INBA-NUMA (LAKE INBA)

A lake view on the artificial bank

Photo.
Photo: A. Kurata


A. LOCATION

  • Chiba Prefecture, Japan.

  • 35:44N, 140:11E.


B. DESCRIPTION

    This lake and Tega-numa (ASI-28) are shallow sister lakes which are more or less the same in their geological origin, physiographic conditions and recent environmental changes. Inba-numa had been a W-shaped lake of about 2,900 ha until the 1950's, but was completely reformed by the reclamation project which was started in 1963 and completed in 1969 for the purpose of flood control, rice field improvement and water resource development. The present lake consists of two parts, North and West Lake connected with each other by creeks, measuring 1,155 ha in the summed area.
    The lake is extensively utilized as a source of water for irrigation and industrial and urban waterworks. It serves as one of the prefectural nature parks providing for fisheries and recreational activities.
    This lake experienced progressive eutrophication since the beginning of the 1970's when the population in its catchment area also began to increase rapidly. This was associated with the blooms of blue-green algae, overgrowth of water chestnuts, unpleasant odor of tap water, and other troubles. Although the domestic wastewater from an increasing number of households is the main cause of eutrophication, some 60% of the total 54,000 inhabitants of the catchment area have not yet been covered by the existing sewerage system. Therefore, together with educational campaign to the public, various measures similar to the case of Tega-numa are being taken to reduce nutrient loads carried into the lake.


C. PHYSICAL DIMENSIONS (Q)

    Surface area [km2] 12
    Volume [km3] 0.0277
    Maximum depth [m] 2.5
    Mean depth [m] 1.7
    Water level Regulated
    Normal range of annual water
    - level fluctuation [m] 0.8
    Length of shoreline [km] 26.4
    Residence time [yr] 0.1
    Catchment area [km2] 487

D. PHYSIOGRAPHIC FEATURES (Q)

D1 GEOGRAPHICAL
  • Sketch map: Fig. ASI-29-01.

  • Names of main islands: None.

  • Number of outflowing rivers and channels (name): 1 (Nagato R.).

D2 CLIMATIC
  • Climatic data at Sakura, 1981-1985

    Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann.
    Mean temp. [deg C] 2.2 3.1 6.4 12.3 17.3 19.4 23.7 22.8 21.3 15.9 10.2 4.9 12.5
    Precipitation [mm] 29 71 114 126 81 190 91 87 194 127 76 40 1,226
  • Number of hours of bright sunshine: 2,370 hr yr-1.

    Fig. ASI-29-01
    Sketch map of the lake (Q).

  • Water temperature [deg C]

    Station 1, 1981-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0.2 4.3 5.2 8.1 14.9 19.8 22.5 26.3 29.2 25.8 19.6 13.0 7.5
  • Mixing type: Polymictic.


E. LAKE WATER QUALITY (Q)

E2 pH
    Station 1, 1981-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0.2 8.0 8.3 8.6 9.0 8.6 8.3 8.6 8.7 8.6 8.1 8.0 8.0
E3 SS [mg l-1]
    Station 1, 1981-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0.2 17 33 26 20 18 10 12 16 21 25 17 13
E4 DO [mg l-1]
    Station 1, 1981-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0.2 14 15 15 13 10 10 13 10 12 10 12 14
E5 COD [mg l-1]
    Determined by KMnO4 method.
    Station 1, 1981-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0.2 11 12 12 12 11 10 10 14 14 13 9.6 11
E6 CHLOROPHYLL CONCENTRATION [micro l-1]
    Station 1, 1981-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0.2 93 125 141 186 130 113 127 129 157 170 162 198
E7 NITROGEN CONCENTRATION
  • Total-N [mg l-1]

    Station 1, 1981-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0.2 2.25 2.92 3.05 2.63 1.79 1.97 1.64 1.56 2.00 2.41 2.73 2.96
E8 PHOSPHORUS CONCENTRATION
  • Total-P [mg l-1]

    Station 1, 1981-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0.2 0.12 0.13 0.13 0.13 0.12 0.12 0.10 0.14 0.14 0.13 0.098 0.11
E10 PAST TRENDS: Fig. ASI-29-02, 03 and 04.

    Fig. ASI-29-02
    Trend of COD.

    Fig. ASI-29-03
    Trend of chlorophyll a.

    Fig. ASI-29-04
    Trends of nutrients.


F. BIOLOGICAL FEATURES (Q)

F1 FLORA
  • Emerged macrophytes
    Phragmites communis, Zizania latifolia, Typha angustifolia, Nelumbo nucifera.
  • Floating macrophytes
    Nymphoides indica, N. peltata, Trapa incisa, T. natans var. japonica, T. natans var. rubeola, Hydrocharis dubia.
  • Submerged macrophytes
    Ceratophyllum demersum, Potamogeton malaianus, Vallisneria gigantea, V. denseserrulata, Najas marina, Cabomba caroliniana, Elodea nuttallii.
  • Phytoplankton
    Blue-green algae (Microcystis), diatoms (Cyciotella), green algae (Chlamydomonas).
F2 FAUNA
  • Fish
    Cyprinus carpio*, Acheilognathus morioka, Gnathopogon elongatus elongatus, Misgurnus anguillicaudatus, Hypomesus olidus*, Hemiramphus sajori, Salangichthys microdon, Anguilla japonica, Sarcochelichthys variegatus, Opsariichthys uncirostris, Zacco platypus, Pungtungia herzi, Pseudogobio rivularis, Carassius*, Gobius. * Economically important.
F5 FISHERY PRODUCTS
  • Annual fish catch [metric tons]
    1985: 851.
  • Fishery products other than fish [metric tons]
    1980: 31.
F6 PAST TRENDS: Fig. ASI-29-05.

    Fig. ASI-29-05
    Trend of fish catch.


G. SOCIO-ECONOMIC CONDITIONS (Q)

G1 LAND USE IN THE CATCHMENT AREA
    1980
    Area [km2] [%]
    Natural landscape 163.95 33.7
    Agricultural land 207.46 42.5
    Residential area 115.77 23.8
    Total 487.18 100.0
  • Main kinds of crops: Rice.

G2 INDUSTRIES IN THE CATCHMENT AREA AND THE LAKE
    1980
    No.of persons engaged No.of establishments
    Primary industry (17,559)
    - Crop production 17,459
    - Fisheries 85
    - Others 15
    Secondary industry 41,375 2,099
    Tertiary industry 92,014 9,945
  • Numbers of domestic animals in the catchment area
    Cattle 12,911, swine 15,164, horse 125.
G3 POPULATION IN THE CATCHMENT AREA
    1985
    Population Population density [km-2] Major cities (population)
    Urban 434,380 - Sakura, Yachiyo, Chiba, Funabashi,
    Rural 105,790 -Narita, Yotsukaido,
    Total 540,170 1,109Kamagaya

H. LAKE UTILIZATION (Q)

H1 LAKE UTILIZATION
    Source of water, sightseeing and tourism (no. of visitors in 1980: 380,000), recreation (sport-fishing) and fisheries.

H2 THE LAKE AS WATER RESOURCE
    1985
    Use rate [m3 sec-1]
    Domestic 0.004
    Irrigation 0.015
    Industrial 0.016
    Power plant -

I. DETERIORATION OF LAKE ENVIRONMENTS AND HAZARDS (Q)

I2 TOXIC CONTAMINATION
  • Present status: No information.

I3 EUTROPHICATION
  • Nuisance caused by eutrophication
    Algal bloom (Microcystis), disturbed filtration in cleaning bed, foul smell of tap water.
  • Nitrogen and phosphorus loadings to the lake [t yr-1]

    1980
    Sources Industrial Domestic Agricultural Natural Total
    T-N 214.6 669.0 - 588.4 1,472
    T-P 30.1 87.5 - 26.0 143.6
I4 ACIDIFICATION
  • Extent of damage: None.


J. WASTEWATER TREATMENTS (Q)

J1 GENERATION OF POLLUTANTS IN THE CATCHMENT AREA
    d) Measurable pollution with limited wastewater treatment.
J2 APPROXIMATE PERCENTAGE DISTRIBUTION OF POLLUTANT LOADS
    COD [kg day-1] [%]
    None-point sources
    - (agricultural, natural and 9,638 95.9
    dispersed settlements)
    Point sources
    - Municipal 117 1.2
    - Industrial 297 3.0
    Total 10,052 100
J3 SANITARY FACILITIES AND SEWERAGE
  • Percentage of municipal population in the catchment area provided with
    adequate sanitary facilities (on-site treatment systems) or public sewerage: 55% (nightsoil treatment for the rest 45%).
  • Percentage of rural population with adequate sanitary facilities (on-site
    treatment systems): Nearly 100%.
  • Municipal wastewater treatment systems
    No. of secondary treatment systems: 1. No. of other types: 2.
  • Industrial wastewater treatment systems
    No. of industrial wastewater treatment systems: 113.

K. IMPROVEMENT WORKS IN THE LAKE

K1 RESTORATION
    Dredging is carried out at mouths of the inflowing rivers.

M. LEGISLATIVE AND INSTITUTIONAL MEASURES FOR UPGRADING LAKE

    ENVIRONMENTS
M1 NATIONAL AND LOCAL LAWS CONCERNED
  • Names of the laws (the year of legislation)

    1. For national laws see "Biwa-ko".
    2. Ordinance for Determining Effluent Standards on the Basis of Water Pollution Control Law (1976)
    3. Chiba Prefecture Environmental Pollution Control Ordinance (1971)
  • Responsible authorities

    1. Chiba Prefectural Government
    2. Chiba Prefectural Government
  • Main items of control

    1. BOD, COD
    2. BOD, COD
M3 RESEARCH INSTITUTES ENGAGED IN THE LAKE ENVIRONMENT STUDIES
  1. Chiba Prefectural Laboratory of Water Pollution, Chiba

N. SOURCES OF DATA

  1. Questionnaire filled by Chiba Prefectural Government.