Water Management on Drinking Water Reservoirs in the Aspect of Climate Variability: A Case Study of the Dobromierz Dam Reservoir, Poland

Abstract

Water reservoirs are important sources of drinking water in many parts of the world. The aim of the article is to check how water management is carried out in the Dobromierz reservoir (southwestern Poland) in the aspect of climate variability and defining recommendations for water management of this object. The reservoir was put into operation in 1986 and supplies drinking water to the city of Świebodzice. The analysis of water management (expressed by characteristic flows) showed that in most cases it is carried out in accordance with the water management manual (average compliance of approximately 93%). The main problems in the proper operation of this facility, based on literature analysis, were a lack of constant water monitoring inflow and outflow from the reservoir, lack of a fish ladder, and unsatisfactory water quality due to agricultural pollutants. The solution to these problems would be to install monitoring devices, build a fish ladder, and regulate the use of arable lands. It was shown that the construction of the reservoir had an impact on the dynamics of annual flows in the Strzegomka River (reduced fluctuations in flows after the reservoir was put into operation; daily data from Łażany water gauge, 1951–2022). Moreover, climate variability has an impact on water management (changes in temperature and sunshine duration, which affect the dynamics of flows) Water management in reservoirs should be adapted to local conditions, as well as strategies for dealing with climate variability, recommendations, documentation, and policies at various levels of management.

1. Introduction

Water reservoirs are important sources of drinking water in many parts of the world [1,2]. The increase in human demand for water services has resulted in the construction and operation of over 1 million artificial water reservoirs around the world [3,4]. As a result, artificial reservoirs have reached the level of 6% to 11% of the global area of standing water [3,5]. The development of the construction of artificial water reservoirs has increased the ability to periodically retain water in the catchment area of a given region [2,3,4,5,6,7]. In addition to collecting increasingly valuable water resources, the creation of artificial water reservoirs results from various socioeconomic needs, i.e., flood protection, energy, recreation, and drought mitigation needs [2,8,9,10]. Over 36,000 large dams have been identified worldwide, with the largest number (approximately 28%) located in Asia [11]. Over the years, there has been a gradual shift away from the construction of artificial water reservoirs in developed countries, with a simultaneous intensification of these types of activities in the regions of Africa, Asia, and South America [2,11]. This approach results primarily from a greater public awareness of the environmental impact, the promotion of projects that are at least environmentally neutral, the high costs of hydrotechnical investments, and national or international regulations. However, the complete abandonment of the development of a multifunctional reservoir structure is contrary to the needs of many countries in terms of the growing demand for water [2].

It is estimated that on a global scale, water reservoirs increase river water resources by over 700% (assuming that reservoirs with a storage from >5000 km³ to >10,000 km³ have a total storage capacity of 8400 km³, working at maximum efficiency, they could retain the equivalent of 20% of the global average annual runoff; assuming a volume of natural river water of 1200 km³, the value of 8400 km³ of damming means a 700% increase in water storage in the global river system) [4,12]. The role of artificial water reservoirs is extremely important due to the decreasing resources of water intended for consumption on a global scale (by 2030, the global demand for fresh water will exceed the supply by 40%) [13]. According to the Global Commission on the Economics of Water (GCEW) report Turning the Tide: A Call to Collective Action, we are dealing with a serious water crisis as a result of the excessive use of water resources, their ongoing pollution and climate change. Without implementing rational water management strategies consistent with environmental policies and sustainable development goals, water supplies around the world will be seriously threatened. It is therefore necessary to place emphasis on counteracting water shortages through a number of activities resulting in increasing the ability to periodically retain water in the catchment area of a given region, one of the basic forms of which is the construction of retention reservoirs [2,14,15]. An artificial water reservoir can effectively serve as a tool to counteract the negative effects of climate variability [16], but its operation must require the adoption of comprehensive and flexible operating and water management instructions, assuming adaptation to various atmospheric scenarios [2].

The impact of climate variability on artificial water reservoirs is reflected in the deterioration of physical parameters, such as an increase in surface water temperature or a change in stratification [1,16]. An example is the research conducted in the Lake Tahoe basin. The analysis of hydrometeorological data (1950–2007) in the Tahoe basin showed that the minimum and maximum air temperatures increased by 0.048 °C/year and 0.028 °C/year, respectively, and the duration of stratification increased by more than 3 weeks in the years 1968–2014. The consequences of this change for the sensitive ecosystem of Lake Tahoe are the exacerbation of water quality problems, i.e., reduced mixing, lower dissolved oxygen, release of biostimulating nitrogen and phosphorus, and increased water surface temperature [16]. Due to climate variability, they deteriorate biological properties (e.g., changes in the structure of phytoplankton) [14,17] and increase the risk of cyanobacterial blooms [18]. Climate variability also enhances processes leading to the eutrophication of water reservoirs [18,19]. For example, in studies on Elodea canadensis it was found that an increase in water temperature by 2–3 °C causes an increase in their biomass by 300–500% [20]. Moreover, in the studied American reservoirs it was proven that the average duration of algal blooms may increase from 16 to 23 days by 2050 and from 18 to 39 days by 2090 [21]. Most of these changes are associated with a reduction in water quality and threaten the reliable production of drinking water [1]. The reliable production of drinking water plays a fundamental role in the research and planning of water management in drinking water reservoirs. It is important to study reliability and variability metrics that aim to improve the system’s performance [22,23]. The performance criteria of water reservoirs refer to the probability of system failure (its reliability) and how serious the consequences of failure may be (its vulnerability) [22]. Often, the designs of constructed dams were made without taking into account the future increase in rainfall caused by climate variability [24]. By analyzing the vulnerability of reservoirs, the main exposure factors can be reduced and the necessary adaptability can be increased through vulnerability analysis [22,23].

Studies confirm the high sensitivity of the ecological status of reservoirs to land use in their catchment, with growth in built-up or agricultural areas significantly contributing to the increase in the concentrations of undesirable elements, which is confirmed by research on the Antoninek reservoir in Poland in an urbanized area (higher concentrations of Ca, Sr, Mn, Al and Ni, Zn) [25]. In turn, research conducted on lakes and artificial reservoirs in China showed that the greater the percentage of arable land in the catchment, the higher the concentration of nitrogen compounds in the reservoir waters [26]. In the case of small reservoirs located in ecologically sustainable and forested areas, such risks are lower [27]. Water quality measurements carried out in small retention reservoirs located in the Prudnik Forest District in Poland show that in terms of nitrates, nitrites, dissolved oxygen, COD, calcium, magnesium, and general hardness, the tested reservoirs belong to the first class of water quality, and in terms of ammonia and phosphate temperatures, the tested reservoirs belong to class II of water quality [27]. Internal sources of pollution of reservoirs may be accumulated bottom sediments releasing, among others, phosphorus in the pelagic zone [28] or the phenomenon of seasonal stratification causing the mobility of negatively interacting elements and deoxygenated bottom waters [29].

The article reviews the tasks of water management carried out on the Dobromierz drinking water reservoir on the Strzegomka River, located in southwestern Poland (Lower Silesian Voivodship). The catchment area of the Dobromierz reservoir is intensively used for agriculture, which increases the threat to the water quality in the reservoir and accelerates the phenomenon of eutrophication [30]. The sources of these pollutants include, among others, intensive animal production in the reservoir catchment area, insufficiently organized water and sewage management, and atmospheric precipitation [30,31]. Research was conducted on the water chemistry in the Strzegomka waters and in the reservoir itself [32]. The analysis of the reservoir’s water quality in the period 2000–2001 showed a seasonal dependence on changes in nitrate concentrations. The highest values occurred in spring and autumn rainfall. High nitrate concentrations also occurred during July’s torrential rainfall. The lowest values were recorded in winter during low flows and in summer during plant vegetation. The average nitrate concentrations downstream of the Dobromierz reservoir were over 37% lower than the values at the inflow, which proves the influence of the reservoir on reducing nitrate concentrations in the Strzegomka waters. Research on the concentration of total, nitrate, and ammonium nitrogen conducted by Dąbrowska in the waters of Strzegomka above the reservoir in the years 2000–2014 showed a decreasing tendency. In the case of total Kjeldahl nitrogen, total phosphorus, and phosphates, no statistically significant trend was observed. For the Dobromierz reservoir, the most important thing in proper water management is maintaining a flood reserve, which will allow for a reduction in the flood wave below the reservoir. Ensuring drinking water supplies for the Water Supply and Sewerage Plant in Świebodzice is of strategic importance. The operation of the reservoir is based on maintaining the flow values at the level necessary for its proper operation, i.e., supplying water to the municipal water intake in the amount of 0.17 m³/s and discharging the inviolable flow in the amount of 0.15 m³/s [33].

The article analyzed the available documentation, i.e., the water operation and management manual and the water law permit for the damming, retention, and economic use of water [33,34] in the aspect of analyzing the principles of water management at this facility, as well as long-term meteorological and hydrological data regarding this area (including water level, flow, temperature, and precipitation). The aim of the article is to answer the following questions: (1) How is water management carried out in the Dobromierz reservoir, taking into account long-term meteorological and hydrological data? (2) How does climate variability affect the parameters of long-term meteorological and hydrological data in the area of the Dobromierz drinking water reservoir? (3) What are the recommendations for water management in drinking water reservoirs?

In the face of decreasing water resources and ongoing climate variability, research on the water resources of artificial drinking water reservoirs seems important, but has not been thoroughly researched in the Central European region. The analysis of water management presented in this article, as well as the planned assessment of water quality and characteristics of water treatment technologies in artificial dam reservoirs, may fill this research gap.

2. Materials and Methods

2.1. Study Area

The Dobromierz water reservoir is located in southwestern Poland and was built in 1977–1986 by dividing the bed of the mountain river Strzegomka, which is a left-bank tributary of the Bystrzyca (tributary of the Oder). The reservoir with a capacity of 10 million m³ and an area of 103 ha (at normal damming level) is located in the northern part of the catchment, in the 62 km course of the Strzegomka River [33]. Administratively, it is located in the Dobromierz commune in the Lower Silesian Voivodship, on the border of the Bolkowskie and Wałbrzych Foothills. It is located in protected areas, i.e., in the Natura 2000 Dobromierz PLH020034 area (an area which is protected due to the presence of habitats dominated by Central European and subcontinental oak-hornbeam forest—40% coverage and less than 10% of the area is covered by lowland and mountain fresh meadows) and the Książański Landscape Park (a compact forest complex protecting natural values along with the diversity of flora and fauna this area and unique fossils of fossil fauna) [33]. The basic facilities forming the reservoir are an earth dam, discharge devices—2 bottom inlets and 2 water intakes, and a surface spillway. Its main function is to supply drinking water to the city of Świebodzice. The daily water production at the treatment plant is approximately 4500 m³/day [34]. Other functions of the Dobromierz reservoir are flow maintenance of the Strzegomka River (covering the minimum flow of 0.15 m³/s), flood protection of the areas below the dam, electricity production, and fish breeding [34].

The catchment area of the Dobromierz reservoir is located in the southern part of the Lower Silesian Voivodship, in the communes of Stare Bogaczowice (approx. 90%) and Dobromierz (approx. 10%). It covers an area of 80.7 km². Geographically, the catchment area is located in the Central Sudetes macroregion. The catchment area is located at an altitude of approximately 300 m above sea level in the north up to 600–700 m above sea level in the south, with a predominance of areas whose altitudes range from 400 to 500 m above sea level. The lowest point of the catchment area is located at the dam of the Dobromierz reservoir (approx. 283 m above sea level). The maximum elevation of the area is 495 m. The river network of the catchment area consists of the Strzegomka River and three tributaries (Chwaliszówka, Sikorka, and Czyżynka). The length of the Strzegomka River from its sources to the dam is 15.8 km. The average catchment decline is 5.2% [30,31]. The water chemistry of the Strzegomka River is highly variable. In particular, the concentration of nitrates varies under the influence of biogeochemical processes in the reservoir, the concentration of chlorides depends on the quality of atmospheric air, and oxidation is strongly influenced by the state of the Strzegomka waters. This is caused by the mountainous nature of the catchment area and the uncontrolled, irregular inflow of pollutants from area and point sources [31].

Semi-permeable formations predominate in the catchment area. Areas intensively used for agriculture dominate here. The structure of land use in the reservoir catchment area is as follows: agricultural land, including meadows and pastures—60%, forests—32% (current mixed forest and coniferous forest habitats), built-up areas—8% [31].

In the catchment area, the soil base is mainly composed of weathered sedimentary, igneous and metamorphic rocks. On their basis, brown, podzolized soils with a mechanical composition of medium and heavy silty clays were developed. These soils are characterized by a high humus content, high hydrolytic acidity, significant K content and low P content, a strongly acidic reaction below 4.5, a degree of saturation with basic cations below 20%, and a C/N ratio of approximately 20 [30].

The crop production of this area is dominated by winter wheat, spring, and winter barley and winter rapeseed. The consumption of nitrogen, phosphorus, and potassium fertilizers is high here and in 2020 it amounted to over 157 kg/ha. Intensive animal production takes place in the catchment area, mainly breeding laying hens. The poultry population exceeds 850,000. The nitrogen load in poultry manure produced in the catchment corresponds to the load produced with the excrement of 100,000 people, and the load of phosphorus corresponds to 400,000 people. The catchment area is largely covered by the sewage network, and in 2006 a sewage treatment plant was opened in Chwaliszów. The area is inhabited by approximately 3000 people [31].

The location of the Dobromierz water reservoir is shown in Figure 1. It also shows the measurement and observation network in the immediate vicinity, i.e., hydrological and meteorological stations. The characteristics of the hydrological and meteorological stations are presented in Section 2.2.

2.2. Materials Related to Water Management of the Dobromierz Reservoir and Measurement and Observation Data

Water management in the Dobromierz reservoir is specified in the water management manual [34] and the applicable water law permit for the damming, retention, and economic use of water [33]. These documents determine the operation of the reservoir in specific hydrological conditions. The Water Management Instruction (hereinafter referred to as the Instruction, 2002) specifies, among other things, data on the determined flows and outflows from the reservoir, the expenses of the discharge devices, and the basic parameters of the reservoir. For the efficient functioning of the reservoir, the manual distinguishes characteristic flows (their values are listed in

Table 1. List of flows important for the proper management of the Dobromierz reservoir.

Parameter Name	Q Limit Value (m3/s)
Reliable flow Q0.3%	≤64
Control flow Q0.5%	≤112
Inviolable flow Qin	≥0.15
Available (disposal) flow for the Municipal Service Plant in Dobromierz Qav	≥0.030
Available (disposal) flow for the Water Supply and Sewerage Plant in Świebodzice Qav	≥0.14
Allowed flow Qal	≤15
Flood flow Qf	≤25
Catastrophic flow Qc	≤180
Guaranteed flow Qg	≥0.15–0.17

):

• reliable Q_0.3% and control Q_0.5% (to maintain the safety of hydrotechnical structures);
• inviolable Q_in (providing aquatic organisms with conditions for existence);
• available Q_av (for drinking water supply);
• allowed Q_al (flow not causing flood damage);
• flood Q_f (adapted to the capacity of discharge devices; may cause flood damage);
• catastrophic Q_c (beyond the ability to control relief devices; causes catastrophic losses to property and threatens people’s life or health);
• guaranteed Q_g (the sum of the inviolable flow and the flow necessary to cover the water needs of various water recipients).

The definition of individual flows is included in legal acts, i.e., in the Regulation of the Minister of the Environment of 20 April 2007 on the technical conditions to be met by hydrotechnical structures, and in the Regulation of the Minister of Maritime Economy and Inland Navigation of 21 August 2019 on the scope of water management instructions [35,36].

The administrator of the reservoir is the Regional Water Management Board in Wrocław, the Catchment Board in Legnica (the part of the State Water Management “Wody Polskie”). Direct management is carried out by the reservoir manager based in Dobromierz [34]. Additional information regarding water intake from the reservoir for public supply purposes is included in the water law report for water services from 2022. The validity of the water service permit is set for 30 years. The Water Supply and Sewerage Plant LLC in Świebodzice is responsible for abstracting surface water from the reservoir [33].

To conduct proper water management in the reservoir, hydrological and meteorological measurements are necessary. There are currently three hydrological stations monitoring water levels and flows on the Strzegomka River, under the supervision of the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB), i.e., Chwaliszów at km 67.79 of the river, measured from the mouth (above the Dobromierz reservoir); Łażany at km 40.00; and Bogdaszowice at km 3.81 (both below the Dobromierz reservoir). In the past, there was also a hydrological station in Dobromierz below the water reservoir; however, current hydrometric measurements have not been performed there for over 30 years [37,38]. The article uses data from the Chwaliszów and Łażany stations, i.e., flows—2006–2022; water levels—1992–2022, interpolating the values based on the increase in the catchment area (areas read on the SCALGO Live platform) [39].

Table 2. List of hydrological stations on the Strzegomka River.

Hydrological Station	River	Station ID	Latitude (°N)	Longitude (°E)	Data Availability—Flows Q	Data Availability—Water Table H	Catchment Area—A (km2)
Chwaliszów	Strzegomka	150160030	50.87741728	16.23320126	2006–2022	1987–2022	66.67
Dobromierz (inlet) *	Strzegomka	-	50.90887948	16.24112438	2006–2022 *	1992–2022 *+	69.14
Dobromierz (outlet) *	Strzegomka	-	50.91034126	16.24394404	2006–2022 *	1992–2022 *+	80.56
Dobromierz	Strzegomka	150169995	50.91034126	16.24394404	1971–1980	1981–1992	80.87
Łażany	Strzegomka	150160090	50.95292776	16.49246334	1951–2022	1981–2022	365.01
Bogdaszowice	Strzegomka	151160180	51.08654103	16.79244022	2020–2022	2009–2022	557.62

Symbols in the table: *—values interpolated based on a comparison of the catchment growth in these points and in the Chwaliszów and Łażany stations; points that are not hydrological stations, ⁺—the analysis of data from 1987 to 1991 was omitted due to the probable change in the zero level of the water gauge at the Chwaliszów station (up to 10-fold differences in interpolated water levels compared to later years).

presents a list of hydrological stations on the Strzegomka River.

With regard to meteorological stations in Poland, there are three types: precipitation (measurement of atmospheric precipitation, the most common in the country), climatological (precipitation, temperature), and synoptic (precipitation, temperature, sunshine duration, other meteorological variables, the least common of this type of station in Poland) [40]. Due to the purpose of the article and the availability of data, it was decided to use data from the Szczawno-Zdrój station in relation to the sum of precipitation and average air temperatures (1956–2014), as well as from the Jelenia Góra station in the context of sunshine duration (1966–2022) [41].

Table 3. List of meteorological stations (climatological and synoptic) in the immediate vicinity of the Dobromierz reservoir on the Strzegomka River.

Meteorological Station	River	Station ID	Current Station Type	Latitude (°N)	Longitude (°E)	Data Availability
Szczawno-Zdrój	Strzegomka	250160130	climatological	50.80669316	16.2412491	1956–2014
Jelenia Góra	Bóbr	350150500	synoptic	50.90048974	15.7882868	1951–1965 c, 1966–2022 s
Śnieżka	Łomniczka	350150510	synoptic	50.73639952	15.7397161	1951–1965 c, 1966–2022 s

Symbols in the table: ^c—climatological data, ^s—synoptic data.

contains information about the meteorological stations in the immediate vicinity of the Dobromierz reservoir, excluding precipitation stations.

2.3. Data Analysis

The work carried out analyses characterizing the variability of the data. In this case, descriptive statistics were performed for hydrological and meteorological parameters (minimum, maximum, mean, median, standard deviation, and coefficient of variation).

The hydrological regime of the Strzegomka River at the outlet from the Dobromierz reservoir was also assessed by dividing the average monthly water flows by the annual average and classifying them according to the divisions according to Dębski (the number and period of floods during the year are taken into account) and Dynowska (the main criterion is the degree of variability of the average monthly flows per year) [42].

The non-parametric Mann–Whitney U test [43] was also performed for data with a non-normal distribution to compare the periods before, during, and after the construction of the Dobromierz water reservoir on the Strzegomka River (the test compares medians in individual data sets). For this purpose, data from the Łażany station below the Dobromierz reservoir were used, for which there are data from all three periods (26, 9, and 37 years, respectively).

Climate variability was estimated by checking the variability of rainfall, sunshine, and temperature over individual years. A practical way to present the first variable is the relative precipitation index RPI [44], which compares the sum of rainfall in a given year to the average annual sum of rainfall over many years—in this case, the classification is as follows: extremely dry year = <25%, very dry = 25–49%, dry = 50–74%, normal = 75–125%, humid = 126–150%, very humid = 151–200%, extremely humid = >200%. In other cases, the variability of parameters over the years at specific meteorological stations was analyzed. Additionally, a Spearman correlation matrix was prepared for non-normal distributions, in which hydrological (water levels, flows) and meteorological (precipitation, air temperature, sunshine) variables were compared in pairs.

In statistical analyses (creating tables and charts), Origin Pro 2022b, IBM SPSS Statistics 26, and Microsoft Excel 2021 were used. QGIS 3.10.0 was used to create maps.

3. Results and Discussion

3.1. Hydrological and Meteorological Conditions

Flows at the outlet from the Dobromierz reservoir in the years 2006–2022 varied from 0.04 m³/s (2018) to 46.60 m³/s (2006). The average value in this period was 0.60 m³/s. The years with the highest median were 2010 and 2013 (0.56 m³/s), and the year with the lowest was 2020 (0.17 m³/s). Analyzing the variability of the results, expressed by the CV coefficient of variation, it was high (above 30%) and ranged from 75.57% (2008) to 284.08% (2006). Higher flow values in Strzegomka (higher than the average annual flow over many years) are characteristic of the period from January to June with a maximum in March (165% compared to the average annual value over many years), which suggests the occurrence of dominant floods caused by the melting of snow and ice. This is confirmed by the division of the hydrological regime according to Dębski—for this section of the Strzegomka River it will be a complex rain–snow regime in the mountain variety, in which the maximum floods occur in March–April (thaws), and immediately after this period, rain recharge occurs, extending the flood period. According to Dynowska’s classification, it is a moderately developed nival river system, where the flows in the spring months are 130–180% of the average annual flow, while in the summer months there is no increase in the flow (>100% MAF) [42]. The variability of flows at the outlet from the Dobromierz reservoir on the Strzegomka River is presented in

Table 4. Descriptive statistics for flows (m³/s) on the Strzegomka River—outlet from the Dobromierz reservoir in the period 2006–2022.

Year	Min	Median	Max	Mean	SD	CV
2006	0.10	0.39	46.60	1.19	3.39	284.08%
2007	0.12	0.41	7.00	0.65	0.76	117.04%
2008	0.08	0.46	2.30	0.56	0.42	75.57%
2009	0.12	0.35	20.10	0.76	1.64	216.73%
2010	0.16	0.56	7.93	0.89	1.05	117.42%
2011	0.14	0.50	10.37	0.78	1.04	132.11%
2012	0.09	0.25	3.83	0.42	0.52	125.01%
2013	0.09	0.56	13.15	0.98	1.37	139.14%
2014	0.16	0.31	4.65	0.46	0.47	101.30%
2015	0.06	0.22	1.49	0.25	0.19	77.34%
2016	0.08	0.27	4.47	0.50	0.53	107.47%
2017	0.08	0.37	3.14	0.52	0.45	87.14%
2018	0.04	0.20	1.65	0.27	0.23	86.33%
2019	0.06	0.20	3.20	0.41	0.48	115.83%
2020	0.08	0.17	15.68	0.46	1.07	235.47%
2021	0.11	0.37	6.47	0.61	0.64	105.09%
2022	0.10	0.31	5.91	0.46	0.49	105.53%

and Figure 2.

In the case of water levels, this variability is not so clear; taking into account data from 31 years, they range from 88 cm (1993) to 338 cm (20 July 1997—the period of great flooding in the Oder basin, caused by intense rainfall in its upper sections), with an average of 125 cm. The CV variability index varied from 5.33% (2015) to 21.14% (1997), which indicates small deviations of daily values from the average in each year. The median value was the lowest in 1993 (101 cm), and the highest in 2003 (141 cm). With regard to the variability of the average monthly levels, a similar phenomenon occurs as in the case of flows; the highest values were recorded from January to April (maximum in March—134 cm), and the lowest from June to November (minimum in August—119 cm). These are not significant; however, there are large differences (of the order of 12.6%). In March, the higher levels were most likely caused by snowmelt, while the low levels in August were caused by low rainfall and high air temperatures. The shift in time compared to the flows may result from taking into account data from different research periods in which the variability was different. The described relationships are summarized in

Table 5. Descriptive statistics for water levels (cm) on the Strzegomka River—outlet from the Dobromierz reservoir in the period 1992–2022.

Year	Min	Median	Max	Mean	SD	CV
1992	93	106	177	110	14	12.35%
1993	88	101	160	103	10	9.29%
1994	93	106	158	109	11	10.18%
1995	95	105	142	106	8	7.38%
1996	90	110	175	113	14	12.55%
1997	84	112	338	115	24	21.14%
1998	104	119	165	120	11	8.83%
1999	105	117	185	121	13	10.93%
2000	108	115	173	119	10	8.63%
2001	111	117	216	122	13	10.73%
2002	108	124	306	130	20	15.15%
2003	126	141	193	141	12	8.66%
2004	123	133	217	137	14	9.88%
2005	112	130	242	134	16	11.64%
2006	110	131	314	135	21	15.33%
2007	125	136	189	140	12	8.27%
2008	122	138	166	137	10	7.30%
2009	122	134	261	138	16	11.90%
2010	124	138	195	140	12	8.62%
2011	123	136	210	138	12	8.81%
2012	118	127	190	130	11	8.55%
2013	116	133	217	135	15	10.80%
2014	112	122	172	123	8	6.31%
2015	109	117	143	117	6	5.33%
2016	110	118	171	122	10	8.19%
2017	108	121	159	123	9	7.42%
2018	109	118	145	119	7	6.06%
2019	110	119	168	123	11	8.69%
2020	112	118	239	123	12	9.88%
2021	112	123	181	127	11	8.45%
2022	111	121	183	124	9	7.18%

and Figure 3.

The Dobromierz reservoir is located in the transitional warm temperate climate zone, with four distinct seasons (spring, summer, autumn, winter) and the related variability of air temperatures and rainfall. The amplitude of air temperatures at the Szczawno-Zdrój station in the period 1956–2014 ranged from 41.2 °C (1977) to 59.4 °C (1956). The lowest temperature was −30.4 °C (1956), and the highest was 34.9 °C (1983). The average daily air temperature in the analyzed period was 7.6 °C and varied in the range from 5.9 °C (1990) to 9.8 °C (2006). The average annual rainfall in the analyzed period was 704.8 mm and ranged from 394.4 (1990) to 1040.2 mm (2002). The described characteristics are shown in Figure 4.

Referring to the monthly values (Figure 5), the mentioned seasonal variability is visible. Taking into account data from the period 1956–2014 for the Szczawno-Zdrój station, the highest average air temperatures are recorded from June to August (from 15.2 to 17.0), which corresponds to late spring and summer, and the lowest from December to February (i.e., from—1.7 to −0.4), which is equivalent to late autumn and winter. The maximum precipitation is observed in July and August (105.5 and 96.7 mm), and the minimum in January and February (33.3 and 33.1 mm).

3.2. Implementation of Water Management Assumptions on the Dobromierz Reservoir

To ensure proper water management, the Dobromierz reservoir was equipped with a hydrological protection network, for example, a telelimnigraph on the discharge tower and the outflow canal, a limnigraph in Chwaliszów, and a rainfall station in Lubomin and Gostków. The facility is guaranteed permanent operation by the management operating the discharge devices and has constant technical and conservation control. The reservoir administration is obliged to carry out ongoing, periodic inspections after and before the winter period, and commission inspections every 5 years or at the request of RZGW Wrocław. This results from the provisions of the Polish Construction Law [45]. The Dobromierz reservoir has a small hydropower plant with a capacity of 40 kW. There is no fish ladder [34].

According to the information contained in the Instruction (2002), the water resources stored in the Dobromierz reservoir are currently used by three users, i.e.,

• Supply and Sewerage Plant in Świebodzice for the purpose of supplying drinking water to the population of Świebodzice;
• Municipal Service Plant in Dobromierz for the water supply of the Dobromierz commune;
• Fishing district of the Polish Fishing Association in Legnica for the needs of the fishing economy.

Water management in the Dobromierz reservoir is divided into two periods, i.e., normal and flood. Water management in the reservoir during the normal period involves supplying water to the municipal water intake in the amount of 0.17 m³/s and discharging the inviolable flow in the amount of 0.15 m³/s. These tasks can be implemented by applying the principle of not lowering the damming level in the reservoir below the minimum damming level (283.50 m above sea level) and not exceeding the normal damming level (298.50 m above sea level) until flood protection measures are initiated. During the period with the highest probability of flood occurrence (from 15 June to 15 September), it is recommended to maintain the damming level at an elevation of no higher than 296.50 m above sea level. Water is discharged from the reservoir through a surface overflow and a bottom sluice. With a reliable flow, the overflow and discharge flows at the maximum damming level are 53.55 m³/s and 22.9 m³/s, respectively. With the control flow, the overflow and discharge flows at the maximum damming level are 53.96 m³/s and 72.5 m³/s, respectively. When operating discharge devices, it is important to ensure that a biological outflow of Q = 0.15 m³/s is provided below the reservoir. During normal operation, the water level in the reservoir is maintained between the normal damming level and the minimum level [34].

Flood management in the reservoir begins when the water level in the reservoir is equal to the normal damming level (298.50 m above sea level) and the inflow to the reservoir exceeds the outflow, i.e., 0.15 m³/s. Water management during this period involves supplying water to the municipal water intake in the amount of 0.17 m³/s and safely conducting the flood wave. During this period, the reservoir damming is in the zone between the normal damming level (298.50 m above sea level) and the maximum damming level (299.70 m above sea level). During flood operation, the water level in the reservoir is maintained between the normal damming level and the maximum level [34].

The most important factor in the proper water management of the Dobromierz reservoir is the existing flood reserve, which allows for effective reduction in the flood wave below the reservoir. Ensuring the supply of drinking water for the Świebodzice Water Supply and Sewerage Plant is of strategic importance, while the energy use of resources in a small hydroelectric power plant is of secondary importance.

Analyzing the water management in the reservoir in terms of maintaining characteristic flows (Q_0.3%, Q_0.5%, Q_in, Q_f, Q_c, and Q_av), the limit values were not exceeded on all days (

Table 6. List of flows important for the proper management of the Dobromierz reservoir along with determining the degree of their maintenance in the years 2006–2022.

Parameter Name	Q Limit Value (m3/s)	Meeting Standards (%)
Reliable flow Q0.3%	≤64	100.00
Control flow Q0.5%	≤112	100.00
Inviolable flow Qin	≥0.15	81.91
Available (disposal) flow for the Municipal Service Plant in Dobromierz Qav	≥0.030	100.00
Available (disposal) flow for the Water Supply and Sewerage Plant in Świebodzice Qav	≥0.14	84.36
Allowed flow Qal	≤15	99.92
Flood flow Qf	≤25	99.98
Catastrophic flow Qc	≤180	100.00
Guaranteed flow Qg	≥0.15–0.17	77.85–81.91

). This is due to the fact that in the analyzed period there were no extremely low or extremely high flows, and the reservoir maintained flow values necessary for its proper operation, as well as for basic nature protection and flood protection. The average compliance with the standards regarding flows in the reservoir was determined by comparing the limit values of flows with actual data at the outlet from the Dobromierz reservoir (daily data from the period 2006–2022). The lower guaranteed flow efficiency (from 77.85% to 81.91%) suggests a potential risk to water supply reliability and ecosystem health. This may affect the quality of life of residents using the waters of the Dobromierz reservoir, whose needs may not be fully met. Despite this, the average compliance with the standards was close to 93%; therefore, the water reservoir mostly functions properly in terms of water management.

The main problem in managing the Dobromierz reservoir is the lack of constant hydrological and water quality monitoring directly at the inflow and outflow from the reservoir, which would contribute to improving the use of water resources in the reservoir. Another problem is the agricultural nature of the catchment area, which causes surface runoff water with nutrients to enter the reservoir, which intensifies eutrophication processes. There is also no fish ladder, which limits the possibility of fish migration. Figure 6 presents a summary of information about the water management carried out in the Dobromierz water reservoir.

An example of another multi-functional reservoir intended to supply drinking water to the population is the Bukówka water reservoir. The Bukówka reservoir is located on the Bóbr River (left tributary of the Oder), in the Lubawka commune, Lower Silesian Voivodship. It was created in 1978–1989. The reservoir is one of the highest dam reservoirs in Poland above sea level (crown elevation 537.10 m above sea level). The basic parameters of the reservoir are a capacity of 12.01 million m³ and an area of 167 ha (at normal damming level). The reservoir serves as flood protection and energy and water supply (the water intake is currently unused). The Bukówka reservoir has a small hydroelectric power plant with a capacity of 0.109 MW and a fish pass [46,47].

In the Bukówka reservoir, water management is carried out based on four periods: normal, flood, drought, and ice phenomena. Water management of the reservoir during normal operation involves maintaining the water level in the reservoir between the minimum damming level (521.30 m above sea level) and the normal level (534.30 m above sea level). Water management in the reservoir during normal periods consists of supplying water to the water intake during low flow for the water intake in Marciszów in the amount of 0.56 m³/s and discharging the inviolable flow in the amount of 0.10 m³/s. The flood management in the reservoir involves supplying water to the municipal water intake in the amount of 0.56 m³/s and safely conducting the flood wave. During this period, the reservoir damming is in the zone between the normal damming level (534.30 m above sea level) and the maximum damming level (536.30 m above sea level). A drought period occurs when the degree of damming in the reservoir is less than or equal to the minimum damming level (521.30 m above sea level), and the inflow to the reservoir is lower than the flow (0.10 m³/s). However, during periods of ice cover, it is recommended to keep the reservoir damming at a normal level so that ice is not drawn into the spillway (dumping ice through the spillways is prohibited). Water is discharged in winter through the lower outlet. The problem in the water management in the Bukówka reservoir is the lack of constant hydrological and water quality monitoring in the tributaries and in the reservoir itself, which would contribute to the proper use of water in the reservoir. It is also at risk of deepening low pressure due to the lack of increase in flow in the Bóbr River. Another problem is the agricultural nature of the catchment area, which causes surface runoff water from fields laden with nutrients to enter the reservoir [46,47].

There are two more multi-functional drinking water reservoirs in Lower Silesia: Lubachów on the Bystrzyca River, in the Walim commune, and Sosnówka on the Czerwonka River, in the Podgórzyn commune.

The Lubachów reservoir was put into operation in 1917. The basic parameters of the reservoir are a capacity of 9.1 million m³ and an area of 51 ha (crown elevation 352 m above sea level). The reservoir serves, among other things, in flood control and energy (small hydroelectric power plant with a capacity of 1.2 MW) and water supply functions for Dzierżoniów, Pieszyce, Bielawy, and adjacent towns. There are recreation centers and summer houses in its catchment area. Additionally, the reservoir is used for fishing [48,49]. Below the Lubachów reservoir, at km 80.74 of the Bystrzyca River, there has been a hydrological station since 1975, which is part of the Polish measurement and observation network IMGW-PIB, where current measurements of water levels and flows are made.

According to research conducted by Rędowicz [48], the basic problem of the existing flood water management system through the reservoir is the measurement of the volume of water flowing into the reservoir by increasing the reservoir filling. This method often leads to incorrect decisions regarding the volume of water discharged. It may happen that the peak moment is not registered and more water is dropped from the reservoir than flows into it. The existing situation can be radically changed by installing devices for continuous measurement of the water level in the reservoir and in the cross-section of the Bystrzyca River at the inlet to the reservoir. It is also important to use data from observations of the water gauge in the Jugowice profile and rainfall stations in Walim, Lubachów, and Wałbrzych. In order to increase the safety of the dam itself and the towns located below, it is necessary to change the rules of water management during the passage of a flood wave through the reservoir, consisting mainly of earlier discharge of larger volumes of water and appropriate maneuvers closing the bottom outlets.

The Sosnówka reservoir was put into operation in 2002. The reservoir has a capacity of 10.9 million m³ and an area of 100 ha (at normal damming levels). It serves as flood protection and a water supply for the Jelenia Góra agglomeration. The Sosnówka reservoir is a relatively young reservoir, fed by waters from the Czerwona and Podgórzyn streams. In the Sosnówka reservoir, water management is carried out based on three periods: normal, flood, and during ice phenomena. The water management of the reservoir during normal operation involves maintaining the water level in the reservoir between the minimum damming level (364 m above Kr.) and the normal level (371 m above Kr.). Water management in the reservoir during the normal period includes, among other things, supplying water to the Jelenia Góra waterworks in the amount of 0.289 m³/s and discharging the inviolable flow below the reservoir in the amount of 0.048 m³/s. Flood management in the reservoir involves supplying water to the municipal water intake in the amount of 0.289 m³/s and safely conducting the flood wave. During this period, the reservoir’s damming is in the zone between the normal damming level (71 m above Kr.) and the maximum damming level (373.40 m above Kr.). During periods of ice cover, it is recommended to keep the reservoir impoundment at normal levels to prevent ice from being drawn into the spillway. Water is passed through bottom bleeds in winter. For the Sosnówka reservoir, there is no management to prepare a flood reserve due to the lack of inflow forecasts and information from the hydrometric network (no measurements of flows in the catchment area above the reservoir; since 2010, there has been a water gauge on the Sośniak River above the reservoir, where water levels are observed), information is provided from IMGW-PIB in the form of rainfall warnings and weather forecasts [50].

Another important issue in the case of the described drinking water reservoirs is the functions expected at the design and construction stage in relation to the functions performed during operation. In the case of the eutrophication process that takes place in these reservoirs, the planned functions cannot be performed properly. This is particularly important in the case of water retention reservoirs for water supply purposes. In such a case, sewage management should be fully organized in the reservoir catchment area—but it is not, despite the investments in the construction of sewage treatment plants [51]. In this case, the solution may be other technologies that remove pollutants, including chemical, physical, and biological [52,53].

Another important reservoir from the point of view of water supply is the Goczalkowice reservoir. The reservoir is located at the foot of the Silesian Beskids and is the basic reservoir of drinking water for the Silesian agglomeration (the most industrialized region in Poland). It was built in 1955 on the Mała Wisła River. The basic functions of the reservoir are water supply to the Upper Silesian agglomeration, flood protection of the Vistula River valley, maintenance of low-flow flows during drought, fisheries management, and nature protection. Water supply is the basic task of the Goczalkowice reservoir. The Goczalkowice Water Treatment Plant collects water from the reservoir and supplies it to the group water supply of Górnośląskie Przedsiębiorstwo Wodociągów S.A. (Upper Silesian Water Supply Company Joint-stock Company). The company supplies water to 66 communes of the Silesian Voivodship and 3 communes of the Lesser Poland Voivodship. In total, approximately 3.4 million inhabitants use the water supply system (including 45% of them using water from the Goczalkowice reservoir) [54,55].

The catchment area of the reservoir with an area of 523.1 km² includes mountain and foothill areas characterized by sudden, intense floods as a result of heavy rainfall on the slopes of the Silesian Beskids. Therefore, an important issue in water management in the reservoir is to make discharges at the beginning of the flood and to maintain a large flood reserve in the reservoir throughout the year. The only way to conduct rational water management on the Goczalkowice reservoir during flood season is to work according to the so-called “rigid” water management instructions. Due to the location of the Goczalkowice Reservoir, it is not possible to clearly determine how big the flood is until the end of the flood. Therefore, the algorithm for managing water in the reservoir is identical for each flood, hence the name “rigid” water management instruction. Water management in the reservoir during normal periods involves supplying water to the water intake in the amount of 1.5 to 4.0 m³/s and discharging the inviolable flow in the amount of 1.0 m³/s. Flood protection measures in the reservoir include water management within the permanent flood reserve, located between the normal damming level (255.50 m above sea level) and the maximum damming level (257.00 m above sea level). The flood protection period begins when the reservoir’s useful capacity is filled (elevation 255.50 m above sea level) or when the inflow to the reservoir exceeds 55 m³/s and lasts until the flood reserve is completely restored [54,55].

The multi-purpose function of the Goczalkowice reservoir necessitates the need to reconcile conflicting interests. In connection with water supply, it is important to maintain a high level of damming to ensure the appropriate quality and quantity of water abstracted. For flood protection purposes, it is beneficial to maintain a significant flood reserve, i.e., the lowest possible damming level. Taking into account the need to compensate for low-flow outflows, a reserve of water should be accumulated in order to have it available during periods of drought. Water management in the reservoir requires periodic changes in the damming level depending on current needs, up to several meters high. This is an unfavorable action from the point of view of fisheries management and nature conservation, where it is important to maintain stable conditions for protected natural habitats and species. Water management in the Goczalkowice reservoir therefore requires compromise and the reconciliation of often divergent requirements to ensure the current drinking water needs of the Silesian Agglomeration, as well as the protection of bird habitats and spawning grounds of predatory fish and flood protection [54,55].

Compromise and the reconciliation of divergent interests also occur in water management at the Vranov reservoir in the Czech Republic. It should be emphasized that in the Czech Republic, approximately 50% of the total water demand of public water supply systems is covered by water reservoirs [56]. One of them is the Vranov reservoir located on the Dyje River. Vranov was built in 1930–1934. The basic parameters of the reservoir are a capacity of 132.69 million m³ and an area of 765 ha. The reservoir serves as flood protection, energy, water supply (water intake on a floating pontoon), balancing flows in the watercourse below the reservoir, and for recreational purposes. The reservoir is located in an area with irregular summer flows and large spring floods. Water management in the reservoir involves mitigating numerous spring floods on the Dyje River, caused by melting snow and its subsequent surface runoff throughout the year. The energy function is also important. An important aspect of energy production at the Vranov reservoir is the connection built with the Dukovany nuclear power plant in order to provide power to the pumps of the fourth reactor block of the power plant in the event of a power failure. Due to the construction of a water supply network in 1982 in the towns surrounding the reservoir, it later acquired the function of water supply. This function became so important that a decree was issued stating the need to slow down the decline in the water level to ensure the operation of the raw water pumping station and water treatment plant in Štítary. As a result, the energy function of the Vranov reservoir was limited only during periods of extreme drought, and the water supply function was prioritized over the energy function [56,57].

Problems in water management in the reservoir arise during extreme hydrological events, during which the reservoir cannot perform all functions at the same time with normal water levels and outflows. The most important thing is to maintain a minimum level of damming to ensure the water level to supply the water treatment plant in Štítary. Another problem for water management in the reservoir is the large number of recreational facilities. In this way, a significant load of nitrogen and phosphorus is discharged into the reservoir along with the sewage. This promotes eutrophication and subsequent problems with raw water quality [57].

A noticeable problem in the management of drinking water reservoirs is the lack of advanced monitoring of information from the hydrometric network directly on the inflow and outflow of the reservoir, which would facilitate water management in these facilities. An example of a reservoir equipped with modern hydrometric monitoring is the Rappbode reservoir, the largest drinking water reservoir in Germany, put into operation in 1959. The Rappbode system consists of a network of six reservoirs, but the core of the system is the Rappbode reservoir. This water reservoir has a capacity of over 113 million m³ and an area of 395.3 ha. It provides drinking water to over a million inhabitants of central and eastern Germany. The Rappbode Reservoir is supplied directly with water from the Hassel and Rappbode tributaries, where pre-reservoirs have been built to capture sediment and remove nutrients. The water supplied by these two tributaries was not sufficient to meet the drinking water demand, and a water transmission gallery was built from the Königshütte reservoir, from which the water could flow by gravity to the Rappbode reservoir. In addition to transmitting raw water, this gallery also enables more efficient use of the large capacity of the Rappbode reservoir for flood protection purposes. The Königshütte reservoir receives its water from two rivers, Warme Bode and Kalte Bode, which coalesce a few meters above the reservoir’s inlet. Flood defenses in front of the Königshütte reservoir are integrated with the Kalte Bode reservoir, which is usually kept almost empty to maximize flood protection potential. The outlets of the Königshütte and Rappbode reservoirs are discharged into the Wendefurth reservoir located in the lower part of the entire system [58,59].

Rappbode reservoir’s monitoring consists of a suite of online sensors for measuring physical, chemical, and biological parameters, complemented by a biweekly limnological sampling schedule. Measuring stations are located on the four main inflows to the system, at the outlets of all feeder reservoirs and in the main reservoir. The newly installed monitoring system is used both for scientific monitoring and process research, as well as for water management. Particular emphasis is placed on monitoring short-term dynamics. The installed meteorological station at the Rappode reservoir provides the required meteorological data on site (wind, radiation, humidity, temperature, etc.), and the inflow stations provide information on the quantity and quality of incoming water. Additionally, measurement stations in the feeder and main reservoirs provide high-frequency data on the stratification and ecosystem dynamics that can be used to calibrate and validate the model. Such a modern monitoring system of the Rappbode reservoir is a key research material for the application and development of model lake studies [58].

3.3. Adaptation Strategies to Improve Water Management in the Dobromierz Reservoir

Adaptation activities aimed at improving water management in the Dobromierz reservoir should be carried out based on the hydrological and hydrochemical monitoring of water. In the current water management of the reservoir, periodic monitoring is carried out (once a quarter), covering the station at the drinking water intake. In the years 1981–1992, additional hydrological monitoring was carried out below the reservoir through readings from the installed water gauge staff. A desirable adaptive action in conducting hydrological and hydrochemical monitoring in the Dobromierz reservoir would be the use of automatic multi-parameter probes using teletransmission (GPRS) and positioning (GPS) systems. These devices enable immediate access to the collected results and remote measurement configuration. The location of probes at characteristic points of the reservoir provides information about the quality of incoming and outgoing water. The results from appropriately placed remote probes make it possible to perform ongoing balancing of the nutrient load between the inlet and outlet of the reservoir water based on the observed concentration differences [60,61,62]. The use of such a solution in the Dobromierz reservoir would allow for a more accurate representation of hydrochemical and hydrological phenomena in the reservoir models and would provide a chance to detect momentary and quickly changing phenomena such as chemical incidents and failures. The challenge in this regard is the financial resources necessary to purchase these devices. The cost of purchasing one YSI EXO2 multi-parameter probe with accessories is approximately USD 33,500 [63]. However, compared to the costs of building the reservoir, this is not a large expense, but it allows for improvements in water management in dam reservoirs. For example, the cost of building the last Czorsztyn reservoir in Poland (total capacity 234.5 million m³) together with the equalizing reservoir amounted to over USD 1 billion [64]. The challenge for the people managing the Dobromierz reservoir will be to obtain funds for the purchase of probes. This may be facilitated by grant or loan programs from EU or national ecological funds [65].

Adaptation activities aimed at improving the water quality of the Dobromierz reservoir should focus on reducing area pollution of agricultural origin. The solution in this regard is to educate farmers on the use of good agricultural practices in the field of organic fertilizers and to introduce solutions that limit surface runoff into the reservoir from areas with excessive concentration, including by creating buffer zones along the stream, which is the main connection between agricultural areas and the reservoir [31].

In the absence of a fish pass on the Dobromierz reservoir, it is difficult to specify appropriate adaptation measures. Fish ladders are an element of hydrotechnical structures, ensuring the possibility of migration of aquatic organisms, which should already be planned at the design stage of the dam reservoir [66]. In the case of a water reservoir with a small hydropower plant, fish passes are designed at the small hydropower plant (in light of the applicable Water Law implementing the assumptions of the Water Framework Directive, this is a mandatory element for new hydropower investments [67]). With an existing hydropower plant, building a fish pass is a huge technical and financial challenge. In particular, in the case of the Dobromierz reservoir, where there are area restrictions for the construction of this type of solution. This requires an in-depth technical analysis with experts in the field of hydrotechnical structures. According to researchers, another solution may be, for example, the use of artificial stocking at separate positions in the river [68,69,70].

3.4. Assessment of the Impact of the Construction of the Dobromierz Reservoir on Flows in the Strzegomka River

The analysis carried out using the Mann–Whitney U test before, during, and after the construction of the Dobromierz water reservoir (respectively, 1951–1976, 1977–1985, 1986–2022) on flows in the Strzegomka River at the Łażany station showed that between each of the analyzed pairs periods, statistical significance was recorded at p < 0.01 (

Table 7. Summary of the results of the non-parametric Mann–Whitney U rank test for pairs of periods before, during, and after the construction of the Dobromierz reservoir (results for the Łażany station, Strzegomka River).

Compared Periods of Construction of the Dobromierz Reservoir	Median (m3/s)	Mean Rank	U	Z	p
Before and during	1.52/1.83	6096.98/7246.34	1.28017 × 107	−15.39083	<0.01
During and after	1.83/1.58	9326.19/8165.97	2.5214 × 107	12.19467	<0.01
Before and after	1.52/1.58	11,087.2/11,800.3	6.0194 × 107	−8.01717	<0.01

). Delving into the results in more detail, however, it can be seen that the difference between the medians was the largest between the period before and during the construction of the reservoir and amounted to 20.4%. During and after construction it was 15.8%, and before and after construction it was 3.9%. The highest amplitude of daily flow values was recorded after the construction of the water reservoir, in the range from 0.55 to 114 m³/s, and the smallest during its construction (0.55–66.8 m³/s). The median and average values were the highest during the construction of the reservoir, the lowest average was found after construction, and the lowest median was found before the construction of the reservoir.

Observing the variability of hydrographs in individual years (Figure 7), it is visible that before the construction of the reservoir, flow fluctuations were greater than during and after its construction—this is reflected in the coefficient of variation CV and in the interquartile range (Q3-Q1), i.e., before reservoir construction: 1.43 and 1.38; during construction: 1.36 and 1.2; after construction: 1.16 and 1.22 m³/s. This is due to the fact that after the construction of water reservoirs, in order to conduct rational water management the variability of flows becomes flattened on an annual basis and increases on a daily basis (specific hydrological parameters of the reservoir must be maintained—Table 2 and Table 3). This is confirmed by numerous works of scientists who have studied the phenomenon of changes in flow fluctuations after the construction of water reservoirs (so-called hydropeaking). Based on research conducted in 1920–2019 on rivers in the USA and Canada (Mississippi, St. Lawrence, Mackenzie, Ohio, Columbia), it has been shown that the activation of water reservoirs as a result of damming rivers for various functions affects changes in weekly flows (lower flows on weekends, higher on weekdays, which reflects the demand for water). Depending on the function performed by the water reservoir, the intensity of this phenomenon varies—it is most intense in the case of the hydropower function, and less intense, e.g., for reservoirs supplying drinking water [71]. Moreover, as noted by Tian et al. [72], the construction of water reservoirs (in addition to changes in land use) causes changes in the hydrological regime of the river below them—in particular, the amplitude of flows on an annual scale is flattened, as well as the size, duration, frequency, time, and pace of changes in daily flow (Wuding River in China, flows from 1960 to 2016). A general decrease in average low annual flows was also noted after the construction of dams in selected rivers of the Iberian Peninsula in Spain, and it was also noticed, among other things, that there was a decrease in average monthly flows in the wet season in dry river catchments and an increase in these values in the dry season in wetter catchments [73]. The scale of hydropeaking depends on the water management in the reservoirs, which was proven by the analysis of hourly flows in regulated rivers in the Nordic countries (Norway, Finland, Sweden), where smaller hydropeaking was visible in the winter months, with greater demand for water, and greater hydropeaking in the summer months and in autumn when the demand is higher [74]. A comprehensive review of the literature based on the results of over 30 studies from various countries around the world [75] showed that small water reservoirs reduce the average annual flow in the rivers below by an average of 13.4 ± 8%, and the severity of these changes depends on the hydrological conditions and climatic and other impacts in dry and wet years (and thus from climate variability).

Analyzing the average monthly flows in these three periods (Figure 8), the most different is the one occurring in the years of construction of the Dobromierz reservoir. The biggest difference concerns the course of average flows in February, June and August—in the first and second cases it is clearly lower, and in the third it is much higher than in the other two periods (dividing by the average annual value in these periods, it is as follows: before construction reservoir—1.13, 1.10, and 0.82; during construction—0.93, 0.80, and 1.54; after construction—1.12, 1.02, and 0.93). The hydrological regime itself in Dębski’s classification has not changed, i.e., combined snow and rain, mountain variety (floods caused by snowmelt in March–April and later extended by rainfall in the following months), but a change in the flood months during the construction of the reservoir is visible, i.e., from March to May, and then in July and August, and in other cases from February to July. According to Dynowska’s typology, the regime has changed—it is nival–pluvial before and after the construction of the reservoir (dominance of thaw floods), and pluvial–nival during construction (dominance of rainfall floods). In all cases, the average flow of the spring month is 130–180% of the average annual flow (April: before construction = 144.0%, during construction = 129.3%, after construction = 132.4%), but the difference is that in the summer months this flow was clearly lower before and after the construction of the reservoir compared to the spring months (112.4 and 118.7% in July, respectively), and during the construction of the reservoir it was higher than in spring (154.2% in August). In this case, however, the reason for such an increase in the flow (even to the value of 66.8 m³/s, recorded on 3 August 1977) was most likely the high rainfall in that region, and not the process of building the water reservoir itself (160.5 mm in three days: 31, 1 July, and 2 August 1977, which constituted 18.72% of annual precipitation at the Szczawno-Zdrój meteorological station). Moreover, the analysis for this period covers 9 years and such episodes of variable outliers will have a greater impact than in the other two periods, which cover longer periods.

3.5. The Impact of Climate Variability on Water Management

In the described region, climate variability is visible in the context of increasing sunshine duration over the years (Figure 9a)—for the Jelenia Góra station in the period 1966–2022, this value increased from 1230.8–1566.6 h in the years 1966–1970 to 1956.2–2226.5 h in the period 2020–2022 (change according to the trend line at R² = 0.7708; average sunshine duration at 1654.24 h). In the case of moisture classification based on RPI at the Szczawno-Zdrój station in the period 1956–2014 (Figure 9b), in most cases (84.74%, 50 years) there were normal years, with precipitation in the range of 75–125%. There were four dry years (1962, 1982, 1983, 1990) and five wet years (1958, 1974, 1997, 2001, 2002). Over the years, there has been some fluctuation in the variability of rainfall, expressed in RPI; however, at the end of the analyzed period (1995–2014) there were more wet years, and this period was preceded by drier years (1980–1994). The correlation analysis carried out between the hydrological and meteorological parameters in the years 2006–2022 for p < 0.05 (Figure 9c) suggests that in most cases there was statistical significance between them (the only exception: temperature/precipitation). Strong correlations (|R| ≥ 0.7) occurred in the following pairs: flow/sunshine duration (−0.83), flow/water level (0.79), flow/precipitation (0.75), sunshine duration /precipitation (−0.71), and flow/temperature (−0.70). A moderate correlation (0.4 ≤ |R| < 0.7) was recorded in the remaining statistically significant pairs, i.e., water level/ sunshine duration (−0.69), temperature/sunshine duration (0.61), water level/precipitation (0.59), and water level/temperature (−0.49). Especially with respect to highly correlated parameters, their directions seem intuitive, e.g., an increase in sunny hours will tend to result in lower flows in the river, or with an increase in rainfall totals the sunshine duration will usually decrease. The statistical analysis carried out proves that climate variability, characterized by various variables, may affect the water management in the Dobromierz reservoir (e.g., maintaining appropriate levels of damming or required flows).

Climate change scenarios under the Coupled Model Intercomparison Project Phase 6 (CMIP6) for the Lower Silesian Voivodship (southwestern Poland), within which the Dobromierz reservoir is located, suggest changes in the values of meteorological parameters and their variability. For example, in the year 2100 compared to the reference years 1950–2014, the median average annual air temperature will increase from 0.62 to 5.07 °C, depending on the scenario. The number of hot days (Tmax > 30 °C) will increase from 49.6% (SSP1–2.6) to 1100% (SSP5–8.5), and in the period 1950–2014 this value was equal to 4.74 days. The forecast length of the growing season in this region of Poland will also increase—from 239.83 days in the reference period to 252.56–327.09 days in 2100. At the same time, uninterrupted dry periods will also last longer in 2100 compared to 1950–2014—by up to 2.4 days (SSP5–8.5) [76]. It should be mentioned that no more detailed analyses have been carried out for the region in question and an attempt could be made to fill this gap in the future.

The observed climate variability for the Dobromierz reservoir in the form of increased sunshine duration will in the future result in scenarios such as increased water temperature and increased evaporation from the water surface. As researchers emphasize, increasing water temperature results in an increase in the frequency of algal and cyanobacterial blooms. It also stimulates the growth of the biomass of bacteria and vascular plants in the reservoir, which translates into an increase in the amount of bottom sediments and accelerates the settling of the reservoir [1,19,31,77].

Increasing periods with a large number of sunny days will favor the occurrence of droughts in the reservoir catchment region, which increase the demand for water for agriculture, which may additionally result in a decrease in the water level in the reservoir and problems in maintaining appropriate levels of damming or required flows [77].

High air temperature will encourage people to use the reservoir for recreational purposes despite the swimming ban. This will translate into contamination of the reservoir with active substances from sun filters, other cosmetics, and biogenic substances [77].

The occurrence of humid years with a tendency for heavy rains increases the pressure exerted on the reservoir by changing the physicochemical and biological conditions of the reservoir’s waters. This translates into higher concentrations of dissolved and biogenic substances in the water. Heavy rains cause increased surface runoff and the washing out of pollutants from the catchment area (arable fields—nutrients) [31].

The issue of the impact of climate variability on the water management of water reservoirs has been studied in various regions of the world. Symptoms of climate variability include changes in evaporation dynamics, rising air temperatures, an increased frequency and intensity of hydrological extreme phenomena (floods, droughts), melting snow and ice, and rising sea levels [78,79]. In the context of drinking water reservoirs, climate variability is one of the causes of variability in the dynamics of water demand [80]. Studies conducted on water reservoirs in Turkey proved that seven out of eight climate variability scenarios showed insufficient water levels in water reservoirs, especially during periods of increased water demand; the importance of sustainable water management in the era of climate variability and lower water availability in the Mediterranean climate zone was also emphasized [81]. In higher places, where the increase in air temperatures will result in the melting of snow and ice, an increase in water resources is expected—e.g., in South Asia (upper course of the Indus River, Pakistan) or in Scandinavia; in these regions, the greater importance of water management in reservoirs should focus on flood protection [82]. Yasarer and Sturm [83] also emphasize that climate change will make it difficult to conduct proper water management of water reservoirs as a result of a reduction in their volume caused by an increased inflow of sediments and nutrients from nearby watercourses, a change in the flow regime, and an increase in water temperatures during the summer (which may intensify the phenomenon eutrophication, which worsens water quality) [84]. In drinking water reservoirs, one way to reduce the impact of climate variability is, among others, to enact dynamic control of the amounts of discharged water, as well as adjustment of the volume of retained water, adapted to the natural regimes of the rivers below water reservoirs [1]. Water management needs should be adapted to local conditions, resulting, for example, from the climate zone, and strategies for coping with climate variability, adaptations to these changes, and the development of recommendations, documentation, and policies at various levels of management should be adapted to them [85,86].

4. Conclusions

4.1. Key Findings

The analysis of water management of the Dobromierz drinking water reservoir showed that in most cases it is carried out in accordance with the applicable documentation and water is provided for other water users (Water Supply and Sewerage Plant in Świebodzice, Municipal Services Plant, fishing district of the Polish Fishing Association in Legnica) and for aquatic organisms (average compliance with standards of approximately 93%), which is reflected in the characteristic flows, damming levels, and volumes of water in the reservoir. Flow fluctuations were greater before the construction of the reservoir than after it was put into operation—the coefficient of variation and the interquartile range decreased from 1.43 and 1.38 to 1.16 and 1.22 m³/s, respectively. This resulted from the conducted water management, which assumed the maintenance of appropriate characteristic flows.

Moreover, the correlation matrix between the hydrological and meteorological parameters (e.g., maintaining appropriate levels of damming or required flows) in the described region showed the occurrence of strong or moderate correlations. This means that climate variability characterized by these parameters may affect water management in the Dobromierz reservoir (e.g., maintaining appropriate damming levels or required flows).

4.2. Implications

The main problems in the proper operation of the Dobromierz reservoir were a lack of permanent hydrological and water quality monitoring in the inflow and outflow from the reservoir, lack of a fish pass enabling the free migration of aquatic organisms, and unsatisfactory water quality caused by agricultural pollutants flowing from neighboring areas (in the Strzegomka River catchment).

Climate variability in the region will affect the reservoir, especially in the context of the need to adapt its functioning to more frequent phenomena of heat and heavy rains and the inflow of nutrients from surface runoff and algae blooms, which deteriorates water quality. Changes in thermal conditions in this region are also confirmed by climate models in various scenarios, e.g., an over-11-fold increase in the number of hot days (T_max > 30 °C) in 2100 compared to 1950–2014. The issue of the impact of climate variability on water management is also confirmed by research on other drinking water reservoirs, e.g., in Turkey (decrease in water levels in most cases) or in Pakistan (increase in available water resources as a result of melting snow in the mountains).

4.3. Recommendations

The solution to the above-mentioned water management problems of the Dobromierz reservoir would be to install devices monitoring the quantity and quality of water in the inflow and outflow from the reservoir (automatic multi-parameter probes), install a fish pass or artificial stocking of river sections, and regulate the issue of the use of arable land (legislative solutions, farmer education, instruments economic incentives to switch to sustainable agriculture). Water management itself should be carried out in accordance with technical documentation specifying social, economic, and environmental issues related to the operation of the reservoir, thanks to which it can perform its intended functions. This is consistent with both national law (e.g., Water Law), international law (e.g., Water Framework Directive, Birds and Habitats Directive), and local regulations (e.g., agreements and contracts with water users).

Water management in the Dobromierz reservoir should be adapted to local conditions, resulting, for example, from the climatic zone, socio-economic needs, or the location in naturally valuable areas. Strategies for dealing with climate variability, adaptation to these changes, and the development of recommendations, documentation, and policies at various levels of management should be tailored to them. Rational water management in this type of facility is consistent with, among other things, the sustainable development goals, the Water Framework Directive, and the Birds and Habitats Directive, and it will become increasingly important in the face of ongoing climate variability. Moreover, ensuring the appropriate quality of drinking water is consistent with Goal 6 of the 2030 Agenda for Sustainable Development, i.e., clean water and sanitation (“ensure availability and sustainable management of water and sanitation for all”).

4.4. Significance and Future Research Directions

The presented results show the specificity of water management in a dam reservoir, the dominant function of which is water supply. The conducted data analysis provides valuable information about water management problems in this facility, including adaptation strategies to improve water management. The presented analysis of climate variability scenarios indicates special attention to the quality of water in the reservoir and prudent management of water resources.

In the future, it is planned to publish further articles based on the analysis of water quality in the Dobromierz reservoir and the description of its treatment technology, including indicators for assessing the efficiency of water reservoirs (e.g., vulnerability and reliability index).

In the future, research could focus on water reservoirs with other functions, hydrographic, hydrological, and meteorological characteristics, or a different origin (e.g., natural water reservoirs), located in other climatic zones. The analyses could cover more detailed climate variability scenarios provided by, among others, the IPCC, and policies related to water management could be analyzed in a broader context—Poland, Europe, or the world.