Trends in changes of lake water levels in the Pomerania Lakeland

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1 Limnological Review 4 (2004) Trends in changes of lake water levels in the Pomerania Lakeland Department of Hydrological-Meteorological State Service in Gdynia, Ciołkowskiego 2/3, Białystok Abstract: Directions of changes in the level of lakes in the Pomerania Lakeland in the last quarter of the 20 th century are presented. Trends in the variation of water levels in outflow-, flow-through, and coastal lakes and also in lakes with the hydrological system changed by anthropogenic factors are characterized. Key words: changes of lake water levels, trends. Introduction Since a long time tendencies in the changes of lake water levels have been discussed in the limnological literature (Skibniewski, 1954; Pasławski, 1972; Jańczak, Choiński, 1988; Bajkiewicz-Grabowska, 1994, 2001; Borowiak, 1997; Dąbrowski, 2002). Directions of these changes are varied depending on the length of the compared periods since in records used as source materials sufficiently long sequences of observation periods common to a greater number of lakes are hardly available. For the currently studied lakes most observation sequences begin in mid-seventies of the last century. Hence the last quarter of the 20 th century is the period in which changes of the level in the greatest number of lakes can be compared. The period used in the investigation is identical with that covered by Dąbrowski (2002) in analyses of changes of the water level in lakes of north-eastern Poland. This assumption permitted the determination of tendencies in the changing level of various lakes in lake districts lying in the area of the last glaciation. Study area The investigated lakes of the Pomerania Lakeland lie between the River Vistula lower course to the east and the Odra lower course and the River Noteć to the west, the Baltic coast to the north and the rivers Warta and Notec lower course to the south (Fig. 1). In this area the Institute of Meteorology and Water Managments conducts daily records of the water level in 28 lakes. Of this number 24 lakes with a common observation period were selected and divided in groups of outflow-, flowthrough, and coastal lakes and lakes of the anthropogenically changed hydrologic system (Tab. 2). Natural variation in the lake level depends on many factors, the most important of them being the and air s. The directions in changes of these elements were analysed on the basis of records from the meteorological stations of Lębork, Resko, and Chojnice (Fig. 1). The tendencies in changes of and s were evaluated using the method of linear regressions. The compared period was too short to observe tendencies in the changes of such stable climatic

2 76 elements as the and s still it was indispensable to refer the observations to the conditions affecting the water level at that time (Tab. 1). For these measurement stations parameters of equations of the and regressions describing the study area show that the trends in the chief climatic factors are fairly insignificant. However, in each case they show an ascending direction. The comparison of the analysed 26-year long period with the longest observation periods recorded by the meteorological stations of this region shows similar tendencies in the changes of and (Tab. 1). It can be stated that in the investigated region the dynamics of changes in lake water levels occurred in similar climatic conditions, the more so that in this region the evaporation did not show significant changes (Jurak, 1992). Table 1. Tendencies in changes of the main climatic elements Station Elements of the climate Period (number of years) Equation of linear regression Determination coefficient (R 2 ) y = x Lębork y = x y = x y = x Resko Chojnice y = x y = x y = x y = x y = x y = x y = x y = x Fig. 1. Location of the lakes (numbered according to Table 2) Tendencies in changes of lake water levels Analysis of the natural changes of lake water levels in the Pomerania Lakeland was carried out on the example of six outflow lakes and 18 flow-

3 Trends in changes of lake water levels in the Pomerania Lakeland 77 through lakes (Fig. 1, Tab. 2). Against this background trends in changes of water level of six lakes under the impact of anthropogenic factors and of four coastal lakes whose water level was periodically affected by the sea were also analysed. The investigation of changes in the level of lake water was carried out using the method of linear regressions based on the assumption that the chronologically examined annual water level showed certain deviations from the regression linear. The dependence between the time (x) and mean annual water level (y) is expressed by the straight line y = ax + b, determined on the basis of the least-square theory where the regression coefficient (a) describes the magnitude of change of the mean annual water level. Since all the observations of lake water levels were converted into a common standing Kronsztad reference level, the value of the regression coefficient determined the changes in metres per annum. Table 2. Characteristic of changes in the water level of lakes and tendencies in their changes No. Lake Catchment OUTFLOW LAKES Observation No of Average level Amplitude Equation of linear Determination period years (m above Kr) (m) regression coefficient (R 2 ) 1 Rakowskie y = x Nakielno Gwda y = x Wierzchowo y = x Ińsko Ina y = x Borzechowskie Wielkie Wierzyca y = x Jasień Łupawa y = x FLOW-THROUGH LAKES 7 Drawsko y = 0,008x Lubie Drawa y = 0,0026 x Osiek y = 0,0084 x Szczytno y = 0,0019 x Brda 11 Charzykowskie y = 0,003.x Gowidlińskie Słupia y = 0,0072 x Raduńskie Górne Radunia y = 0,0011 x Morzycko Słubia(Odra) y = 0,0052 x LAKES OF THE HYDROLOGICAL SYSTEM AFTER ANTHROPOGENIC TRANSFORMATION 15 Ostrzyckie Radunia y = 0,0047 x Wdzydze Wda y = 0,0008 x Siecino Drawa y = 0,0099 x Bobięcino Wielkie Wieprza y = 0,0015x Wielkie Gwda y = 0,0056 x Sępolińskie Brda y = 0,005 x COASTAL LAKES 21 Jamno Unieść y = 0,0124 x + 0, Bukowo Unieść y = 0,0014 x + 0, Gardno Łupawa y = 0,0013 x + 0, Łebsko Łeba y = 0,0011x 0, Outflow lakes lying near watersheds are natural water bodies where lake rivers begin. The pattern of changes in the water level of these lakes chiefly depends on the transformation processes of in their direct basins. The investigated lakes lie at the altitudes from m to m. The amplitude of water level variation is m. Of the investigated six lakes four are characterized by an ascending trend which is statistically significant in the case of lakes Insko, Nakielno and Jasien (Tab. 2). The remaining two lakes Rakowskie and Wierzchowo show a decreasing trend, statistically significant in the case of Lake Wierzchowo. Flow-through lakes in the drainage area of lake rivers form natural equalizing water bodies which regulate the flow along the river course. In these lakes the rhythm of variation of the water level chiefly depends on the upper part of the catchment area (an affluent) and to an only small degree on the direct catchment. The sites of flow-through lakes are more varied than of the outflow ones. Lake Gowidlinskie lies at the highest position ( m) and Lake Osiek at the lowest (51.34 m). A very narrow range of amplitudes ( m) of the varying water level is observed here. In seven lakes out of eight an ascending tendency is observed (Tab. 2). In five cases the statistical significance of this trend is low. A statistically

4 78 significant trend was only found in lakes Gowidlinskie and Osiek. It should be also noted that in one case (Lake Raduńskie Górne) a decreasing statistically non-significant tendency was recorded. It seems that the distinct dominance of ascending tendencies in the changes of the water level of flow-through lakes can be regarded as a characteristic trait of the Pomerania Lakeland. However, if by way of example the 26-year long sequence is fit in the 48-year sequence for Lake Charzykowskie the above postulate becomes senseless (Tab. 3). The trend of the longer observation sequence is statistically non-significant still it changes the direction into a decreasing one. This shows that in the case of flow-through lakes the selected common comparative period can be characterized by trends quite different from these found for long observation periods. Changes of the water level in lakes with natural variation form a background for the comparison of the degree of human interference in the behaviour of lakes with anthropogenic changes of the hydrological system. Such lakes lie in different parts of the catchment. They are classed in this group on account of damming facilities located on the inflow or the outflow. The investigated lakes lie at the altitudes from m to m (Tab. 2). The amplitude of changes depends on methods of water management both in the catchment and in the lakes. In the case of the investigated six lakes the amplitudes varied from 0.69 m to 1.38 m. In four of them changes of the water level showed a decreasing tendency, however this trend was statistically significant only in the case of Lake Siecino. The remaining two lakes: Ostrzyckie and Wielkie were characterized by a significant increasing tendency (Tab. 2). The differentiation of tendencies in the changing water level of lakes with an anthropogenically deformed hydrological system is not a specific phenomenon. The requirements of water economics are varied still the activities aimed at the prevention of water shortages prevail resulting in continuous decreases in the level of lake waters. The prevailing decreasing trend is corroborated by the 58-year observation sequence of Lake Wdzydze (Tab. 3). This example shows that where human interference in the formation of water level is of long standing even short observation sequences give a very clear picture of the direction of changes in lake waters levels. Table 3. Tendencies in the changes of lake level in the investigation period against the background of longer obserwation series Period Equation of linear Determination Average level Amplitude Lakes (Number of years) regression coefficient (R 2 ) (m above kr) (m) FLOW-THROUGH LAKES (26) y = x Charzykowskie (48) y = x LAKES OF THE HYDROLOGICAL SYSTEM AFTER ANTHROPOGENIC TRANSFORMATION (26) y = x Wdzydze (58) y = x COASTAL LAKES (26) y = x Jamno (49) y = x (26) y = x Bałtyk (Kołobrzeg) (135) y = x Łebsko (26) y = x y = x Bałtyk ( Łeba) (56) y = x Coastal lakes formed in the mouth sectors of rivers which drain their waters to the Baltic Sea, owing to changes in the shoreline are large shallow water bodies under the impact of the sea levels. Short river sectors connecting them with open sea present an easy way for the inflow of sea waters. This most frequently occurs in the case of storm winds from the north. Analysis of changes in the water level was carried out on the basis of four lakes lying at m with almost identical variation amplitudes from 1.30 m to 1.40 m (Tab. 2). An increasing tendency, significant in the case of Lake Jamno, characterizes all these lakes. A general tendency in the level of coastal lakes is confirmed by a distinctly significant increasing trend in the 49 year observation sequence of Lake Jamno (Tab. 3). At the same time it is difficult not to resist the statement that the trends in the lakes are ascending because the same trend is observed in the sea level at Łeba or Kołobrzeg. It is true that

5 Trends in changes of lake water levels in the Pomerania Lakeland 79 almost all of them are statistically non-significant still if we compare the trend in the Baltic Sea at Kolobrzeg, especially for a long sequence, with the trend in the nearest Lake Jamno they both are statistically significant (Tab. 3). Summary In the last quarter of the 20 th century in the Pomerania Lakeland varied changes could have been observed in the water levels of lakes of natural hydrological regimes. In most flow-out lakes (67%) the water level showed an increasing trend. In the remaining 33% of lakes a decreasing tendency was manifested. In flow-through lakes an ascending trend decidedly prevailed (88%). The observed character of changes depended on the selection of the investigation period, individual traits of lake catchments, and on the transformation processes of into inflow occurring there. The trend in changes of water levels of lakes of the anthropologically changed hydrological system is the resultant of natural processes and human activity. Water management prevents the recognition of natural processes occurring in lakes and at the same time causes decreasing trends in water levels which is the dominant feature of lakes of the Pomerania Lakeland. It was also ascertained that all the coastal lakes manifested an increasing trend in the changes of water levels whose magnitude and direction approximated to the varied level of the sea. The discussion of trends in changes of lake water levels in the Pomerania Lakeland can be summed up by the conclusion that in the last quarter of the 20 th century mean annual water levels investigated using the method of linear regressions, usually showed an ascending tendency in out-flow, flowthrough and also coastal lakes. They are also in accordance with the trends in the chief climatic elements of this region. Due to the number of the investigated objects the above conclusion should not have a territorial character. Its application to the remaining types of lakes of the Pomerania Lakeland, not included in the study, could be misleading. References Bajkiewicz-Grabowska E., 1994, Tendencje zmian charakterystyk hydrologicznych jezior Polski Północno- Wschodniej, Przegląd Geof., XXXIX, 2, Bajkiewicz-Grabowska E., 2001, Trends in water level changes in the lakes of North-eastern Poland, Limnological Review, vol. 1 (2001), Wyd. UMK, Toruń, Bajkiewicz-Grabowska E., 2002 Obieg materii w systemach rzeczno-jeziornych, UW, Wydział Geografii i Studiów Regionalnych (rozprawa habilitacyjna). Borowiak D., 1997, Wahania poziomów jezior Polski w latach , Mat. Konf. Nau. Poznań 2 grudnia 1997, Wyd. Homini, Poznań Bydgoszcz, Dąbrowski M., 2002, Changes in the water lewel of lakes in northeastern Poland, Limnological Review, vol. 2 (2002), Wyd. UMCS, Lublin, Jańczak J., Choiński A., 1988, Wahania poziomów wody wybranych jezior Polski w latach , Wyd. UMK, Toruń. Jańczak J., 1996, Atlas jezior Polski, Tom I, Poznań. Jańczak J., 1997, Atlas jezior Polski, Tom II, Poznań. Jurak D., 1992, Temperatura wody i parowanie w świetle róŝnych scenariuszy klimatycznych, Przegląd Geof., XXXVII, 3 4, Pasławski Z., 1972, Wieloletnie wahania i tendencje zmian poziomu wód jezior odpływowych w Polsce Północnej, Przegląd Geof., XVII, Skibniewski L., 1954, Wahania poziomów zwierciadła wody większych jezior Pojezierza Pomorskiego i Mazurskiego, Przegląd Meteo., VII, 3 4. Streszczenie Poglądy na temat zmian poziomów jezior są róŝne i zaleŝne od długości analizowanych ciągów obserwacyjnych. Artykuł niniejszy poprzez dobór jednakowego okresu i metodyki badań ( ) jest uzupełnieniem tej problematyki przedstawionym przez autora w roku 2002 dla jezior północno-wschodniej Polski. Stwarza moŝliwość jednolitego spojrzenia na trendy róŝnych typów jezior połoŝonych w pasie obu największych pojezierzy w ostatnim ćwierćwieczu XX wieku. Zmiany poziomów jezior Pojezierza Pomorskiego przedstawiono na przykładzie 24 jezior, z których 14 reprezentowało naturalne reŝimy hydrologiczne (jeziora odpływowe 6, jeziora przepływowe 8). Pozostałe jeziora znajdowały się pod wpływem antropopresji ( 6 jezior) i stanów morza (4 jeziora). Analiza kierunków zmian poziomów jezior metodą prostych regresji pozwoliły stwierdzić, Ŝe w badanym okresie jeziora odpływowe, przepływowe i przybrzeŝne charaktery-

6 80 zowały się w większości trendem rosnącym (tab. 2). Był on zgodny z trendem głównych elementów klimatu decydujących o warunkach zasilania jezior. Wnioski płynące z tendencji zmian poziomu jezior Pojezierza Pomorskiego rozpatrywane na przykładzie niewielkiej liczby jezior nie mogą mieć charakteru wniosków obszarowych. Oprócz głównych elementów klimatu nie mniej istotną rolę odgrywa w nich zespół indywidualnych cech zlewni jeziornych.