Water circulation in a lake at extreme water levels: Lake Wigry case

Transcription

Water
Limnological Review
9, circulation
2-3: 63-72 in a lake at extreme water levels: Lake Wigry case study
63
Water circulation in a lake at extreme water levels:
Lake Wigry case study
Elżbieta Bajkiewicz-Grabowska
University of Gdańsk, Department of Limnology, ul. Dmowskiego 16a, 80-264 Gdańsk, Poland, e-mail: [email protected]
Abstract: The paper is based on Lake Wigry and presents the lake’s water balance for an average year as well as for years when the water level
in the lake was extremely high or extremely low.
Keywords: lake water balance, extreme water levels
Introduction
An analysis of changes in average annual water
levels in a lake can show which years were characterized by extremely high and extremely low water levels.
A lake’s water level is determined by the amount of water taking part in its hydrological cycle in a given year.
Changes in a lake’s water level reflect changes in its retention ability, which in turn depends on the amount of
water circulating in a lake.
A variety of publications have shown that the
amount of water present in a lake in a given year determines the lake’s function in groundwater circulation
systems (e.g. Bajkiewicz-Grabowska 2002). This brings
up the following three questions: 1) Are extreme lake
water levels reflected in a lake’s water balance?, 2) Do
lake water levels affect a lake’s role in groundwater circulation systems? and 3) If so, can this role change?
In order to answer the above questions, detailed
lake water balance data was analyzed for an average year
as well as years featuring extremely high and extremely
low water levels. The paper is a case study of Lake Wigry
– one of the larger lakes in northeastern Poland (Choiński
2005) and the largest lake in Wigry National Park.
Lake Wigry is located in the Lithuanian Lake
District (842.7) on the boundary of two meso-regions:
the Eastern Suwałki Lake District (842.73) and the
Augustów Plain (Kondracki 1988). The lake is part of
the Niemen River drainage basin. It is located on the
boundary of the middle and lower course of the Czarna
Hańcza River (Fig. 1). Lake Wigry’s full drainage basin
has an area of 487.24 km2 (Hydrographic Atlas 2005).
Its lake coefficient (C)1 is 23. Finally, its direct drainage
basin has an area of 94.4 km2 (Bajkiewicz-Grabowska
1999)2.
Lake Wigry is a postglacial lake with a complex
history, which has helped shape its current state (Fig. 2).
The lake consists of five parts – each with a diverse morphology and morphometry (Czeczuga 1979; Górniak
2006). The northern part of Lake Wigry is called the
Wigry Basin or the Northern Basin. This is the moraine
part of the lake with a well developed shoreline, which
includes a number of bays (Hańczańska Bay, Zadworze
Bay, Wschodnia Bay) and small islands located close to
the shoreline (e.g. the Grądziki Cimochowskie Islands).
The Wigry Basin is over 50 m deep in some
places with numerous ridges and cavities. Two of the
lake’s main tributaries flow into this Basin: the Czarna
Hańcza (into Hańcza Bay) and the Wiatrołuża, also
known as the Piertanka (into Zadworze Bay). The two
rivers deliver an average of 91.7 hm3 of water to the lake
1
The lake coefficient (C) is the ratio between a lake’s total drainage basin
area and lake surface area. In the hydrobiological literature, it is known as
the Ohle Coefficient.
2
According to the Hydrographic Atlas of Poland (2005), the drainage
basin of the Czarna Hańcza River up to the point where it flows into Lake
Wigry has a surface area of 208.7 km2, the surface area of the drainage basin
of the Piertanka River is 183.51 km2, and the direct drainage basin of Lake
Wigry has a surface area of 74.76 km2.
64
Fig. 1. Drainage basin of Lake Wigry
per year – 52.5 hm3 of it coming from the Czarna Hańcza
(Bajkiewicz-Grabowska 1992, 2002). The inflow data
comes from the 1971-1995 time period. The Basin also
receives some water from a stream draining Lake Leszczewek. The stream’s average annual discharge is estimated
to be about 18 hm3 (Bajkiewicz-Grabowska 1999a).
The Czarna Hańcza River exits the Wigry Basin
through Wschodnia Bay, which is located at the base of
the Klasztorny Peninsula (Figs. 1, 2). It is the only river
that exits Lake Wigry. Its discharge is estimated to be
about 124.0 hm3 per year, based on data from 1971 to
1995 (Bajkiewicz-Grabowska 2002).
Water circulation in a lake at extreme water levels: Lake Wigry case study
65
Fig. 2. Division of Lake Wigry into water areas
The Wigry Basin transitions into a narrow part of
the lake called the Szyja Basin. The point where the two
parts of the lake meet is the deepest point in the lake
(74.2 m). In the Szyja Basin, the lake’s basin narrows,
the shoreline is poorly developed with steep banks.
There are no islands or underwater shallows. This Basin
tends to be over 50 m deep and does not have any surface inflows.
South of the Łapa Peninsula, Lake Wigry again
takes on some characteristics of a moraine lake. The
shoreline becomes well developed and includes two
bays (Wasilczykowska Bay, Krzyżańska Bay). The banks
are often low and wetland-like. This area is called the
Zakąty Basin, also called the Middle Basin. This part of
the lake includes several islands and shallows including
Kamień, Brzozowy Ostrów, Długi Ostrów, Szeroki Ostrów. Maximum depths in this area exceed 40 m (42.5
m in Pod Rzeczką Bay). The Zakąty Basin narrows between the Łukaszowy Róg Peninsula and the Jurkowy
Róg Peninsula and becomes the flat-bottomed Bryzgiel
Basin, also known as the Western Basin. The lake becomes more shallow in this area with a diverse bottom
topography that includes a mosaic of small depressions
and mounds. The maximum depth of this part of the
lake reaches 44 m. The southern part of the Bryzgiel Basin includes a number of islands (e.g. Ostrów, Ordów,
Krowa). The shoreline is well developed and includes
numerous bays (e.g. Słupiańska Bay, Przewłokowa Bay).
The narrowest part of Lake Wigry – called Wigierki Bay – begins at the Łysocha Peninsula and ends
66
Fig. 3. Average annual water levels (SW) in Lake Wigry from 1949 to 2008 (Wigry water level gauging site, Institute of Meteorology and
Water Management data). SSW – Average long-term water level
at Uklei Bay in the west. The Bay is separated from the
rest of the lake by an underwater ridge. Two streams
flow into Uklei Bay – one drains Lake Staw and the
other Lake Czarne near Gawrych Rudy. It is estimated
that the two streams deliver about 5.6 hm3 of water to
Lake Wigry per year (Bajkiewicz-Grabowska 1999a).
Wigierki Bay is connected to several lakes (Okrągłe,
Długie, Muliczne) via a small stream called Dłużanka
or Bystra. The aforementioned lakes do contribute
some water to Lake Wigry but their contribution depends on the water level in Lake Wigry. When water
levels are high in Lake Wigry, inflow from the other
lakes becomes small, while extremely high water levels in Lake Wigry can cause the flow of surface water
from Lake Wigry to the three aforementioned lakes. It
is estimated that average annual discharge from lakes
Okrągłe, Długie, and Muliczne can reach about 5 hm3
(Bajkiewicz-Grabowska 1999a).
Fluctuations in water levels in Lake Wigry
Water level data for Lake Wigry has been collected at a hydrometric site in the Wigry Basin since 19713.
The lake’s average water level from 1949 to 2008 was the
equivalent of 131.92 m above sea level in Kronsztadt or
The recording of water levels in Lake Wigry has been performed with
interruptions since 1926. In 1926-1933 and 1947-1970, water levels were recorded at Stary Folwark.
3
107 cm (Fig. 3). The average annual water level fluctuated 45 cm over the same period of time (high: 130 cm,
low: 85 cm). The maximum fluctuat equels 85 cm (highest: 161 cm, lowest: 76 cm)4.
From 1961 to 2008, the range of extreme annual
water levels in Lake Wigry fluctuated from 52 cm (1975)
to 13 cm (2003). The most frequently encountered range
was 21 cm, while the average range was 28 cm.
Fluctuation ranges for average annual water levels in Lake Wigry (Fig. 3) do not repeat over time (Bajkiewicz-Grabowska 1994; Dąbrowski 2002; and others).
Fourier Analysis of changes in water levels was used to
show the absence of any cyclical patterns (Dąbrowski
and Węglarczyk 2005).
The highest water levels in Lake Wigry were observed in April and May of the average hydrological
year for the 1949-2008 time period of interest, while the
lowest were observed in July of that year (Fig. 4). The
lake’s water level was 8 cm higher than the annual average during the April high, and 4 cm lower during the
summer low (Fig. 4). The average water level in the lake
during the winter season (November – April) was 4 cm
higher than the corresponding water level during the
summer season (May – October).
4
The data were obtained as part of a project designated PBZ-KBN-086/
P04/2003 and titled: “Extreme meteorological and hydrological events in
Poland – event assessment and forecasting of consequences for human communities”.
Average monthly water levels in Lake Wigry have
been shown to vary more during years characterized by
extreme water levels. Extremely high water levels were
recorded in 1975 (average: 130 cm) and 1981 (average:
127 cm) (Fig. 3). The highest water levels in 1975 and
1981 were recorded during the winter season (peak in
67
January), while the lowest during the summer season
(Fig. 4). Extremely low water levels were recorded in
1969 (average: 85 cm) and 2003 (average: 86 cm). The
highest water levels in 1969 and 2003 were recorded in
May (main peak) and November, while the lowest in
March and August (Fig. 4).
Fig. 4. Average monthly water levels in Lake Wigry from 1949 to 2008 and for years with extreme water levels (explained in text)
Water balance in Lake Wigry
The quantity of water circulating in a lake in a
given hydrological year can be described using the water balance equation, which takes into account vertical
water exchange (atmosphere – lake), horizontal water exchange (lake – drainage basin), and underground water
exchange (lake – groundwater). The formula is as follows:
(Pj – Ej) + (SHd – Hw) + DZj = DRj
(vertical exchange) + (horizontal exchange) + (underground exchange) = changes in amount of water in lake (retention difference)
where: Pj – atmospheric precipitation on lake,
Ej – evaporation off a lake’s surface, SHd – river water
and surface water inflow total from a lake’s direct drainage basin (including uncontrolled streams), Hw – river
water outflow from a lake, DZj – change in groundwater inflow, DRj – difference between a lake’s volume of
water held at the beginning and the end of a balance
period. When DZj > 0, the inflow of groundwater dominates underground water exchange and a lake takes in
groundwater. When DZj < 0, lake outflow dominates
underground water exchange and a lake loses water via
underground channels.
The water balance for Lake Wigry was calculated
for an average year from the 2001-2008 time period of
interest using the same method as in previous publications (Bajkiewicz-Grabowska 1982, 1999, 2001, 2002).
The components of the lake’s water balance were calculated based on measurements performed at several
IMGW hydrometric sites – a precipitation gauging site
at Stary Folwark and water level gauging sites at Wigry,
Stary Bród, and Czerwony Folwark. Only the underground water exchange component was calculated as a
difference. The water balance calculations take into account changes in surface area and lake volume caused
by different water levels in the lake.
68
Table 1. Average annual water balance for Lake Wigry for 2001-2008
Inflow of water
Balance elements
Outflow of water
hm3
%
Balance elements
Precipitation (Pj)
12.874
11
Evaporation (Ej)
Total inflow (SHd )
99.052
84
River water outflow (Hw)
Net change in underground water flow
(underground inflow)
5.903
5
(underground outflow)
117.829
100
Total (sum)
Changes in lake volume (Rj)
Total (sum)
Total (sum)
117.829
100
Lake Wigry contained 117.829 hm3 of water (Table 1) during an average hydrological year calculated
based on the 2001-2008 time period of interest. This
yields 5697 mm per unit of surface area. The time period in question ends with a stable water balance and the
lake’s gain of 13 mm (Table 1). It can be argued that the
water balance estimated for 2001-2008 reflects average
conditions for lower than average quantities of water in
the lake (Fig. 3).
Horizontal exchange of water (inflows and outflows) plays a key role in water circulation in Lake Wigry (Table 1, Fig. 5). River water flowing into the lake
constitutes 84% (Table 1) of all inflows and 92% of all
outflows. The ratio between water flowing into the lake
and water leaving the lake is 0.92 for the 2001-2008 period of interest.
In 2001-2008, the average annual inflow of water
to Lake Wigry was 99.052 hm3 (4783 mm). One flood
was recorded to have contributed water to the lake during this period of time. It began in October and resulted
in a peak inflow of water in March (13.822 hm3). High
inflow continued through April (11.548 hm3). Total inflow decreased from May to September. In September,
the inflow of water to Lake Wigry was only 5.224 hm3.
The magnitude as well as dynamics of water inflow to Lake Wigry are decided primarily by two rivers: Czarna Hańcza (50% of total inflow) and Piertanka
(about 37%).
Underground water exchange also plays an important role in water circulation in the lake. The magnitude of net underground inflow is comparable to the
magnitude of vertical water exchange. Underground inflow (DZj > 0) from water-bearing layers drained by the
lake is 5% of the balance total in an average year (5.903
hm3). On the other hand, the loss of water by the lake
feeding groundwater systems (DZj < 0) can reach 1% of
the balance total (1.115 hm3 (Table 1).
Total (sum)
hm3
%
8.715
7
107.711
92
1.115
1
117.541
100
0.288
0
117.829
100
Fig. 5. Volume of water taking part in the lake’s circulation system in
an average year based on the 2001-2008 time period (totals curves)
Total inflow of water into the lake (sum of river
and underground inflow) constitutes 89% of the water
balance. This percentage is comparable to that of total
outflow of water from the lake (sum of river and underground outflow), which is 93%. The ratio of total inflow
to total outflow for Lake Wigry is 0.96 for the time period of interest.
2003 was the hydrological year of extremely low
water levels in Lake Wigry (Fig. 3). The lake’s average
annual water level (SW) was 86 cm. At this level, the
lake occupies an area of 2,034 ha and stores about 331.9
hm3 of water. In 2003, 93.086 hm3 of water circulated in
the lake (Table 2) or 4575 mm per unit of surface area.
The water balance for 2003 was generally stable
with a change in the lake’s hold on water of +20 mm. The
structure of the lake’s water balance for this extreme year
does not differ from that of an average year. The horizontal exchange of water dominated water circulation in the
lake. Inflow and outflow were 81% and 87% of the balance total, respectively (Table 2). The ratio between water
flowing into the lake and leaving the lake via the Czarna
Hańcza River was 0.92 for that particular year.
In 2003, larger amounts of water were exchanged
via underground means. Underground inflow (DZj
> 0) from water-bearing layers drained by the lake is
69
9% of the balance total in an average year (8.840 hm3).
On the other hand, the loss of water by the lake feeding groundwater systems (DZj < 0) can reach 2% of the
balance total (1.850 hm3 (Table 2). In this extreme year,
total inflow to the lake (river water and groundwater)
constituted 90% of the balance total, while total outflow
89%. The ration between total inflow to the lake and total outflow was 1.01 in 2003.
Extremely high water levels were noted in the lake
in 1975 and 1981 (Fig. 3). In 1975, the average annual
water level in the lake (SW) was 130 cm. At this level, the
lake occupies an area of 2,138.3 ha and stores of 339.4
hm3 water. In 1975, 177.087 hm3 of water were in circulation in the lake (8,281 mm) (Table 3). The water balance
for the lake was negative for that extreme year. Finally,
the lake’s ability to hold water was -420 mm.
The structure of the lake’s water balance was somewhat different that year compared to years previously
mentioned. Horizontal exchange still played a key role in
the lake’s water circulation system (Table 3). Inflow from
the lake’s drainage basin made up 64% of the balance total, while outflow via the Czarna Hańcza about 95%. The
lake’s ratio between inflow and outflow was 0.68 for this
extreme year. The drainage of water-bearing layers dominated the lake’s underground water exchange.
Table 2. Water balance for Lake Wigry for a year with extremely low water levels (2003)
Inflow of water
Balance elements
Outflow of water
hm
3
%
Balance elements
hm3
%
Evaporation (Ej)
9.641
10
Precipitation (Pj)
9.296
10
Total inflow (SHd )
74.951
81
81.186
87
1.850
2
92.677
100
8.840
9
Total (sum)
93.086
100
Total (sum)
0.409
0
93.086
100
Total (sum)
93.086
100
hm3
%
Total (sum)
Table 3. Water balance for Lake Wigry for a year with extremely high water levels (1975)
Inflow of water
Balance elements
Outflow of water
hm3
%
Balance elements
Precipitation (Pj)
12.668
7
Evaporation (Ej)
Total inflow (SHd )
114.172
64
41.310
24
Total (sum)
168.150
95
Total (sum)
Total (sum)
8.937
5
177.087
100
9.303
5
167.784
95
177.087
100
177.087
100
Total (sum)
70
Underground inflow to the lake exceeded underground outflow by 41.310 hm3 that year (24% of balance
total). Total inflow to the lake (river water and groundwater) was 88% of the balance total, while total outflow
was 95%. The ratio between total inflow to the lake and
total outflow was 0.93 for that particular year.
In 1981, the average annual water level (SW) in
Lake Wigry was 127 cm. At this level, the lake occupies
an area of 2,129.6 ha and stores 338.890 hm3 of water.
178.695 hm3 of water (Table 4) were in circulation in
the lake, which corresponds to 8,375 mm per unit of
surface area. The lake’s water balance was negative for
that extreme year. Finally, the lake’s ability to hold water
was -160 mm.
Horizontal exchange still played a key role in
water circulation in the lake. Inflow from the lake’s
drainage basin made up 73% of the balance total, while
outflow via the Czarna Hańcza 95%. The ratio between
inflow to the lake and outflow was 0.77 for that extreme
year. The drainage of water-bearing layers dominated
underground water exchange. The inflow of groundwater was 31.502 hm3 or 18% of the balance total. Total
inflow to the lake (river water and groundwater) made
up 91% of the balance total for that extreme year, while
total outflow 95%. The ratio between total inflow to the
lake and total outflow was 0.96.
The data in the paper suggest that Lake Wigry is
a type of lake where water circulation is determined by
the inflow and outflow of river water (horizontal water
exchange). Underground water exchange is also important but it depends on the water level in the lake. On
an annual basis, the lake’s net change in underground
water flow is positive. This indicates that Lake Wigry
drains underground water-bearing layers. Underground outflow from the lake can become dominant
(Fig. 6) during the winter months with lake water levels at an average and extremely low groundwater levels.
The net change in underground water flow indicates
that groundwater inflow to the lake dominates during
periods of extremely high lake water levels.
Water balance data indicates that the net change
in underground water flow to the lake depends on a
so-called discharge increase coefficient (dQ5), which
reflects the relationship between horizontal and underground water exchange in a lake (Borowiak 2008). This
relationship is shown in Fig. 7.
Conclusions
Water circulation in Lake Wigry, as is the case
with all flow-through lakes, is largely determined by the
degree of horizontal water exchange. The magnitude
of this exchange is decided primarily by two rivers: the
Czarna Hańcza and the Wiatrołuża. An important role
in lake water circulation is also played by underground
water exchange – the measure of which is the net change
in underground water flow. Lake Wigry drains groundwater supplies. The role of underground inflow of water in the lake’s water balance grows with increasing
quantities of water being stored in its basin – this being
measured by increasing water levels. The underground
water exchange component of Lake Wigry’s water balance ranges from 18% to 24% of the total quantity of
water circulating in the lake during years of extremely
high water levels in the lake. Extremely low water levels
in the lake do not alter the structure of the annual water
balance of the lake.
5
The discharge growth coefficient (dQ) is the ratio of the difference between the average annual discharge of the river flowing out of the lake (Qw)
and the average annual discharge of streams feeding the lake (Qd) and average annual discharge of streams feeding the lake (Qd); dQ = (Qw-Qd)/Qd.
Table 4. Water balance for Lake Wigry for a year with extremely high water levels (1981)
Inflow of water
Balance elements
Outflow of water
hm3
%
Balance elements
Precipitation (Pj)
13.401
7
Evaporation (Ej)
Total inflow (SHd )
123.446
69
38.410
22
Total (sum)
175.257
98
Total (sum)
Total (sum)
3.438
2
178.695
100
hm3
%
9.087
5
169.608
95
178.695
100
178.695
100
Total (sum)
71
Fig. 6. Monthly variability in net changes in ground water flow to Lake Wigry in an average year based on the 2001-2008 time period and
for years with extreme water levels
Fig. 7. Relationship between the net change in groundwater flow to Lake Wigry and discharge increase coefficient (dQ)
References
Atlas podziału hydrograficznego Polski (Hydrographic Atlas
of Poland), 2005, IMGW, Warszawa (in Polish).
Bajkiewicz-Grabowska E., 1994, Tendencje zmian charakterystyk hydrologicznych jezior Polski Północnej (Change
patterns in hydrological characteristics of lakes in northern Poland). Prz. Geof. 39(2): 151-168 (in Polish, English summary).
Bajkiewicz-Grabowska E., 1999, Charakterystyka hydrograficzna Wigierskiego Parku Narodowego (Hydrographic
characteristics of Wigry National Park), [in:] Zdanowski
B., Kamiński M., Martyniak A. (eds), Funkcjonowanie i
ochrona ekosystemów wodnych na obszarach chronionych (Functioning and protection of water ecosystems in
protected areas), Wyd. IRS, Olsztyn: 99-113 (in Polish).
72
Bajkiewicz-Grabowska E. 1999a. Bilans wodny jeziora Wigry
w latach 1981-1995 (Water balance for Lake Wigry for
1981-1995), [in:] Zdanowski B., Kamiński M., Martyniak
A. (eds), Funkcjonowanie i ochrona ekosystemów wodnych na obszarach chronionych. (Functioning and protection of water ecosystems in protected areas), Wyd. IRS,
Olsztyn:113-129 (in Polish).
Bajkiewicz-Grabowska E., 2002, Obieg materii w systemach
rzeczno-jeziornych. (Circulation of matter in river-lake
systems), WGSR UW, Warszawa, p. 274 (in Polish, English summary)
Borowiak D., 2000, Reżimy wodne i funkcje hydrologiczne
jezior Niżu Polskiego (Water regimes and hydrological
functions of Polish Lowland lakes), Bad. Limnol. 2, Wyd.
KLUG, Gdańsk, p.164 (in Polish, English summary).
Dąbrowski M., 2002, Changes in the water level of lakes in
northeastern Poland, Limnol. Rev. 2: 85-92.
Dąbrowski M., Węglarczyk S., 2005, Cyclical nature of fluctuations in the levels of lakes of Northern Poland, Limnol.
Rev. 5: 61-67.
Kondracki J., 1988, Geografia fizyczna Polski (Physical Geography of Poland), PWN, Warszawa, p. 463 (in Polish).

Water circulation in a lake at extreme water levels: Lake Wigry case

Transcription

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