Changes in water resources in selected lakes in the middle and lower catchment of the River Warta
Lakes play a very important function in water retention, particularly in areas with strong water deficiency. Water resources in lakes determine the possibility of using them for different purposes, e.g. economic development (irrigation or drinking water, water supply for industry). With ongoing processes causing a reduction in lake capacity (shrinking and shallowing), it is necessary to investigate the scale and rate of decreasing lake resources, in terms of the economic and natural consequences of this process. The aim of the study was to test changes in water resources in lakes situated in the middle and lower River Warta catchment. Special attention was paid to lakes characterised by reduced water resources in order to identify factors contributing to this phenomenon. Our current studies of 127 lakes showed a 10% decrease in water resources over 50 years (1910-1960) in the analysed part of the catchment. Our results indicated that the predominant process causing the disappearance of lakes is shallowing. This factor should be monitored carefully to avoid further lake loss.
The objective of this paper was to analyse the effects of catchment area forestation on the temperature of river waters. Two rivers, Czerna Wielka and Szprotawa, were covered in this research project and both are located in south-west Poland, running through the largest compact forest complex in the country, Bory Dolnośląskie. Both of the rivers are located within the same climatic region, and are similar in terms of their hydrology. Land use in the catchment area however is very diverse with 71.6% forest cover in the case of the Czerna Wielka River and direct contact of the forest with the river occurs over 68.3% of its length. For the Szprotawa River, the indices amount to only 39.3% and 21.6%, respectively. Data on the daily water temperatures for both of the rivers were obtained from the Hydrological Yearbooks of Surface Waters for the period 1969–1983. The mean annual water temperature in the analysed period in the case of the Czerna Wielka River amounted to 8.0°C, and in the case of the Szprotawa River it equalled 9.2°C. During the cooler half of the year (November-April), the mean water temperature for the entire examined period amounted to 3.8°C for Czerna Wielka, and 3.6°C for Szprotawa. Higher variability was recorded for the warmer seasons (May-October), where mean temperatures amounted to 12.1°C and 14.7°C, respectively. The obtained results concur with other similar research conducted around the world, and in the context of climate change are of practical value. The important role of forests in the moderation of thermal conditions is evidently unquestionable. Therefore, minimising the impact of unfavourable climatic changes on river ecosystems and their surroundings requires striving for appropriate forest management in the near-bank (buffer) zone along banks of both larger rivers and their tributaries
In the work, medium-scale topographic maps from the first half of the 20th century and aerial photographs from the first decade of the 21st century were used to determine the rate of overgrowing of lakes. A comparison of results for lakes located in different parts of Poland demonstrated that the process of proliferation of emergent plants dominated - such a situation was recorded for 14 out of the 15 lakes analysed in the work. The calculated average overgrowing rate for all the lakes in the years 1945-2010 was 0.4 ha yr-1. Lake Karaś (1.83 ha yr-1) and Lake Oświn (1.2 ha yr-1) were characterized by the highest overgrowing rates. Moreover, the work presents an attempt at discovering the factors which determine the scale of the lake overgrowing process. To this end, the main lake parameters were analysed and it was concluded that the rate of overgrowing of lakes was influenced, inter alia, by water level fluctuations and the shape and depth of a given lake. However, each lake should be treated as an individual object which responds to the processes occurring in its environment in a unique way.
Based on cartographic materials, three lakes drained in the previous centuries have been reconstructed. It was found that by restoring them to their original state their total surface area would rise by 140 ha. Knowing the water surface level, determined on the basis of the range of peat soils and contour lines, the author reconstructed the bathymetric plans of the lakes. This, in turn, allowed a determination of the volume of water stored in their basins. It amounted to ca. 2.5 million m3. The approach adopted in the paper draws attention to an important element, i.e. to lakes which no longer exist in Poland and which have disappeared as a result of hydrotechnical works. Their restoration to their original state could become an important link in water retention and delaying the land phase of the water cycle. Activities aimed at retaining water in reception basins are carried out in Poland, among others, through the small retention program. Within this project, facilities such as artificial storage reservoirs are being built. In the light of the foregoing, before making a decision to build such a reservoir, it is worth considering the possibility to restore the natural components of the environment, which is more justifiable, both from the point of view of subsequent functioning of the environment and economic factors.
The evolution of lakes, which, in the final stage, leads to their disappearance, is mainly based on the analysis of surface area change. However, there is one more, frequently overlooked process, which determines lake disappearance - namely the lake shallowing. The present paper presents the direction and rate of such evolution, taking both these processes into account. It is a comparative analysis of 9 lakes in the Żnin region - which has the greatest water deficit in the whole of Poland. Based on bathymetric plans from two periods (early 20th century and early 60’s of the 20th century), the author was able not only to evaluate the scale of surface area change but also to determine the value of lake basins volume change. Both these values were negative and amounted 9.1% and 14.9% respectively. Assuming that the said processes are invariable, the prospective period of lake functioning is about 500 years when taking into consideration surface area change only, and is 150 years shorter when allowing for aquatic resources change. The presented approach to lake disappearance analysis, which treats this phenomenon as a decline in aquatic resources rather than a simple decrease in surface area, is more reliable and therefore, scientific works in this field should take the results of lake shallowing into consideration.
Lake Śniardwy is the largest among more than 7000 Polish lakes. So far, it has not been a subject of detailed investigations concerning long-term changes in water temperature or ice regime. A considerable change in thermal and ice conditions has been observed in the period 1972–2019. Mean annual water temperature increased by 0.44°C dec−1 on average, and was higher than an increase in air temperature (0.33°C dec−1). In the monthly cycle, the most dynamic changes occurred in April (0.77°C dec−1). In the case of ice cover, it appeared increasingly later (5.3 days dec−1), and disappeared earlier (3.0 days dec−1). The thickness of ice cover also decreased (2.4 cm dec−1). Statistical analysis by means of a Pettitt test showed that the critical moment for the transformations of the thermal and ice regime was the end of the 1980’s. In addition to the obvious relations with air temperature for both characteristics, it was evidenced that the occurrence of ice cover depended on wind speed and snow cover. The recorded changes in the case of Lake Śniardwy are considered unfavourable, and their consequences will affect the course of physical, chemical, and biological processes in the largest lake in Poland.
The paper presents the dynamics of daily water temperature fluctuations in Lake Kierskie (West Poland) and determines the effect of air temperatures and wind speed on the variability of the parameter (water temperature). The analysis was based on hourly water temperature values from the period from May 2012 to January 2018. The data analysis permitted the description of the thermal regime of waters in the lake in the hourly cycle in reference to each month, and designation of months characterised by the highest and lowest stability of water temperatures. More than half of the analysed days was characterised by a change in water temperature not exceeding 1°C. The course of water temperature showed strongly positive correlations with air temperature, and weak correlations with wind speed. Water temperatures in the lake were correlated the strongest with air temperatures with a 24 h time step.
The character of phenomena related to freezing depends on macroscale factors and mainly on thermal conditions, but local conditions can also be of great importance in shaping the course of these phenomena. Building of ice cover can be substantially different even for neighbouring lakes as a result of different individual features such as, for example, morphometric parameters. In this study the variation in the thickness of ice cover on Lake Samołęskie (29 ha) localised in western Poland, is analysed. According to measurements performed the differences in the thickness of ice cover was significant as it varied from 20 to 33 cm. A characteristic feature was that along the west bank of the lake the ice cover was thinner and its thickness increased towards the east. Analysis of the hydrogeological data revealed that this phenomenon is related to the supply of water to the lake basin by efficient water-carrying formations. The influx of water affects the bottom layer of the ice cover. This effect is aggravated by the fact that the water is supplied under elevated pressure as evidenced by the presence of a polynia of about 10 m in diameter, over the deepest (over 20 metres deep) site in the lake. The polynia is never frozen, even during the coldest winters. The specific features of the lake illustrate the influence of local geological conditions on the character of processes taking place in lakes in general.
Dissolved oxygen in water (DO) is one of the primary parameters determining its quality. It remains in close relation to water temperature. The article presents results of DO and water temperature measurements performed for five stratified lakes in Poland in the years 2007-2014. The measurements were performed with monthly frequency in the summer half-year (May-October) in the deepest place of each of the lakes. The compared data show the complex character of the course of both of the parameters. The analysis of depth profiles revealed the prevalence of a decreasing tendency, or no tendency was recorded - both in the case of DO and water temperature. Due to the incoherent character of the water in the case of stratified lakes, a more detailed comparison was performed, referring to changes occurring in zones developed during summer stratification. It was determined that the temperature of the epilimnion was directly dependent on air temperature, and in all cases it showed an increasing tendency. Simultaneously, along with an increase in temperature, in the majority of cases, DO concentration in the zone decreased. In reference to the waters of the hypolimnion, increasing, decreasing, and no tendencies were recorded. The above distribution is determined by individual factors of the lakes - morphometry, trophic status, groundwater alimentation, etc. Several years of measurements of DO and water temperature in depth profiles of several lakes in Poland constitute valuable information on the current transformations of the environment of this part of Europe. In the future, the information can provide the basis for long-term analyses over the next decades, enriching global knowledge on those parameters of key importance for water ecosystems.
The paper presents an attempt to assess the effect of damming of lakes on water resources, based on the example of Lake Powidzkie (Central Poland) and its catchment. The region in which the analysed object is located has the greatest water deficits in Poland. The co-occurrence of unfavourable natural and anthropogenic factors contributed to a considerable reduction in the water resources retained in the lake. Particularly low water levels were recorded in the 1990s and in the first decade of the 21st century. The situation was not improved by a water dam constructed on the out-flow from the lake in the 1960s, due, among other things, to neglect in its exploitation. With regard to the modernisation of the dam in 2010 and the favourable hydrological situation in the winter of 2011, it was possible to retain the excess water and substantially reduce its outflow. The restoration of the lost water resources, and the hydrological benefits of this were still observable several years later. Problems related to water deficits are becoming increasingly common in many regions of the world. One solution to mitigate such a situation may be, among others, to dam natural lakes. This is simpler, cheaper, and less invasive for the environment than the construction of new water reservoirs.