The Open Hydrology Journal, 2008, 2, 15-24

15Spatial Distribution of Water Quality and Load Units in the Iinashi River
Basin, Shimane Prefecture, Japan
Hiroaki Somura*,1, Daisuke Nakayama2, Morihiro Aizaki1, Yasushi Seike3, Minoru Okumura3 andKumiko Nishikata1
1

Faculty of Life and Environmental Science, Shimane University, Matsue 690-8504, Japan

2

Research Center for Coastal Lagoon Environment, Shimane University, Matsue 690-8504, Japan

3

Interdisciplinary Faculty of Science and Engineering, Shimane University, Matsue 690-8504, Japan
Abstract: The spatial distributions of water quality and load units were determined for each small sub basin of the Iinashi River basin asa first step in water environment management. In addition, a GIS database was established to comprehensively resolve the issue of water
quality degradation. It was found the concentrations of water quality parameters in the Iinashi River basin were relatively low at almost
all sampling points. The average concentrations of each parameter are 1.86 mg·L-1 of chemical oxygen demand, 0.73 mg·L-1 of dissolved
organic carbon, 18.5 % of biodegradation ability, 6.58 mg·L-1 of silica, 519.8 μgN·L-1 of total nitrogen, and 27.4 μgP·L-1 of total phosphorus. In addition, the load units discharged from the small watersheds were obtained. The load units varied greatly regardless of similar
land use in the basin. Moreover, discharged loads from observation point 1-31 in the Yamasa River basin were higher than those fromobservation point 1-5 in the upstream of the Iinashi River basin regardless of high percentage of forest area (91.51%), low percentage of
agricultural land (2.3%) and low population density (24 people·km-2). Finally, the discharged loads from observation point 1-3 were determined as 53.2 g·ha-1·day-1 of suspended solid, 45.4 g·ha-1·day-1 of chemical oxygen demand, 20.9 g·ha-1·day-1 of dissolved organic carbon, 8.1 g·ha-1·day-1 of particulate organic carbon, 16.3 g·ha-1·day-1 of total nitrogen, 5.0 g·ha-1·day-1 of dissolved organic nitrogen, 1.3
g·ha-1·day-1 of particulate organic nitrogen, and 0.3 g·ha-1·day-1 of total phosphorus.

Keywords: Water environment management, Lake Nakaumi, GIS, non-point sources.
1. INTRODUCTION
Integrated management of the water environment fromriver basins to downstream areas such as lakes is very important in conservation and the sustainable use of resources. In
recent years, there has been an attempt to improve the water
quality in lakes by establishing adequate sewage systems andcreating legislation so that pollutant loading to lakes andrivers meets environmental standards. However, the water
quality has not improved as much as expected. One of thereasons is considered to be pollutant loads discharging from
non-point sources such as agricultural land.
Load units from non-point sources such as paddy fields,
upland fields, forests and urban areas are important in thatthey are used to estimate the impact of currently considered
conservation methods on downstream areas and lakes. However, these values are too difficult to determine with certainty because pollutant loads from non-point sources areaffected by the conditions of sites and precipitation (Takeda,
2001). In addition, as a load unit is defined by one constantvalue or one equation for one land use, it is very difficult toconsider a detailed plan for the protection of downstream
and lake environments. Furthermore, few studies have been
done on the load units of divided small watersheds. Thus, it
is very important to present information on the load units forsmall watersheds flowing into a target site.

*Address correspondence to this author at the Faculty of Life and Environmental Science, Shimane University, Matsue 690-8504, Japan;
E-mail: som-hiroaki@life.shimane-u.ac.jp

1874-3781/08

The water quality of Lake Shinji and Lake Nakaumi in
Shimane Prefecture, Japan, has not improved greatly. Lake
Shinji and Lake Nakaumi were designated as one of the Wetlands of International Importance by the Ramsar Convention
in November 2005.
Many researchers have studied the water quality of Lake
Shinji and Lake Nakaumi from several perspectives (e.g.
Seike et al. 2006; Sakuno et al. 2003; Nonaka et al. 2003;
Senga et al. 2001; Sakuno et al. 1998). In addition, there
have been studies on the Hii River, which accounts for about
70% of the total catchment area of Lake Shinji (e.g. Takeda
et al. 1996; Ishitobi et al. 1988). However, there have beenfew studies on the water environment of rivers flowing into
Lake Nakaumi. One of the rivers is the Iinashi River that
accounts for about 30% of the total catchment area of Lake
Nakaumi. When considering watershed management and theimprovement of the water environment in lakes, information
for both lakes and rivers is necessary. Thus, we attempt to
produce a Geographic Information System (GIS) databasefor point and non-point sources and investigate the Iinashi
River basin to understand the spatial distribution of water
quality and load units for small watersheds that make up thelarger basin as a first step in water environment management.
2. STUDY AREA
The Iinashi River basin is located in the eastern part of
Shimane Prefecture, Japan (Fig. 1). The river flows from Mt.
Tamamine. The catchment area is about 210 km2 and thelength of the river is about 40 km. The river flows throughYasugi City and then into Lake Nakaumi, which is a promi2008 Bentham Science Publishers Ltd.

16 The Open Hydrology Journal, 2008, Volume 2nent brackish water lake. The river basin has two reservoirs:
the Fube and Yamasa dams. The dams are sources of drinking water for the cities of Matsue and Yasugi and the town ofHigashi Izumo. Forest is dominant in the area. Over 80% of
the area is forest and only 6% is used for agriculture.
Along the downstream of the Iinashi River, the Tayori
River flows to Lake Nakaumi. Tayori River is an agriculturaldrainage river with a catchment area of about 12 km2. It isvery difficult to distinguish the Iinashi River and Tayori
River basins because of the complexity of their drainage, andthus the investigation was conducted for the rivers combined.
3. METHODS
3.1. Field Investigation and Water Quality Analyses
A field investigation was conducted on the 5th of August
2006 by three groups. The weather was fine but record heavy
rain of about 250mm over two days had fallen on the inves-Fig. (1). Location of the Iinashi River Basin.

Somura et al.

tigation site two weeks previous. One group investigated theupper main stream, another group investigated the branch of
the river (Yamasa River basin), and the last group surveyed
the downstream of the river. Flow velocities and river crosssections were measured to calculate water volume, and riverwater was sampled to analyze water quality. Water samples
at dam reservoirs were collected with a Van Dorn type sampler of 3L capacity, at about 0.5m, 3m, and 10m from thesurface. Multiple water quality sensors (Hydrolab MS4a,Hydrolab Quanta, Horiba U-21XD) were used to measurewater temperature, dissolved oxygen (DO) concentration,and electrical conductivity (EC). There was a total of 30 observation points.
Water quality was analyzed immediately after the field
investigation in laboratories. Suspended solid (SS) was immediately filtered through glass fiber-filters (Whatman
GF/F) preignited at 450 °C. Particulate organic carbon(POC) and particulate organic nitrogen (PON) concentrations were determined using a YANACO MT-5 type CHN

Spatial Distribution of Water Quality

The Open Hydrology Journal, 2008, Volume 2analyzer. The chlorophyll-a (Chl-a) concentration was determined by the SCOR/UNESCO method (1966). Alkalinity
was determined using a method presented in Limnological
Analysis (Saijo & Mitamura, 1995). The dissolved organiccarbon (DOC) concentration was determined using a totalorganic carbon analyzer (SHIMADZU TOC-VCSN). Biodegradation ability (BD) was determined using the same method
as for DOC with samples stored for 100 days in a constantcondition (20 °C and shielded). The chemical oxygen demand (COD) was measured using the potassium permanganate method (JIS K 0102). The Ammonia (NH4) concentration was determined using the indophenol method (Sagi,
1966). Nitrite (NO2) and nitrate (NO3) concentrations were
determined using the methods of Bendscheider & Robinson(1952) and Wood et al. (1967), respectively. The total nitrogen (TN) concentration was measured with a nitrogen analyzer (Mitsubishi Kagaku model TN-05). The phosphate(PO4) concentration was determined by the method of Murphy & Riley (1962). The total phosphorus (TP) concentration was determined by the method of Menzel & Corwin(1965). Dissolved organic nitrogen (DON) and dissolved
organic phosphorus (DOP) concentrations were calculated
from the relationship between dissolved organic and inorganic carbon. Colon bacillus was detected using a method of
JIS K 0102. Cation concentrations such as those of iron (Fe),calcium (Ca), magnesium (Mg), and manganese (Mn) were
determined by inductively coupled plasma (ICP) emission
Table 1.

17spectrometry (SHIMADZU ICPS-2000). The silica (Si) concentration in the filtrate was determined by ICP emission
spectrometry. Anion concentrations such as those of chlorine(Cl) and sulfuric acid (SO4) were determined by ion chromatography (DIONEX DX-120).
3.2. GIS Data Development
GIS data were found for the basin using existing statistical data and considering emission control measures from
non-point sources. Details of the GIS data are shown in Table 1. To develop the GIS, national spatial data of the infrastructure of the river and reservoirs, population data, forest
data, a topographical map and aerial photographs were prepared as the existing datasets. Using these datasets, the survey points, watershed, agricultural areas, residential areas
and moorland areas were developed in the GIS. The Iinashi
River basin was divided into 31 sub-basins using the locations of observation points and the topographical map.
Wherever it was difficult to determine the watershed, a subbasin was determined by terrain analysis. In addition, theforested area in each sub-basin was calculated using datafrom the Shimane Forest information station. Moreover, agricultural, residential and moorland areas were tabulated foreach sub basin after land use units were subediting using theaerial photographs. Population data for each sub basin was
estimated from village data. Population data for the Tayori

GIS Data Information Established in the Basin
Data Type

Used Data

Original Sources

Possessors
Line

Polygon

Point

RasterRiver

National Spatial Data Infrastructure

Reservoir

National Spatial Data Infrastructure

Population

Statistical Data Base

ForestShimane Forest Information
Station1/25000 Topographic
Map

Digital Geographic Map*

Geographical Survey Institute

Aerial Photograph

Aerial Photograph*Shimane Prefecture

Developed Data

Original Sources*

Geographical Survey Institute*

Geographical Survey InstituteShimane Prefecture Mountainous Region
Research Center*

Department of Agriculture, Forestry and
Fisheries in Shimane Prefecture*

Data Type
Manufacture Methods
Line

Polygon

Point*

RasterSurvey Points

Original

Based on 1/25000 Topographic Map

Watershed

Original*

Based on 1/25000 Topographic Map andTerrain Analysis

Agricultural Land

Original*

Interpretation of an Aerial PhotographResidential Area

Original*

Interpretation of an Aerial Photograph

MoorlandOriginal*

Interpretation of an Aerial Photograph18 The Open Hydrology Journal, 2008, Volume 2

Table 2.

Dam site

Fube

YamasaSomura et al.

Water Quality at Fube and Yamasa Dams

Depth
(m)

Water
temp.

DO

pH

-1

(°C)

(mg·L )

SS

-1

BD

COD
-1

TN

TP

(mg·L )

(μg·L )(%)

(mg·L )

(μgN·L )(μgP·L-1)

-1

0.5

27.8

9.9

9.0

2.0

4.9

32.8

1.3

473.9

15.7

3

24.7

9.8

9.6

2.7

6.9

23.9

1.4

516.9

19.2

10

21.7

8.2

9.6

2.9

1.7

8.4

1.8

493.4

9.7

0.5

26.5

8.2

7.9

1.5

2.7

38.3

1.5

397.1

6.5

3

29.4

8.8

8.5

2.1

7.3

18.1

1.9

465.6

6.9

10

13.1

5.1

8.8

4.0

2.4

-0.4

1.4

639.7

5.7

River basin could not be obtained at the time of this study.
Thus, no data was prepared in the GIS database.
4. RESULTS AND DISCUSSIONS
4.1. Water Quality at Dam Sites
The water quality in two artificial reservoirs at the Fubeand Yamasa dam sites were measured at three depths (Table
2). As the capacities of both reservoirs are less than 10 million m3, the reservoirs belong to the river category in Japan.
The maximum Chl-a concentration, 7.3 μg·L-1, was recorded at a 3m depth at the Yamasa reservoir. The highest
value recorded at the Fube reservoir (6.9 μg·L-1) was also at
a 3m depth. The lowest values were measured at 10m depths
for both dam reservoirs, 1.7 μg·L-1 at Fube and 2.4 μg·L-1 at
Yamasa, respectively. As average values of Chl-a concentrations were 4.5 μg·L-1 at Fube and 4.1 at Yamasa, both reservoirs are considered to be mesotrophic (OECD, 1982).
As pH values for the Fube and Yamasa reservoirs exceeded 7 at all depths, the alkaline state of the water is confirmed. The alkaline state was stronger in the Fube reservoir
than in the Yamasa reservoir. The pH values at a depth of 10
m in both reservoirs were higher than those at 0.5 m. No
photosynthesis would occur at this depth. The pH values at a
depth of 10 m were higher than those at a depth of 0.5 m
Table 3.

Chl-a
-1

because of vertical mixing. Vertical mixing due to heavy rain
two weeks previous is considered to be the reason for thesmall vertical variation in pH values.
Water temperature and DO concentrations decreased
with depth from 0.5 to 10 m in the Fube reservoir. In addition, water temperature and DO concentration dramatically
dropped to their lowest values at a 10 m depth in the Yamasa
reservoir. The DO concentration is low when the pollution
from organic matter is considerable. The SS concentration inboth reservoirs increased with water depth. It is consideredthat SS in water began to settle gradually after heavy rain.
This means that water quality characteristics at each depth in
the reservoirs become complex in a transitional period.
Biodegradation ability was calculated using the DOC
concentrations measured before and after the 100 day decomposition experiment. This value is an index of how much
persistent organic matter in the water can be broken down
under a given condition. The biodegradation ability for bothdams decreased with depth from 0.5 to 10m. 32.8% in the
Fube reservoir and 38.3% in the Yamasa reservoir were seen
at a 1m depth and 8.4% in the Fube reservoir and -0.4% in
the Yamasa reservoir were seen at a 10m depth. It seems
there are relatively large amounts of easily decomposableorganic matter in the upper layer and more persistent organic
matter at depth.

N/P Ratios for Fube and Yamasa Dams
Dam site

Fube

YamasaDepth (m)TIN/PO4-P

TN/TP

0.5

20.0

30.0

3

16.0

27.0

10

44.0

51.0

0.5

66.0

61.0

3

106.0

67.0

10

165.0

112.0

Spatial Distribution of Water Quality

Table 4.

The Open Hydrology Journal, 2008, Volume 2

19

Average Water Quality in the Basin

COD

DOC

-1

-1

BD

Si

TN
-1

-1

NH4-N

NO2-N

NO3-N

-1

-1

-1

TP

PO4-P
-1

-1

Colon Bacteria(mg·L )

(mg·L )(%)

(mg·L )

(μgN·L )(μgN·L )(μgN·L )(μgN·L )(μgP·L )(μgP·L )(per 100ml)

1.86

0.73

18.5

6.58

519.8

26.0

1.0

214.7

27.4

15.2

910.0

As the average concentrations of TN were about 494.7
μg·L-1 in the Fube reservoir and 500.8 μg·L-1 in the Yamasa
reservoir, it is possible for Lake Nakaumi to be eutrophic
and have a TN concentration higher than the environmental
standard of 0.4 mg·L-1.

water quality parameters in the Iinashi River basin were relatively low. However, even an average value of TN concentration exceeded the environmental standard for Lake Nakaumi. The TN was on average 46% inorganic and 54% organic nitrogen. Phosphorus was about 56% inorganic andabout 44% organic. By the way, NO2-N concentration atsome observed points showed under a detection limit (<1μgN·L-1). Thus, in this case, the value was treated as zero
when the average NO2-N concentration was calculated.
The composition of total nitrogen at each sampling pointis shown in Fig. (2). The concentration in the basin did not
differ greatly at each observation point except for the downstream of the basin. A high concentration of nitrogen wasobserved at the river mouth of the Tayori. TN concentrationsof about 1263.6 μg·L-1 and 1183.5 μg·L-1 were detected at

Fig. (2). Composition of total nitrogen at each sampling point.

N/P ratios are an index of eutrophication and are shownin Table 3. Both the ratios of TIN/PO4-P and TN/TP werelargest at a 10 m depth in the dam reservoirs. It was thus
clarified that phosphorus limits eutrophication in both dam
reservoirs because the ratios were over 20, especially at
depth.
4.2. Water Quality in the Basin
Average values for several water quality parameters arepresented in Table 4. It is considered the concentrations ofFig. (3). Composition of total phosphorus at each sampling point.

20 The Open Hydrology Journal, 2008, Volume 2observation points 2-1 and 2-2, respectively. The TN concentration at the river mouth of the Tayori was about 2.5
times that of the Iinashi. The inorganic nitrogen in the TNvaried from about 26% at observation point 1-20 to 63% at
observation point 1-18.

Somura et al.

fects on lacustrine ecosystems and water quality (NIES,
2001), the biodegradation abilities of river water, sewage
treatment water, mountain stream water, percolating waterfrom upland fields and miscellaneous drainage are 27.6%,
18.1%, 6.3%, 12.4% and 65.7% respectively. As the degradation ability in the upstream of the basin was low (6.0 –8.0%), it is considered that persistent organic matter flows
from the forest basins. Conversely, a high degradation abilityof about 47% was observed in the tributary river at Hirose.
From the report by the NIES, it is considered that miscellaneous drainage flows into the river.

Fig. (4). Spatial distribution of DOC.

A high concentration of phosphorus was observed at not
only the town of Hirose and in the downstream of the Tayori,
but also in the upstream of the Yamasa (Fig. 3). The organic
phosphorus percentage composition of TP was highest at
observation point 1-36 in the upstream of the Yamasa, being
about 93%. Dissolved phosphorus in the upstream of the
Iinashi was more dominant in the TP compared with the upstream of the Yamasa.

Fig. (5). Spatial distribution of BD.

The DOC concentration trend changed around Hirose(Fig. 4). A low concentration was observed in the upstreamof the basin. Conversely, the DOC concentration increased in
the downstream of the basin. In addition, the biodegradation
ability was determined from the analysis of the DOC concentration before and after the 100 day decomposition experiment (Fig. 5). According to studies on the origin anddynamics of recalcitrant organic matter in lakes and its ef-

Coliform bacteria were detected not only in the downstream but also in the upstream of the river (Fig. 6). A highconcentration was observed especially in the upstream of themain channel. It is possible the river was polluted by humanwaste just before water sampling because Coliform bacteria
are detected acutely when water is polluted by minute quantities of human waste, or by constant human waste considering a low sewage system adoption ratio.

Spatial Distribution of Water Quality

The Open Hydrology Journal, 2008, Volume 2

21basin was not prepared in the GIS database. The percentageof residential area was between 0% and 20.1% in the basin.
Almost all sub basins were 0% to 0.5% residential except forthe downstream of the Iinashi River and some sub basinslocated in the upstream of the Iinashi River. The populationis very much concentrated in downtown Hirose. However,the population is spread all over the basin along rivers.

Fig. (6). Spatial distribution of CB.

The spatial distribution of Si is shown in Fig. (7). Si is
supplied by weathering, such as that associated with rainwater or river water, and silicate mineral or rock. Si is very
common and has an important role in the food chain. However, several researchers reported that Si concentration decreases from upstream to downstream of a dam (Vörösmarty
et al. 1997; Venugopalan et al. 2000). Likewise, Si was observed to decreases slightly from upstream to downstream
for the basins in this experiment.
4.3. Land Use
The percentage of land used for agriculture in the GIS
data is shown in Fig. (8). The percentage varied from 0% to
69% in the basin. Especially in the Tayori River basin, there
was a high ratio of agricultural land. Relatively high percentages from 6% to 13% were seen around not only Hirose but
also in the upstream of the Iinashi River. Agricultural land
percentages from 0% to 5% were dominant in the basin.
The relationship between residential area and populationin the basin is shown in Fig. (9). As it is very difficult to obtain population data for the Tayori River, the data for the

Fig. (7). Spatial distribution of Si.

The forest area ratio and breakdown of forest types areshown in Fig. (10). There is a recognizable spatial distribution of forest in the basin. As there is much habitation along
the rivers, the forest area ratio decreased in the central part ofthe basin. However, the ratio was over 50% for almost all thebasin. Not only artificial forests but also natural forests remain to a large extent. In addition, a few bamboo groves
remain in the Yamasa River basin.
4.4. Load Units
Load units were calculated from concentration data anddischarge data for several parameters as shown in Fig. (11).
Fig. (11) shows variations in the load unit for small watersheds. The x-axis represents each small sub basin watershed22 The Open Hydrology Journal, 2008, Volume 2Somura et al.

located from the upstreams to the downstreams of the Iinashi
and Yamasa Rivers, and each sub basin is independent. In
other words, small watersheds were selected using the criteria that there is no inflow from other watersheds. Watersheds
1 to 8 are located in the upstream of the Iinashi River, watersheds 9 to 11 are in the upstream of the Yamasa River, andthe remaining watersheds are in the downstream of the
Iinashi River. The graphs in the figure show the ratios offorest and agricultural areas in target small watersheds andload units of the COD, TN and TP. Load units were calculated by two methods. One method was a calculation using
the results of our investigation, and the other method was a
calculation using the L-Q equation for the load unit estimations of the forest area defined by Shimane Prefecture. In
addition, maximum and minimum values for each parameterof the forest area were quoted from the literature (Takeda,
2001). As shown in the figure, forest occupied over 80% of
all watersheds except for watersheds 6 and 8, and over 60%
of watersheds 6 and 8. The proportion of agricultural area isvery small and varied from 0.94% to 8.29%.

Fig. (8). Agricultural land in each sub basin.

Fig. (10). Forest areas and types.

Fig. (9). Residential areas and population.

Load units for all parameters varied for each watershed
regardless of the similar components of land use. The load
unit of the COD changed from 34.94 (watershed 13) to129.46 (watershed 2) g·ha-1·day-1, that of TN from 7.57 (watershed 12) to 35.15 (watershed 2) g·ha-1·day-1, and that of
TP from 0.19 (watershed 7) to 1.78 (watershed 2) g·ha-1·day 1. The TP load for watershed 9 was discounted becauseSpatial Distribution of Water Quality

The Open Hydrology Journal, 2008, Volume 2

23

Fig. (11). Difference in discharged load units for each small watershed.

it was an outlier. Compared with load units calculated using
the L-Q equation, almost all values calculated using the results of the investigation were higher, but trends of changes
in load units are also represented by L-Q equations. COD
load units for watersheds 1, 2, 4, 5, 6, 8 and 9 exceeded the
maximum value in the literature of 91.23 g·ha-1·day-1. TNload units for watersheds 1, 2, 3, 4, 5, 6, 8 and 9 exceeded
20.0 g·ha-1·day-1 and TP load units for watersheds 2, 4, 6 and12 exceeded 0.79 g·ha-1·day-1. In addition, there is a trend forload units in the downstream of the Iinashi River (watersheds 12 and 13) to be lower for COD, TN and TP, except
for TP in watershed 12, compared with load units in the upstream of the basin. Though it is very difficult to determine
the reasons for the spatial differences in load units, the ratiosof agricultural and residential areas are one because the percentage of agricultural area exceeded 4.5% in watersheds 2
and 6 and these watersheds had higher load units. Moreover,
the TP concentration in watershed 12 exceeded the maximum value, 0.79 g·ha-1·day-1. It is considered that the 0.87%
residential area, which was the highest value in the target
small watershed, was a reason for the high load units.
Finally, pollutant loads flowing from upstream to downstream were calculated as shown in Fig. (12). As there are nodischarge data for around the river mouth (1-1, 1-2) andTayori River, the loads were not calculated in this paper.
These values were calculated at observation points 1-3, 1-5
and 1-31. Observation points 1-5 and 1-31 are located immediately before the confluence of the upstreams of the
Iinashi and Yamasa Rivers. Observation point 1-3 is locatedin the downstream of the Iinashi River and is the final pointof discharge data. Because there is no inflow from anotherriver after observation point 1-3 for the Iinashi River, thecalculated load unit at 1-3 is not affected by the inflow of apollutant load from another river until the river mouth.
Though the Yamasa River basin has a higher percentage offorest area (91.5%), lower percentage of agricultural land
(2.3%) and lower population density (24 people·km-2) thanthe upstream Iinashi River basin as shown in Table 5, discharged loads from observation point 1-31 were higher thanthose from observation point 1-5. The discharged loads fromobservation point 1-3 were 53.2 g·ha-1·day-1 of SS, 45.4 g·ha1·day-1 of COD, 20.9 g·ha-1·day-1 of DOC, 8.1 g·ha-1·day-1 of
POC, 16.3 g·ha-1·day-1 of TN, 5.0 g·ha-1·day-1 of DON, 1.3

Fig. (12). Pollutant loads from upstream to downstream.

24 The Open Hydrology Journal, 2008, Volume 2

Table 5.

Somura et al.

Forest Area Percentage, Agricultural Area Percentage and Population Density
Forest area (%)

Agricultural area (%)

Population density (people·km-2)

Upstream of Iinashi River basin

89.1

4.3

26

Yamasa River basin

91.5

2.3

24

Downstream of Iinashi River basin

60.6

15.8

-

g·ha-1·day-1 of PON, and 0.3 g·ha-1·day-1 of TP respectively.
5. CONCLUSION
Water quality was investigated to determine the characteristics of the water environment in the basin. A GIS database was created to consider the relationship between landcharacteristics and water environment. This research issummarized as follows.
1. From the Chl-a concentration, it is considered the damsof Fube and Yamasa were mesotropic reservoirs. Considering of N/P ratio, phosphorus limits eutrophication.
2. The concentrations of water quality parameters in the
Iinashi River basin were relatively low at almost all sampling points. The average concentrations of each parameter are 1.86 mg·L-1 of COD, 0.73 mg·L-1 of DOC, 18.5 %of BD, 6.58 mg·L-1 of Si, 519.8 μgN·L-1 of TN, 26.0
μgN·L-1 of NH4-N, 1.0 μgN·L-1 of NO2-N, 214.7 μgN·L-1
of NO3-N, 27.4 μgP·L-1 of TP, and 15.2 μgP·L-1 of PO4P.
3. Some substances such as phosphorus and colon bacteria
had high concentrations not only at Hirose but also in the
upstream of the basin.
4. Information for the load units discharged from small watersheds was obtained. Load units varied regardless ofsimilar land uses in the basin.
5. Discharged loads from observation point 1-31 in the Yamasa River basin were higher than from observation
point 1-5 in the upstream of the Iinashi River basin regardless of the high percentage of forest area (91.51%),
low percentage of agricultural land (2.3%) and low population density (24 people·km-2).
6. The discharged loads from observation point 1-3 were
determined as 53.2 g·ha-1·day-1 of SS, 45.4 g·ha-1·day-1 of
COD, 20.9 g·ha-1·day-1 of DOC, 8.1 g·ha-1·day-1 of POC,
16.3 g·ha-1·day-1 of TN, 5.0 g·ha-1·day-1 of DON, 1.3 g·ha1·day-1 of PON, and 0.3 g·ha-1·day-1 of TP.
From this research, an outline of the water environment
for the Iinashi River basin was clarified for the first time.
Moreover, a prototype GIS database was built for the basin.
With the development of an integrated approach for water
environment management in the basin, it will be necessary tostore such research results to estimate the quantitative impact
on the lake.
ACKNOWLEDGEMENTS

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[18]

This work was supported by intensive field work andwater quality analysis by students at Shimane University.
We would like to express our gratitude to Mr. Ken-ichiro
Kido, Mr. Tokusei Matsushima, Ms. Makiko Sakurai, Mr.
Received: December 21, 2007

Masaaki Hirao, Mr. Kiyoshi Fujii, Mr. Satoshi Ishida, Mr.
Ken-ichi Sawata and Mr. Yuya Hirai. We also wish to convey our special thanks to Mr. Ko Fujiyama of the Shimane
Prefecture Mountainous Region Research Center who collected GIS and statistical data. The statistical population database and forest data were obtained from the Shimane Prefecture Mountainous Region Research Center and Department of Agriculture, Forestry and Fisheries in Shimane Prefecture, respectively. Aerial photographs were obtained from
Shimane Prefecture. This research was supported by a Grantin-aid of the Shimane University Priority Research Project.

[19]

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Revised: January 16, 2008

Accepted: January 18, 2008