improvement andManagement, Graduate School of Agriculture, Hokkaido University.
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