Water Management Instructions as an Element of Improving the State of the Pakoski Reservoir (Central–Western Poland)

Abstract

The management of reservoir dams in the context of observed climate changes and changing environmental conditions is becoming an increasingly significant challenge. Changes in the regimes of rivers feeding the reservoirs, sudden floods, long periods of drought, shallowing of reservoirs, water pollution, and algal blooms create unprecedented threats to the operation of these reservoirs. Among these challenges, the most crucial seems to be the proper management of available water resources, which condition the existence of the reservoir. The location of the reservoir has a significant impact on how water management is conducted. In the case of mountain and upland reservoirs created for flood protection of areas downstream, water management practices differ significantly from those for lowland reservoirs, which primarily serve to retain water for industrial and agricultural needs in the area, with an additional flood protection function. The aim of this study was to assess the factors determining the supply of lowland reservoirs using the example of the Pakoski Reservoir (Central–Western Poland) and to propose actions that would allow more efficient management of water resources in the catchment and reservoir, enabling the preservation of the current morphometric parameters in the face of climate change, adverse environmental phenomena, and increased anthropogenic pressure in the catchment area. This study focused on the Pakoski Reservoir, located in the southern part of the Kuyavian–Pomeranian Voivodeship. It was constructed fifty years ago as a result of damming water in the river systems of the Noteć and Mała Noteć Rivers. For decades, it served its functions, and its management posed no major issues. However, over the past decade, due to environmental changes and human activities in the catchment area, the reservoir has increasingly faced problems with filling.

1. Introduction

Managing reservoir dams in the face of observed climate change and environmental transformations is becoming an increasingly challenging task [1,2,3,4]. As shown in the last decade, many of these hydrotechnical structures, which have been in place for decades, are encountering more and more problems related to their operation [5,6,7]. The water management guidelines created for them several decades ago, as well as the water law permits issued based on these guidelines, are becoming increasingly difficult to implement under current conditions and, in some cases, are even harmful to the reservoirs and the areas located downstream [8,9]. The lack of snow cover, long periods without rainfall, and rainfall events of unprecedented intensity make reservoirs managed according to established schemes fail to meet their role in flood protection and retention. The regime of rivers is also changing, with rivers that were once snow- and rain-fed increasingly becoming solely rain-fed [10]. As a result, many of these rivers, particularly those in the lowland areas, no longer experience a clear winter–spring rise, and the maximum flows now occur in months previously considered winter months [10,11,12,13]. On the other hand, events becoming increasingly frequent are heavy, localised rainfalls that cause local flooding [14,15,16] or extensive, prolonged low-pressure rainfalls, such as those in the Sudetes in September 2024. These events lead to water saturation in the catchment area and widespread, multi-day runoff to lower-lying areas. Additionally, significant changes in land use, particularly urbanisation in cities and smaller towns, result in the loss of natural soil permeability or the intensive irrigation of agricultural land, which drains deeper aquifers [17]. These phenomena and processes contribute to increased water inflow to reservoirs during wet periods and reduced inflow during dry, hot days. As a result, many reservoirs face problems in filling and maintaining stable water levels. Furthermore, in some cases, the flood reserves designed in earlier years are now found to be too small. For lowland reservoirs closing catchments with low water capacity, the first of these issues is especially important, as many of these reservoirs were designed as retention reservoirs primarily for agricultural or industrial water supply. In Poland, this issue is particularly relevant for reservoirs located in regions like Greater Poland and Kuyavia, as these areas experience the lowest rainfall and have the highest groundwater withdrawals for agricultural purposes. Observations show that water reservoirs such as Środa Reservoir on the Maskawa River, Września Reservoir on the Wrześnica River, Kowalskie Reservoir on the Główna River, Słupecki Reservoir on the Meszna River, and Pakoski Reservoir on the Mała Noteć River have faced similar challenges. The inability to capture water from their catchments or the total loss of inflow during the summer months resulted in an inability to reach or maintain normal impoundment levels during the summer half-year. This, combined with poor water quality and high temperatures, contributed to accelerated eutrophication. Consequently, this led to algal blooms, which in shallower reservoirs resulted in massive fish kills. Another negative aspect of prolonged low-water levels in reservoirs was the limitation or complete cessation of feeding the river valleys that pass through these reservoirs. It should be emphasised that this historical catchment was cut off from the reservoir in the 19th century due to drainage works in the upper Noteć River carried out by the Prussian authorities at the time [18,19].

Among all the reservoirs mentioned above, the largest and most economically significant is the Pakoski Reservoir, located in the southwestern part of the Kuyavian–Pomeranian Voivodeship. It is of great importance for the local economy, as it provides water to several industrial plants in the area and serves as a flood protection measure for the towns located in the middle of Noteć River valley. This reservoir has been operational for 50 years, but in its entire history, the past 5 years have proven to be the most challenging, as it has not been able to reach its normal impoundment level even once during this period. The problems described above seem fully justified when the changes in the land use structure and groundwater conditions of the main rivers feeding the reservoir are analysed. In this region, the climatic water balance has significantly worsened, there is no longer snow cover, and the upper parts of catchment areas, especially those with significant lake areas, have become non-drainable. With the constant water demand in the region, increased evaporation from the surface of the reservoir, and water feeding into the Noteć River, it is not surprising that this reservoir has remained only half-filled throughout the year in recent years, despite having the potential to replenish its water. To address the water deficits, it is necessary to modify the water management guidelines created in the 1970s to make them more flexible and reflect the potential for capturing excess water from both the current catchment of the reservoir and its historical water supply area.

The aim of the study is to assess the factors influencing the water supply to lowland reservoirs in Europe, using the Pakoski Reservoir as a case study, and to identify actions that will enable more efficient water resource management in the catchment and reservoirs, allowing for the maintenance of the current morphometric parameters in the context of climate change, adverse environmental phenomena, and increased anthropogenic pressure occurring in the catchment area. The obtained results can be applied to other lowland reservoirs in Europe in a similar climatic zone.

2. Materials and Methods

The physical–geographical description of the analysed area was based on information from orthophotomaps, topographic and hydrographic maps available on the Geoportal website [20], and raster geological maps at a scale of 1:50,000 provided on the website of the Polish Geological Institute—National Research Institute [21]. Land use forms were determined using data from Corine Land Cover 2021, downloaded from the website of the Chief Inspectorate for Environmental Protection [22]. The historical extent of the Pakoski Reservoir was determined using maps obtained from the Archive of Western Polish Maps [23]. The hydrological and meteorological characteristics of the Pakoski Reservoir catchment were based on data from the Institute of Meteorology and Water Management—National Research Institute (Figure 1), and water level fluctuations in the reservoir were plotted based on data from the State Water Holding Polish Waters. All hydrological and meteorological analyses were conducted using hydrological years, i.e., from 1 November of the previous year to 31 October of the year under analysis. In the development of hydrological characteristics, daily data on water levels and water flows from water gauge stations located in Goryszewo on the Panna River, Gębice on the Mała Noteć River, Pakość on the Noteć River, Kruszwica on Gopło Lake and data from water gauge observations conducted on the Pakoski Reservoir were used. Meteorological characteristics were based on daily data from the climatological station in Kołuda Wielka and the precipitation station in Gębice. Based on daily data, characteristic monthly and annual values were determined and shown in charts presenting trends for individual parameters.

The analysis of the possibility of supplying the Pakoski Reservoir from the Noteć River was carried out based on the location of the water table above the Pakość weir on the Noteć River and in the Pakoski Reservoir. The days on which the water table in the Noteć River was higher than in the Pako Reservoir were indicated as potential periods of water supply to the reservoir from this river. New damming elevations for the Pakoski Reservoir were determined based on the history of elevation changes in the reservoir in relation to changes in flow rates in the Mała Noteć River and the Panna River, as well as climatic trends determined on the basis of analyses carried out at the nearest climatological stations. The descriptions and diagrams of the hydrotechnical structures were based on the current water law permit, which allows for the operation of the Pakoski Reservoir, and the attached water management instruction for the reservoir [24].

The resulting maps were created with reference to the Kronstadt ’86 height reference system and the current Polish State Geodetic Coordinate System 1992.

The analyses of numerical data were carried out in MS Office and the maps were drawn in QGIS.

3. Study Area

The Pakoski Reservoir is located in the Kuyavian–Pomeranian Voivodeship, within the Inowrocław and Mogilno counties, spanning the municipalities of Pakość, Janikowo, Mogilno, and Strzelno. The reservoir begins south of Pakość at kilometre 1 + 825 of the Mała Noteć River, where the main dam is situated, and extends to kilometre 22 + 200 of this river near the village of Nowy Młyn (Figure 1). In addition to the main dam and the reservoir basin, the Pakoski Reservoir’s hydrotechnical infrastructure includes the Skalmierowice side dam, which separates the reservoir basin from the old Noteć riverbed, as well as the Pakoski hydrotechnical complex structure with channels distributing water among the Noteć, Mała Noteć, and the reservoir.

The catchment area of the Pakoski Reservoir, calculated as the drainage area of the Mała Noteć River, amounts to a total of A_całk = 727.0 km², of which the direct catchment area is A_bezp = 118.9 km². By considering the potential for water diversion into the Pakoski Reservoir from the Noteć River through the Pakoski hydrotechnical complex, the effective catchment area increases by an additional A = 1567.45 km². As previously mentioned, the primary river supplying the reservoir is the Mała Noteć, originating from Lake Niedzięgiel. Its largest tributary is the Panna River, which flows into the Mała Noteć 1.3 km upstream of its entry into the Pakoski Reservoir (Figure 1).

The catchment areas of both rivers encompass lakeland regions. They have a sandy–clayey character and are utilised in diverse ways. Larger forest complexes dominate the southern and western parts of these areas, while the central and northern regions are primarily agricultural lands. The direct catchment of the Pakoski Reservoir and its smaller tributaries, forming the northern supply area, mainly consists of fields established on drained clay deposits. This land structure contributes to uneven water supply patterns to the reservoir and its tributaries. During wet periods and the winter–spring thaw, there is a rapid runoff toward local watercourses. These watercourses converge, causing distinct short-term flow peaks (Figure 2 and Figure 3). This is reflected in unit runoff values during very wet periods, reaching q > 32.0 L s km⁻² in the Mała Noteć catchment and q > 20.0 L s km⁻² in the Panna catchment. In contrast, during rainless periods with heightened evapotranspiration, these rivers carry minimal water, with flows of just a few litres per second. This results in unit runoff values in dry months of q = 0.04 L s km⁻² for Mała Noteć and q < 0.26 Ls km⁻² for Panna. In the upper sections of these rivers and their tributaries, prolonged episodes of complete flow cessation have been recorded over many years. This issue is particularly severe in the Mała Noteć, where its headwater section has been dry since 2014 [25], and in Panna’s spring zone, where significant declines in lake water levels have also been observed [26].

According to Kondracki’s classification [27], the analysed water body is located within the Gniezno Lakeland, with its catchment area additionally covering the Kuyavian Lakeland and the Inowrocław Plain. This region has long been characterised by some of the lowest precipitation levels in Poland, averaging 500–550 mm per year (Figure 4). An additional factor contributing to the area’s low water availability is the long-standing trend of rising temperatures (Figure 4). This warming, combined with the increasing frequency of dry continental air masses with higher vapour pressure deficits, results in an unfavourable climatic water balance [28,29,30]. Consequently, the reservoir faces limited recharge potential. These adverse atmospheric conditions are exacerbated by significant water withdrawals for agricultural and industrial purposes, creating one of the most severe water deficits in the country [25,31,32,33].

Given the uneven distribution of river flows in the region and the growing industry in the southern part of Kujawy, a decision was made in the second half of the 20th century to construct the Pakoski Reservoir. It was established in 1975 as part of a lake damming programme implemented in the Noteć River basin. The damming aimed to regulate the water balance in the Noteć River basin, which had been disrupted by 19th-century Prussian drainage works; meet the growing demand for water due to the development of nearby industrial facilities; supply water for agriculture; provide flood protection for the middle Noteć valley; and sustain river flow during low-water periods to support navigation. The primary water consumers from the reservoir were intended to be the Soda Works in Janikowo and Mątwy, as well as the aforementioned agriculture and navigation sectors.

The Pakoski Reservoir was created by raising the waters of the Mała Noteć River by 4.5 m in its lower section, encompassing its valley and the basins of three associated lakes (Pakoskie Północne, Pakoskie Południowe, and Bronisławskie). The combined surface area of the bodies of water increased by 60%, from 8.1 km² to 13.0 km², while the volume grew by 110%, from 40.6 million m³ to 86.5 million m³. The maximum water level was limited by the topography of the area and the need to preserve existing infrastructure, primarily a road bridge and a railway line crossing the river valley. The impounding of water caused flooding of the floodplain of the Noteć River between the lakes, as well as a 2.5 km section above Lake Bronisławskie. The flooded areas included the reed belt surrounding the lakes, the adjacent and river-connected peatlands (mainly between the Pakoskie Lake and Lake Bronisławskie), and arable land up to a width of several to 100 m. During the creation of the reservoir, approximately 90,000 trees were planted in its immediate vicinity, including mostly willows (Salix sp.), black alders (Alnus glutinosa), and linden trees (Tilia sp.) [34,35].

The reservoir has an elongated shape, resulting from its location within the subglacial trough of the Skorzęcińsko–Pakoska valley, which divides the Gniezno Upland and the Kuyavian Upland [36]. The depth of the upland cut relative to the trough floor ranges from 20 to 50 m. This form is locally filled with 5–10 m thick lake deposits, including gyttja and peat. The western slopes of the trough are higher but gentler, with a width of about 750 m, while the eastern slopes are narrower, about 250 m wide. The eastern shores often feature cliffs [19,34,35]. The reservoir is divided by three artificial embankments used as roads or railway embankments. As a result, it is divided into four distinct parts, which correspond to the former bodies of water and depressions from which it originated. These include Lake Pakoskie Północne (Janikowskie), Lake Pakoskie Południowe (Trląg), Lake Bronisławskie, and Lake Kunowskie (Figure 5). The morphology of the reservoir bed is very varied, influenced by factors such as the numerous shallows and deep areas of the impounded lakes, the winding and incised channel of the Mała Noteć River, and the flattened areas of the reservoir bed created by flooding the river terraces and historical biogenic accumulation plains associated with the old lake bays. It is also worth noting that the reservoir bed slopes asymmetrically. The eastern slope of the basin is steeper than the western one, as are the depths near the shoreline. The western slope is gentler, with extensive shallow areas along the shoreline. The flooded strip, between elevations 74.9 and 79.4 m above sea level, reaches over 100 m in width with a slope of 6–7°. The reservoir basin’s morphology also features two depressions. The first is located in the northern part of Lake Pakoskie Północne, while the second is at the entrance to the Kołudzka Bay in the central part of Lake Pakoskie Południowe.

According to the valid water permit [24], which defines the operational rules of the reservoir, the damming levels of its water level are as follows:

Absolute Minimum Damming Level (Absolute MinPP) = 74.93 m a.s.l.;

Minimum Damming Level (MinPP) = 75.50 m a.s.l.;

Normal Damming Level (NPP) = 78.85 m a.s.l.;

Maximum Damming Level (MaxPP) = 79.40 m a.s.l.

At the characteristic operational elevations, the surface area and capacity of the reservoir are as follows: P_MinPP = 8.35 km² and V_MinPP = 45.10 million m³; P_NPP = 12.20 km² and V_NPP = 80.46 million m³; and P_MaxPP = 13.02 km² and V_MaxPP = 86.46 million m³. The usable capacity of the reservoir (Vu) is 41.36 million m³. The remaining morphometric parameters of the reservoir, determined at the NPP level, are as follows:

Maximum length—20.1 km;

Maximum width—2.0 km;

Average width—0.74 km;

Maximum depth—18.6 m;

Average depth—9.2 m;

Length of shoreline—50.55 km.

According to the instructions mentioned above, the Pakoski Reservoir is filled to the elevation of 78.85 m a.s.l. (i.e., NPP) during the period from 16 September to 15 March. In the case of a spring flood event between 16 March and 15 May, it is intended to capture excess water within the flood storage capacity, which operates between NPP and MaxPP, set at the elevation of 79.40 m a.s.l. From 16 May to 15 September, the reservoir undergoes a gradual emptying phase to the MinPP level of 75.50 m a.s.l. In the case of extremely unfavourable hydrological–meteorological conditions and increased water demand from users located downstream of the reservoir, lowering the water level to the Absolute MinPP of 74.93 m a.s.l. is permitted (see Figure 6).

The recreational functions of the Pakoski Reservoir are limited by its poor water quality, which, according to materials published on the website of the Chief Inspectorate of Environmental Protection [36], has been in poor ecological condition for many years. The poor water quality is a result of long-term contamination from nearby agricultural fields, as well as domestic and industrial sewage. The waters of the reservoir qualify as out of class, among others, due to exceeding the standards for nitrogen and phosphorus compounds. The sediments contained significant amounts of heavy metals, aromatic hydrocarbons, chloride ions, and plant protection products. Despite the poor transparency and frequent algae blooms, the reservoir still has functioning beaches and recreational areas, such as in Janikowo [37]. The reservoir is also an excellent place for fishing, particularly known for its large pike-perch (Sander lucioperca). Other fish species found in the reservoir include perch (Perca fluviatilis), bream (Abramis brama), roach (Rutilus rutilus), rudd (Scardinius erythrophthalmus), common crucian carp (Carassius carassius), silver crucian carp (Carassius gibelio), carp (Cyprinus carpio), white amur (Ctenopharyngodon idella), and eel (Anguilla anguilla). The large surface area of the reservoir also makes it suitable for various water sports, such as sailing and windsurfing.

4. Results

The functions performed by the Pakoski Reservoir, along with its operating instructions, dictate the water level regime in the reservoir. During the winter–spring period, meltwater is collected to prevent flooding in the Noteć valley below the reservoir and to store it for industrial purposes. In the summer–autumn period, the stored water is used for field irrigation and to ensure the flow of water in the Noteć River. Due to its functions and the fact that nearly half of the water stored in the reservoir is usable capacity, significant fluctuations in water levels occur throughout the year (Figure 7). According to the operation instructions for the reservoir, the permissible amplitude of water level fluctuations is 3.9 m, ranging from 75.5 m above sea level (0.6 m above the maximum water levels recorded in the lakes before their impoundment) to 79.4 m above sea level. These changes in water levels occur steadily, interrupted only by heavy rainfall episodes directly over the reservoir. In the autumn–winter period, the trend is upward, and in the spring–summer period, it is downward. The water levels change uniformly by one to several centimetres per day. Over the course of the year, there are no periods of prolonged water levels remaining at the same level. After analysing the water level in the reservoir throughout its operational period (Figure 7), it can be concluded that the permissible water levels were only exceeded in critical situations, with the highest water level reached during the flood of July 1980. Water levels were lowered below the permissible minimum only in the first years of operation (1975–1980). The lowest water levels in the full operational range of the reservoir were recorded at the turn of the 1980s and 1990s, in 2003, and in the years 2019–2021, when significant drops in water levels were also observed in central and western Poland [38]. Based on available data, the annual water level amplitudes in the reservoir were determined, averaging H = 2.12 m, and ranging from H = 0.62 m in 2015 to H = 3.83 m in 1977. The difference between the highest and lowest water levels throughout the entire operation period of the reservoir is H = 4.35 m.

Such significant annual water level fluctuations lead to considerable changes in the reservoir’s surface area over short periods. During years characterised by extreme wet or dry conditions, with maximum water level amplitudes, the reservoir’s surface area can change by 4.7 km² and its volume by 42.6 million m³ within a few months. This means that one-third of the reservoir’s basin can be periodically flooded and dried out. In accordance with the water management instructions, high water levels typically occur during the spring–summer transition, while low levels dominate the autumn–winter period. This operational scheme limits vegetation growth on the reservoir bed by preventing macrophyte colonisation in the lower parts. However, periods of several consecutive years with the reservoir not being filled, coupled with persistently low water levels throughout the year, expose large sections of the reservoir bed during vegetation periods. This fosters accelerated plant succession, which is particularly noticeable in the littoral zone. This process was especially intense during the late 1980s and early 1990s and over the past five years. Exposed sections of the bed were quickly colonised by reed beds and hygrophilous trees, which were subsequently submerged during reservoir refilling. This caused the decomposition of organic matter and an additional influx of nutrients into the reservoir’s waters. A reduced water volume also negatively affects the breeding of various fish species and other organisms, particularly those whose life cycles are tied to reed beds and shallow littoral zones. Moreover, low water levels pose potential risks for reservoir users, limit the possibilities for water-based recreation, and hinder activities dependent on stable water availability.

Difficulties in filling the Pakoski Reservoir are largely the result of climate change and reduced water inflow. As previously mentioned, the average annual precipitation in the reservoir’s catchment does not exceed 550 mm and remains relatively stable both temporally and spatially, with a slight decrease towards the southeast. However, potential evaporation from water surfaces, as well as evapotranspiration responsible for water loss from land, has significantly increased over the past 30 years. From levels of 650–750 mm/year, these values have now consistently exceeded 800 mm/year. The intensified evaporation from water and land surfaces, combined with an extended vegetation period, has led to increased water losses in the catchment and directly from the reservoir. The lack of snow and ice cover further limits water accumulation in the soil during the cold season and prevents winter–spring floods, which historically helped fill the reservoir. Another critical factor in reducing water inflow is the exclusion of upper parts of the catchment area due to lowered water levels in the lakes feeding the Mała Noteć and Panna rivers and their tributaries. Key examples include the non-flowing lakes Niedzięgiel and Białe, located in the Mała Noteć’s source area, and Lake Ostrowickie, part of the Panna Południowa’s source area. High evaporation rates, years of low precipitation, and regional groundwater level declines—linked to hydraulic connections with these lakes—have caused their water tables to drop to the extent that their outflow channels have dried up. Consequently, the Pakoski Reservoir’s supply area, being the final recipient of waters from these catchments, has shrunk. To illustrate the scale of this problem, comparing the flow rates in the Mała Noteć and Panna at their gauge stations just before the Pakoski Reservoir reveals a declining trend in unit runoff. Despite similar environmental conditions (precipitation, land use, soil composition, and lake coverage), the Mała Noteć basin, more affected by surface water and groundwater declines, shows a more pronounced negative trend. Analysis of runoff trends from 1975 to 2023—the operational period of the reservoir—shows stable unit runoff values during the late 20th century when averaged over wet and dry years. However, in the 21st century, a marked decline has occurred, which is more evident for the Mała Noteć. For instance, at the Gębice water gauge station, the unit runoff decline is approximately 0.8 L s km⁻² per decade, compared to 0.6 L s km⁻² per decade at the Goryszewo water gauge station for the Panna River (Figure 8). The decline in groundwater levels, driven by municipal and industrial water withdrawals, long-term mining dewatering associated with lignite extraction in the Konin region, and increasing water use for agricultural irrigation, exacerbates the problem. It not only reduces the inflow from lakes and rivers but also limits the groundwater stream directly feeding the Pakoski Reservoir.

In addition to contemporary anthropogenic factors affecting the water resources of the Pakoski Reservoir, attention should also be paid to transformations that occurred hundreds of years ago. At the end of the 18th century and in the mid-19th century, extensive land reclamation works were carried out in this area to canalise the Noteć River and parts of its tributaries and to convert previous wetland areas into arable land [18,38,39]. These works, in addition to improving navigability on the Noteć River and reclaiming thousands of hectares of fertile soil, resulted in the lowering of surface water and groundwater levels across the entire Noteć catchment and adjacent areas, reshaping the river network and disconnecting selected water bodies from their alimentation zones. The Noteć River, which used to flow through northern Lake Pakoskie, was dammed with an embankment and a sluice in Leszczyce, and its waters were diverted to the Noteć Canal, built in 1774. This canal bypassed the Pakoski Reservoir, directing water northward and connecting to the Bydgoszcz Canal in the Toruń–Eberswalde Ice-Marginal Valley to the east and to the main Noteć River to the west. As a result of these modifications, the primary river feeding the Pakoskie Lake became the Mała Noteć, whose recharge area was also reduced due to the construction of the Ostrowo–Gopło Canal, which redirected the southwestern streams that had previously fed it toward Lake Gopło. Consequently, the catchment area of the former Pakoskie Lake was reduced from its original ~2240 km² to 727 km². Over the next several decades, these works led to a long-term lowering of groundwater levels and the water table in lakes in the region by up to 2.5 m [18,39]. In the former Pakoskie Lake, the water table dropped by 76 cm, and in Lake Bronisław, it dropped by 66 cm [38]. The surface area of the latter decreased by 42%.

Observing such drastic negative changes in the hydrology of the area, the Prussian authorities at the time initiated measures to limit water outflow from selected lakes and rivers, constructing retention and stabilisation structures and simultaneously facilitating flow management during flood periods [19]. This programme also included Lake Pakoskie, whose water level, after stabilisation in the early 20th century, was set at approximately 75.0 m above sea level, with water level fluctuations ranging from 0.5 to 0.6 m.

The next stage of works in the catchment area involved the construction of the Pakoski Reservoir by damming the waters of the Mała Noteć River at the outflow from the northern Pakoskie Lake with a frontal dam and protecting the bed of the Old Noteć River with a side dam to prevent flooding from the impounded lake waters. During this period, the Pakość weir and associated canals were also built, enabling the regulation of water flow and transfer between the Noteć, Mała Noteć, and Pakoski Reservoir (Figure 9). Another function of the weir is to control the water level in Lake Gopło, located about 21.5 km from this hydrotechnical structure.

When describing the Noteć River and Lake Gopło, it is essential to note that significant disruptions have occurred in their hydrological recharge regimes. Due to mining activities associated with open-pit lignite extraction, as well as regional lowering of surface water and groundwater levels, certain areas within the potential recharge zone of Lake Gopło have been periodically or permanently excluded. These areas include the upper parts of the Noteć River catchment [31,32], the catchments of southeastern tributaries of Lake Gopło, the Ostrowo–Gopło Canal catchment located in the Powidzko–Ostrowska trough lake region [40,41,42,43,44,45,46,47], and the catchments of the Skulskie lakes. Partial compensation for the lost water resources has been achieved through the redirection of drainage water from the Lubstów and Tomisławice open-pit mines directly to the Noteć River or its tributaries [31,48] and by supplying the Noteć River through the Gawrony weir, which allows discharges from the summit section of the Ślesiński Canal (Figure 9) [49]. However, it should be noted that in the coming years, as more mines close, discharges of mining water toward the Noteć River will decrease, while the demand for water in areas affected by the mining-induced depression funnel to the east of Lake Gopło will remain constant. A similar situation is currently observed in the Biskupia Struga and Ślesiński Canal catchments, from which it is becoming increasingly difficult to allocate water to the Noteć system. In the coming years, due to water drainage by flooded pits located near the lakes of the Ostrowo–Gopło Canal, it is expected that reduced water flows in this second-largest tributary of Lake Gopło will persist. For many years, the catchment area of this watercourse, spanning over 300 km², has been characterised by the absence of unit runoff. Until the groundwater levels and lake water tables return to their mid-1980s levels, this area is unlikely to contribute to the alimentation of Lake Gopło.

5. Discussion

The intensifying effects of climate change, which result in sudden water surges on one hand and significant water deficits on the other, demand more effective actions in managing water reservoirs [50,51,52]. These efforts must aim to better utilise water resources within the catchment areas of these bodies of water [53]. Studies of the Pakoski Reservoir have shown that deepening water deficits, particularly severe in central Poland, lead to challenges in filling the reservoir, which in turn impacts its functionality. When revising the reservoir’s replenishment and usage scheme, it is crucial to maintain its existing functions, meet the water needs of users reliant on its resources—including those of the upper Noteć River—and ensure the functioning of the Pakoski Reservoir ecosystem. Recent years have demonstrated that the greatest challenge facing the reservoir’s management is filling it to the target operational water level (NPP) during the winter–spring period and then maintaining that level while ensuring a minimum outflow during the summer months. Snowless winters, high evapotranspiration, and groundwater withdrawals reduce the inflow of water to the reservoir and its tributaries. Consequently, its retention functions and the ability to supply water to the middle Noteć are becoming increasingly difficult to sustain. The intensifying effects of climate change, which result in sudden water surges on one hand and significant water deficits on the other, demand more effective actions in managing water reservoirs. These efforts must aim to better utilise water resources within the catchment areas of these bodies of water. Studies of the Pakoski Reservoir have shown that deepening water deficits, particularly severe in central Poland, lead to challenges in filling the reservoir, which in turn impacts its functionality.

When revising the reservoir’s replenishment and usage scheme, it is crucial to maintain its existing functions [5,51,53], meet the water needs of users reliant on its resources—including those of the upper Noteć River—and ensure the functioning of the Pakoski Reservoir ecosystem. Analyses of the recent years have demonstrated that the greatest challenge facing the reservoir’s management is filling it to the target operational water level (NPP) during the winter–spring period and then maintaining that level while ensuring a minimum outflow during the summer months. Snowless winters, high evapotranspiration, and groundwater withdrawals reduce the inflow of water to the reservoir and its tributaries. Consequently, its retention functions and the ability to supply water to the middle Noteć are becoming increasingly difficult to sustain. At the same time, it should be noted that the Mała Noteć River, due to the desiccation of its southern sections and the exclusion of parts of its recharge area (see Figure 1), currently exhibits a very low probability of flooding. Therefore, it is worth considering adjustments to the reservoir’s water levels and adopting a more flexible approach to water management during periods of reservoir filling and utilisation.

Considering the above, and particularly to increase the permanent water reserve available to reservoir users and entities in the upper Noteć valley, it is proposed to raise the Absolute Minimum Operating Water Level (MinPP) to the current MinPP level of H = 75.50 m a.s.l., and the MinPP to H = 77.00 m a.s.l. No changes are required for other operational water levels. This approach would reduce the reservoir’s water demand during its filling period while maintaining a safe flood reserve. Additionally, a more flexible approach to the dates limiting reservoir filling and utilisation is recommended. Specifically, it is suggested to extend the filling phase until 15 May if the target operating water level (NPP) is not reached by 15 March. This recommendation is driven by changing winter patterns, which no longer feature prolonged snow cover. In earlier years, snowmelt during spring thaws provided a rapid increase in river levels, enabling a swift recovery of the reservoir’s water resources while posing a potential flood risk to areas in the upper and middle Noteć valleys. It should also be noted that, during the autumn and winter, if the Mała Noteć catchment and the reservoir’s recharge zone experience significant rainfall and persistent snow cover exceeding 15 cm by the end of February, winter water intake should be limited. In such cases, the reservoir’s water level should not rise by more than 1–2 cm per day. If rising temperatures lead to rapid snowmelt and a corresponding surge in inflows to the reservoir’s primary tributaries, the Mała Noteć and Panna Rivers, reaching a flow rate of Q = 3.5 m³/s, measures should be taken to suppress the flood wave passing through the reservoir. During such conditions, an increase in the reservoir’s water level of up to 5 cm per day would be permissible.

In the event of a prolonged hydrological drought, when the combined flow rates of the Mała Noteć at the Gębice gauging station and the Panna at the Goryszewo gauging station do not exceed Q = 0.5 m³/s during the early filling phase of the Pakoski Reservoir, measures should be taken to utilise water from the Notecki Canal (Noteć) to support the reservoir’s inundation. As previously demonstrated, until the mid-19th century, Lake Pakoskie Północne, which forms the core of the Pakoski Reservoir, was fed by both the Mała Noteć and the Noteć. The Noteć, with its larger catchment area, is more water-rich. Currently, the Noteć carries mine drainage water from the Tomisławice open-pit mine and benefits from additional discharges from the peak section of the Ślesiński Canal. As a result, water flows in the Noteć at the Pakość weir, particularly during winter and spring, are typically 2–3 times greater than those in the Mała Noteć before its confluence with the Pakoski Reservoir. During these periods, water levels in both the Noteć and Lake Gopło are often higher than in the Pakoski Reservoir (Figure 10 and Figure 11). Over the past six years, such conditions occurred for more than half of the time (1193 days out of a total of 2190).

By utilising these additional water resources from the Noteć, along with the distribution of water levels in the Pakoski Reservoir and the Noteć, and considering that the so-called Pakoski hydrotechnical complex allows for the regulation of water management in the lower section of the Mała Noteć and the canalised upper section of the Noteć from the town of Pakość upstream, through Lake Gopło and the Warta–Gopło Canal to the Koszewo lock, it is worth taking steps to redirect part of the water from the Noteć to the Pakoski Reservoir. This can be achieved by using the existing hydrotechnical infrastructure built in the 1970s as part of the Pakoski hydrotechnical complex, which allows such water diversion either through gravitational flow or by means of pumping stations.

Considering the average flows of the Noteć River below Pakość weir, amounting to Q = 5.5 m³/s (IMGW-PIB data) and the need to maintain water flows in the river at levels that guarantee the water needs of users and preserve the functionality of the waterway on the Noteć, it seems that redirecting Noteć water to the Pakoski Reservoir at a rate of Q = 0.5 m³/s would not deplete its resources. On the contrary, it would allow for a faster filling of the reservoir, at least until the water levels in the system of the Lake Gopło–Noteć–Pakoski Reservoir are equalised.

In the winter–spring period of the 2023 hydrological year, during which there was a shortfall of nearly 2 m to reach the NPP, this intervention would have allowed an additional 43,200 m³/day to be introduced into the Pakoski Reservoir. Given the reservoir’s morphometric parameters, this would result in a daily water level increase of almost 0.5 cm during periods of low water levels. If the water capture from the Mała Noteć were maintained at the same rate as it was in the considered year, and the reservoir was supplemented with water from the Noteć until the water levels in both systems were equalised, it would ultimately lead to an additional several tens of centimetres of water in the reservoir. It is worth noting that the proposed water extraction of Q = 0.5 m³/s would constitute 11–44% of the flow in the Noteć River at the Pakość gauge, located downstream of the confluence with the Mała Noteć. Even better results could be achieved with the modernisation of the Pakoski hydrotechnical complex and the installation of new pumps, which would facilitate water transfer once the water levels in the reservoir and the supply canal equalise. This action would not only increase water resources in the Pakoski Reservoir during drought periods but also enable the storage of water from potential discharges from the peak station of the Ślesiński Canal and mine drainage water from the Tomisławice open pit, which currently flow through the Warta and Oder rivers to the Baltic. Keeping these waters within the river-lake system of the Lake Gopło–Noteć–Pakoski Reservoir would help retain water in a region with the greatest water deficits in Poland [25,26,31,32,33].

6. Conclusions

In the face of observed changes in river hydrological regimes (snowless winters, heavy rainfall, and late rainy autumns and winters) and increasing challenges related to the operation of reservoirs located on these rivers, there is a need for a serious revision of existing water management instructions for many hydrotechnical facilities. These facilities were designed and have functioned for decades under different hydrological conditions and for different water needs. In many cases, the documentation that has been tested does not foresee actions related to water deficits (drought), focusing primarily on flood control functions. However, currently, especially in the lake districts and the central Polish lowlands, we observe a growing need to address the deficit in the hydrological balance, which is the result of changes in the size and distribution of water inflows to the catchment (intensity and character of rainfall and snowmelt) and water usage (including evaporation, transpiration, and water needs of users). Therefore, there is a need for a “flexible” approach to the operation of hydrotechnical facilities, which will allow for the most efficient use of their retention potential while maintaining their flood control function. Many of these hydrotechnical facilities require consideration of new supply conditions, changes in water needs, and associated functionality of the facilities, as well as the interrelationships between them. A particularly important shift in the approach to water resource management is indicated in the upper Noteć River area, which is characterised by a very unfavourable water balance and is subjected to significant anthropogenic pressure. The results of this pressure are changes in the hydrographic network, high water demand from industry and agriculture, and the impacts of mining (surface lignite mining and underground salt extraction). Water needs resulting from the development of tourism and navigation (e.g., ensuring transit depths and water for lock operation on the Noteć River) as well as the functioning of natural habitats associated with rivers and bodies of water, suffering from water shortages, are also of significant importance. These demands include, among others, the restoration of surface water resources, stabilisation of water levels in reservoirs, and extension of the navigation season. Balancing these often seemingly conflicting interests may seem difficult, but not impossible. An example where such positive effects can be achieved through a change in the current water management approach is the Pakoski Reservoir.

The change in the damming levels, dates of achieving the specified water levels, and the method and scheme of supplying the Pakoski Reservoir, including the redirection of water from the Noteć River (upper Noteć Canal) through the Pakoski hydrotechnical complex during periods of higher water levels in the Noteć River and Lake Gopło, induced by increased supply in the upper Noteć catchment or water discharge from the peak position of the Ślesinski Canal, allows for the achievement of these objectives. The proposed changes will have the following effects:

• Increasing the efficient use of water resources available in the catchment area of the Mała Noteć and upper Noteć Rivers;
• Increasing the water resources of the reservoir while maintaining the safe flood capacity;
• Increasing the capacity for water flow supplementation in the Noteć below the reservoir;
• Securing the water resources of the Mała Noteć and upper Noteć River systems after the closure of the Tomisławice open-pit mine;
• Raising the groundwater levels and improving the condition of wetlands within the reservoir catchment area;
• Improving the habitat conditions for fish and other organisms inhabiting the reservoir’s littoral zone;
• Reducing the process of overgrowth in the coastal zones of the reservoir;
• Improving navigation conditions and increasing the reservoir’s tourist attractiveness.

Taking into account the challenges associated with maintaining the water level in artificial reservoirs in various parts of the world and the increasing problems with providing water for various industries and agriculture in the light of recorded climate changes [54,55,56,57,58,59,60,61,62,63,64], it seems reasonable to apply the solutions indicated for the Pakoski reservoir to other reservoirs of this type. The solutions developed for the Pakoski Reservoir in terms of increasing the flexibility of water management instructions created decades ago can be applied in particular to other lowland dam reservoirs located in a similar climatic zone, within areas where changes in their water supply regime have been observed in recent years and increasing water deficits have been observed.