Effect of Hydraulic Structure on Mitigating Extreme Hydrological Conditions of a Small River in the Temperate Zone (Główna River, Central Europe)

Abstract

Water resources are of elementary economic and environmental importance, and the observed global transformations as well as regional environmental conditions necessitate activities aimed at providing an optimal amount of water at different levels. One such solution is hydrotechnical infrastructure that permits the precise control of the amount of water in the catchment. This paper presents results concerning changes in the water flow in the Główna River in Poland before (1955–1983) and after (1984–2021) the construction of the Kowalski Reservoir. In the former period, there were no changes in water flow, and the obtained results were not statistically significant (p 0.05). In the period after the construction of the reservoir, a decreasing trend in monthly flow was observed in December, April, July, September, and October. Moreover, a decrease in 1, 3, 7, and 90 day maximum flow was observed. For maximum 30 day flow, the changes were below the threshold of the adopted significance level. Moreover, minimum flow in the period after the construction of the reservoir showed no significant decreasing trend. In hydrological terms, the reservoir served its purpose by contributing to the stabilisation of the water flow. This information is important from the point of view of an increase in retention and corresponds with a broader programme conducted in the territory of Poland. In the context of the construction of further reservoirs, it is important to investigate the current range of changes in water circulation for objects of the type already functioning in the environment, constituting an actual point of reference.

1. Introduction

As a result of the observed climate change, water resources are gaining increasing importance. They shape both the natural environment and human activity. It should be emphasised that the deficit and availability of water are among the most serious global challenges [1]. The provision of an optimal amount of water, and therefore a reduction in its excess and supplementation in periods of deficits, is the task of water management authorities that face increasingly serious challenges in ongoing dynamic civilisation development and climate change. It is important in the context of areas such as hydropower production [2,3]. Hydrology simulations at the catchment scale show that, in most cases, models forecast considerable changes in river flow [4].

Detailed threats to the guarantee of optimal amounts of water based on the case study of Holland are listed by de Wit et al. [5]. They include climate change, extreme weather conditions, economic growth, urbanisation, soil subsidence, and excess food production. In the context of the water balance, viable possibilities in the scope of increasing water availability are offered in reference to outflow regulation through an increase in retention. These objectives are implemented with the application of natural forms of land use [6] as well as technical solutions (dams, weirs) and reclamation of abandoned open pit mines to water reservoirs, artificial mine lakes, or quarry lakes [7,8,9,10]. The scale of activity primarily depends on the natural features of the environment and the economic needs of a given region. For example, high retention capacity is characteristic of catchments with natural lakes [11] that then show more uniform river outflow on an annual scale [12]. A lack of lakes or small amounts of water accumulated in lakes necessitate human activity aimed at increasing the retention capacity of the catchment. A key solution in the scope is the construction of water reservoirs that enable easy access to water resources and permit control of them. Such a situation is well illustrated by the territory of Poland, where the division into the northern (area of lakelands related to Scandinavian glaciation) and southern parts (outside of the range of the last ice sheet, lack of natural lakes, numerous dam reservoirs) is evident. According to the calculations of the Main Statistical Office [13], Poland is classified as a country poor in water resources, averaging approximately 60 bn m³ and in dry periods even below 40 bn m³. Therefore, activity is conducted that is aimed at a changing the unfavourable components of the water balance, primarily through an Increase in retention [14,15,16]. The determination of the hydrological conditions provides the basis for planning the implementation of hydrotechnical infrastructure [17]. Moreover, in the context of the construction of further reservoirs, it is important to investigate the current range of changes in water circulation for objects of the type already functioning in the environment. According to Lucas-Borja et al. [18], in some cases, the objectives of the construction of reservoirs were not met due to, among other factors, their low structure quality, unsuitable location, or lack of appropriate design criteria. Important to water management authorities is the ability to forecast water availability [19] based on knowledge of the geological conditions of the catchment area [20].

The result of hydrological processes characterising a given region is water flow that offers clear information on the volume of water resources. This parameter provides the basis for further hydrological analyses—such as the determination of the volume of water outflow. As emphasised by Muelchi et al. [21], understanding the transformations of overland flow is of key importance for the economy, including, among other sectors, agriculture, fisheries, hydropower production, and tourism. Dams have an effect on changes in the time, volume, and frequency of high and low water flows, affecting the hydrology of rivers [22]. Changes in the hydrological regime, however, are difficult to assess objectively, due to, among other reasons, the possibility of referring to the original hydrological background [23].

The objective of this study is to show the impact of the Lake Kowalskie dam reservoir on the flow regime of the Główna River. The analysis is of key scientific importance, as it allows for a better understanding of the river’s regime alteration as a result of the construction and operation of the dam reservoir. Each study of this type is unique due to the interactions between the dam reservoir and the river. It also results from the capacity of the dam reservoir and the adopted principles of water management, on the one hand, and on the other hand, from the regime and magnitude of the flow of the river that supplies the reservoir. Moreover, due to the overlapping impact of climate change on the regime of the Główna River, the study also shows the response of the adjacent river (the Cybina River), which has no established dam reservoir. The novelty of this article is the two-stage assessment of changes in the regime of the Główna River induced by the construction of the Lake Kowalskie dam reservoir by comparing IHA indices for the pre-dam (1955–1983) and post-dam (1984–2021) periods, and comparing the direction and extent of flow changes in the post-dam period (1984–2021) between the Główna River and the adjacent Cybina River to answer the question of whether the Lake Kowalskie dam reservoir fulfils a mitigation function in terms of climate change.

It should be emphasised that the discussed area belongs to those with the greatest water deficits in Poland, and activities for an increase in retention are important. A long-term collection of results regarding water flow and the role of the dam in its shaping, with a simultaneous comparison with a case without such infrastructure, offers information on changes in hydrological conditions in the case of undertaking similar hydrotechnical measures for other rivers in the region.

2. Materials and Methods

2.1. Study Area

The Kowalski Reservoir is located in West Poland on the Główna River (Figure 1). The Główna River has its origin in Lake Lednica and flows into the Warta River—the third largest river in Poland. Its length is 43 km, its catchment area occupies 235 km², and the density of the river network is 1.01 km·km⁻² [24]. The land use structure in the catchment is dominated by agricultural land, accounting for 62%. The Kowalski Reservoir was constructed in 1984 [24] as a result of flooding a natural lake. The Kowalski Reservoir has several functions. Its most important tasks include correction of water flow, provision of water retention for agriculture, and reduced flood threat [25]. It also fulfils recreational functions.

The Kowalski Reservoir features a two-stage structure: the preliminary reservoir fulfils the function of a deposition tank closed with a dam in Jerzykowo (Figure 2a). The lower reservoir is closed by the frontal dam (Figure 2b). The main features of the reservoir are presented by Sojka et al. [24]: The flooding area for the maximum and normal water levels is 203 and 195.1 ha, respectively; the volume for the maximum and normal water levels is 6.580 × 10⁶ and 5.970 × 10⁶ m³, respectively; the mean depth is 3.1 m; the maximum depth is 6.5 m; and the type of structure is an earth dam. The water surface level varies from a minimum of 85.00 m a.s.l. to a maximum elevation of 87.30 m a.s.l. The normal water level is 87.00 m a.s.l.

The normal water level in the Kowalski Reservoir is maintained outside of the flood periods and the maximum water level during the flood period. The Kowalskie Reservoir is a multipurpose reservoir. It covers water storage for irrigation, flood protection, recreation, and maintenance of the biological flow downstream of the main dam location [25].

2.2. Data Set

The paper employed data regarding water flow and water level values in the Główna River for Wierzenica station (Figure 1), measured by the Institute of Meteorology and Water Management—National Research Institute in the years 1955–2021.

Data regarding daily water flow in the Główna River in the period of 1955–1974 were reconstructed based on water levels. Hydrometric measurements (several to a dozen measurements at an annual scale) covering water levels, channel geometry, and point-based water flow velocity measurements provided the basis for the determination of the water flow rate in the river, expressed in m³·s⁻¹. Based on the dependency of the amount of water in the river (expressed as water flow) on the water volume in the river channel (water levels), the curve of the water flow rate was determined. This permitted the reconstruction of daily water flow in the river in the years 1955–1974. Moreover, information on water flow for the Cybina River (Antoninek profile, Figure 1) in the years 1984–2021 was used. Such an approach aimed at the determination of differences in the course of the water flow in the river in the case of two neighbouring catchments with similar geological and climatic features but a variable degree of implementation of hydrotechnical infrastructure (presence and lack of a dam reservoir).

2.3. Methods

The assessment of the effect of the Kowalski Reservoir on the outflow regime of the Główna River employed the Indicators of Hydrologic Alteration (IHA). The IHA covers 33 indicators that are divided into 5 groups. The indicators refer to the following: (1) monthly flow magnitude, (2) annual extreme flow magnitude and duration and the base flow conditions, (3) annual extreme flow timing, (4) high and low pulse frequency and duration, and (5) rate and frequency of flow changes (

Table 1. Indicators of hydrological alteration [26].

Group	Hydrologic Parameter (Units)	Abbreviation
Group 1: Magnitude of monthly flow (12)	Median flow (m3·s−1) of:
	November	MedNov
	December	MedDec
	January	MedJan
	February	MedFeb
	March	MedMar
	April	MedApr
	May	MedMay
	June	MedJun
	July	MedJul
	August	MedAug
	September	MedSep
	October	MedOct
Group 2: Magnitude and duration of annual extreme flow (12)	Annual minimum flow (m3·s−1):
	1 day	D1min
	3 day	D3min
	7 day	D7min
	30 day	D30min
	90 day	D90min
	Annual maximum flow (m3·s−1):
	1 day	D1max
	3 day	D3max
	7 day	D7max
	30 day	D30max
	90 day	D90max
	Number of zero-flow (days)	NZF
	Base flow index (-)	BFI
Group 3: Timing of annual extreme flow (2)	Julian date of annual flow (days):
	1 day minimum	DD1min
	1 day maximum	DD1max
Group 4: Frequency and duration of high and low pulses (4)	Number of low pulses in each hydrologic year (-)	NLP
	Median duration of low pulses (days)	DLP
	Number of high pulses in each hydrologic year (-)	NHP
	Median duration of high pulses (days)	DHP
Group 5: Rate and frequency of flow changes (3)	Rise rate (m3·s−1·day)	RR
	Fall rate (m3·s−1·day)	FR
	Number of flow reversals (-)	NFR

).

The IHA indicators were developed by Richter et al. [26]. They permit the assessment of the effect of anthropogenic activity on the hydrological regime of a river. The NZF (number of zero flow) was excluded from the analysis, because the flow values in the period of 1955–2021 were at a level between 0.021 and 15.0 m³·s⁻¹. The IHA indicators were calculated for the period before (1955–1983) and after (1984–2021) the construction of the Kowalski Reservoir. The quantitative assessment of the effect of the reservoir on the hydrological regime of the Główna River was conducted according to the methodology described by Xue et al. [27], involving the subsequent calculation of the deviation degree with the following formula:

$${\mathrm{D}}_{\mathrm{i}}=\frac{{\mathrm{M}}_{{\mathrm{e}}_{\mathrm{i}}}-{\mathrm{M}}_{{\mathrm{o}}_{\mathrm{i}}}}{{\mathrm{M}}_{{\mathrm{o}}_{\mathrm{i}}}}\times 100$$

(1)

where:

Mo_i—median value for the pre-impacted period;

Me_i—median value for the post-impacted period.

A negative Di value indicates a decrease in mean value in the post-impacted period compared to the pre-impacted period, whereas a positive Di value indicates an increase in the mean value in the post-impacted period compared to the pre-impacted period.

Moreover, the degree of hydrological alteration (DHA) for each index was calculated with the following formula:

$${\mathrm{D}\mathrm{H}\mathrm{A}}_{\mathrm{i}}=\left|\frac{{\mathrm{N}}_{\mathrm{o}}-{\mathrm{N}}_{\mathrm{e}}}{{\mathrm{N}}_{\mathrm{e}}}\right|\times 100$$

(2)

where:

No is the observed number of years for which the value of the indicator falls within the RVA target range;

Ne is the expected number of years for which the value of the indicator falls within the RVA target range.

The assessment of the change in the regime in reference to each indicator was conducted on a three-step scale as follows: (1) no alteration, low alteration, DHA_i < 33%; (2) moderate alteration, 33% ≤ DHA_i ≤ 67%; and (3) high alteration, DHA_i > 67% [27].

The overall degree of hydrological alteration (ODHA) was calculated based on the following formula:

$${\mathrm{O}\mathrm{D}\mathrm{H}\mathrm{A}}_{\mathrm{i}}=\sqrt{\frac{{\mathrm{D}\mathrm{H}\mathrm{A}}_{\mathrm{i},\mathrm{m}\mathrm{a}\mathrm{x}}^2+{\mathrm{D}\mathrm{H}\mathrm{A}}_{\mathrm{i},\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}^2}{2}}$$

(3)

where:

DHA_i,max is the maximum value of the degree of alteration of the indicators;

DHA_i,mean is the mean value of the degree of alteration.

The total regime change resulting from the construction of the Kowalskie Reservoir was determined based on a five-step scale as follows: (1) slight alteration (ODHA < 20%), (2) low alteration (20% ≤ ODHA < 40%), (3) moderate alteration (40% ≤ ODHA < 60%), (4) high alteration (60% ≤ ODHA < 80%), and (5) severe alteration (ODHA ≥ 80%) [27].

Because the Kowalskie Reservoir dam was constructed in the period of 1978–1984, a Pettitt test [28] was used to find the change point in mean monthly flow series and minimum and maximum flows of 1, 3, 7, 30, and 90 days.

Due to being in the times of climate change, it was important to analyse the flow series in terms of the occurrence of a trend, and the magnitude trend calculations were conducted in three variants: (1) for the entire measurement series for 1955–2021, (2) for two subseries designated based on the term of commencement of the functioning of the Kowalski Reservoir in 1955–1983 and 1984–2021, and (3) for two subseries designated by the change point calculated based on the Pettitt test. The trend calculations were conducted for mean monthly flows of 1, 3, 7, 30, and 90 days minimum and 1, 3, 7, 30, and 90 days maximum.

The trend analysis was conducted by means of a non-parametric Mann–Kendall test [29,30], and the magnitude of changes was assessed by means of a Sen test [31].

The analysis of the directions and magnitude of changes by means of MK and S tests used a modified mk package developed by Patakamuri and O’Brien [32]. The statistical analysis by means of the Pettitt and Mann–Kendall tests adopted a standard level of significance of 0.05.

The obtained results were compared with the results of trend analyses obtained for the Cybina River in the hydrological profile of Antoninek, which shares its watershed with the Główna River. The comparison covered the period from 1984 to 2021.

3. Results

Mean annual flow of the Główna River in the hydrological station Wierzenica in the period of 1955–2021 varied from 0.14 to 2.98 m³·s⁻¹, with a mean value of 0.90 m³·s⁻¹ (Figure 3a). Mean annual flow in the period of 1955–2021 showed a decreasing trend (Figure 3b). The trend was statistically significant at a level of 0.05.

In the period from 2000 to 2021, mean annual flow was at a level not exceeding 0.25 m³·s⁻¹ as many as seven times. Such situations were also observed earlier, but with lower frequency: twice in the early 1990s and once in the 1980s. Mean monthly flow varied from 0.02 to 8.34 m³·s⁻¹. The highest mean monthly flows occurred in February and March, and the lowest from June to October. It should be emphasised that in the period from June to October, high mean monthly flows also occurred, reaching the maximum value in August (Figure 4).

The analysis of the impact of the Kowalskie Reservoir dam on the flow regime of the Główna River based on the IHA indicators is presented in

Table 2. Change in the hydrological regime of the Główna River in the period before and after the construction of the Lake Kowalskie Reservoir dam.

Indicator	Pre	Post	DD (%)	DHA (%)	Indicator	Pre	Post	DD (%)	DHA (%)
	Impact					Impact
MedNov	0.65	0.28	−56.9	−37.6 M	D90min	0.40	0.20	−50.4	45.7 M
MedDec	1.14	0.31	−72.8	−94.1 H	D1max	5.50	2.10	−61.8	−51.4 M
MedJan	1.55	0.44	−71.6	−87.3 H	D3max	5.37	2.05	−61.9	−51.4 M
MedFeb	1.60	0.55	−65.5	−42.8 M	D7max	4.69	1.86	−60.3	−58.4 M
MedMar	1.75	0.57	−67.4	−65.3 M	D30max	3.56	1.27	−64.3	−58.4 M
MedApr	1.50	0.70	−53.2	−29.6 L	D90max	2.56	0.93	−63.7	−51.4 M
MedMay	1.00	0.43	−57.0	−56.4 M	BFI	0.11	0.25	125.4	−44.5 M
MedJun	0.65	0.23	−64.6	−30.6 L	DD1min	232	220	−5.4	−37.6 M
MedJul	0.65	0.23	−64.6	−37.6 M	DD1max	37	70	87.8	−9.8 L
MedAug	0.63	0.22	−65.1	59.6 M	NLP	2	3	50.0	−36.4 M
MedSep	0.50	0.21	−59.0	38.8 M	DLP	9.0	11.0	22.2	−23.7 L
MedOct	0.55	0.24	−56.4	−9.8 L	NHP	3	2	−50.0	−64.8 M
D1min	0.12	0.11	−8.3	31.8 L	DHP	8.8	4.0	−54.3	−54.2 M
D3min	0.12	0.12	0.0	52.6 M	RR	0.15	0.04	−76.7	−87.3 H
D7min	0.12	0.13	4.2	73.4 H	FR	−0.10	−0.04	−60.0	−88.3 M
D30min	0.15	0.15	2.4	108.1 H	NFR	67	92	37.3	−23.7 L

L—low alteration; M—moderate alteration; H—high alteration.

.

The analysis of the deviation degree (DD) indicators showed that the medians of monthly flow after the construction of the Kowalskie Reservoir were lower by 53 to 73% than in the period preceding its construction. In the second group of IHA indicators, the smallest changes occurred in the scope of minimum flows of 1, 3, 7, and 30 days in a range from −8% to 4%. The 90 day minimum flow and 1, 3, 7, 30, and 90 day maximum flows decreased by 50% and from 60 to 64%, respectively. In contrast, the BFI indicator increased by 125%. The BFI is calculated as the ratio of the annual 7 day minimum flow and the annual mean flow. Minimum values of 1 day flow occurred in approximate terms, and 1 day maximums occurred around 33 days later, after the construction of the Lake Kowalskie Reservoir dam. In the fourth and fifth group of IHA indicators, DD values ranged from −77% to 50%.

The analysis of the values of the degree of hydrological alteration (DHA) showed high alteration in reference to the indicators MedDec, MedJan, RR, FR, D7min, and D30min (Figure 5). For MedDec, MedJan, RR, and FR, the DAH values varied from −94% to −87% (Figure 6a). This means that the flow values occurred more rarely in the RVA target range after the construction of the Kowalskie Reservoir than in the period preceding its construction. D7min and D30min fell within the RVA target range more frequently than before the construction of the reservoir dam. The DHA values were 73% and 108%, respectively (Figure 6b). Low alterations were observed for the indicators MedApr, MedJun, MedOct, D1min, DD1max, DLP, and NFR. The DHA values varied from −31% to 32%. According to the adopted classification, this suggests little or no alteration. In the case of the remaining indicators, the changes were at a moderate level.

Finally, the overall degree of hydrological alteration (ODHA) was calculated by taking all 32 of the analysed indicators into consideration. The result of 84.6% suggests severe alteration. The values of ODHA for groups 1 and 2, including indicators 12 and 11, reached 75.1% and 86.4%, pointing to high and severe alteration, respectively.

The Kowalskie Reservoir was commissioned in 1984, although its construction lasted ca. 6 years. Due to this, a Pettitt test was applied to find the change point in the series of monthly flow (group 1 of the IHA indicators) and extreme flow (group 2 of the IHA indicators). The analysis showed the occurrence of change points that were statistically significant at a significance level of 0.05 for the following indicators: Jul, Aug, Oct, and d90min (1982); and Dec, Jan, Feb, Mar, d1max, d3max, d7max, d30max, and d90max (1983). For the remaining IHA indicators, change points occurred in the period outside the period of construction of the Kowalskie Reservoir: in 1989 for Apr, May, and Jun, and in 1996 for BFI. In the series of Nov, Sep, d1min, d3min, d7min, and d30min, there were no significant change points, according to the Pettitt test. It is evident that the construction of the Kowalskie Reservoir had an impact on the course of more than half of the IHA indicators. Finally, the assessment of temporal flow was conducted in the period before and after the construction of the reservoir by means of the Mann–Kendall and Sen tests. It was determined that the previously adopted division into the pre-impact (1955–1983) and post-impact (1984–2021) periods was also statistically justified (

Table 3. Direction and magnitude of characteristic flow before and after the construction of the Kowalski Reservoir.

	Pre-Dam Period (1955–1983)				Post-Dam Period (1984–2021)
Indicator	S	Z-Value	p-Value	Sen’s Slope (per Decade)	S	Z-Value	p-Value	Sen’s Slope (per Decade)
MedNov	−8	−0.14	0.890		−76	−0.98	0.327
MedDec	8	0.14	0.890		−160	−2.08	0.038 *	−0.105
MedJan	34	0.65	0.514		−138	−1.79	0.073
MedFeb	−98	−1.92	0.055		−136	−1.77	0.077
MedMar	−54	−1.05	0.295		−114	−1.48	0.139
MedApr	−32	−0.61	0.540		−156	−2.03	0.043 *
MedMay	−84	−1.64	0.101		−102	−1.32	0.187
MedJun	−84	−1.64	0.101		−196	−2.55	0.011 *	−0.071
MedJul	−80	−1.56	0.119		−130	−1.69	0.092
MedAug	−88	−1.72	0.086		−142	−1.84	0.065
MedSep	−94	−1.84	0.066		−176	−2.29	0.022 *	−0.063
MedOct	−44	−0.85	0.396		−204	−2.66	0.008 *	−0.070
D1min	74	1.44	0.149		−58	−0.75	0.456
D3min	46	0.89	0.374		−68	−0.88	0.381
D7min	32	0.61	0.540		−70	−0.90	0.367
D30min	4	0.06	0.953		−80	−1.03	0.301
D90min	−56	−1.09	0.277		−118	−1.53	0.126
D1max	−26	−0.49	0.621		−202	−2.63	0.009 *	−0.718
D3max	−22	−0.41	0.678		−188	−2.45	0.014 *	−0.605
D7max	−38	−0.73	0.465		−174	−2.26	0.024 *	−0.491
D30max	−40	−0.77	0.441		−150	−1.95	0.051
D90max	−62	−1.21	0.228		−168	−2.18	0.029 *	−0.208
BFI	82	1.60	0.110		212	2.76	0.006 *	0.057

* Statistically significant trend at the level of 0.05 (Sen’s slope values were not calculated for non-significant trends).

). The obtained results showed no changes in the flow of the Główna River in the period of 1955–1983. The results obtained in all cases were not statistically significant at a significance level of 0.05. In the period after the construction of the Kowalski Reservoir, a decreasing trend for monthly flow was recorded in December, April, July, September, and October. Moreover, a decrease in maximum flow for 1, 3, 7, and 90 days was observed. For 30 day maximum flow, the changes were already below the threshold of the adopted significance level. The results of minimum flow were also interesting. In the period after the construction of the reservoir, there was no significant decreasing trend. One of the functions of the Kowalskie Reservoir is the control of flow, a reduction in spring floods, and an increase in outflow in the summer period.

The results of the trend analysis for the Cybina River at the hydrological station Antoninek showed that in the period of 1984–2021, median flow significantly decreased in December, September, and October (

Table 4. Direction and magnitude of changes in characteristic flow of the Cybina River in the period of 1984–2021.

	Period (1984–2021)
Indicator	S	Z-Value	p-Value	Sen’s Slope (per Decade)
MedNov	−80	−1.03	0.301
MedDec	−188	−2.45	0.014 *	−0.066
MedJan	−128	−1.66	0.097
MedFeb	−70	−0.90	0.367
MedMar	−88	−1.14	0.255
MedApr	−128	−1.66	0.097
MedMay	−140	−1.82	0.069
MedJun	−64	−0.82	0.410
MedJul	−30	−0.38	0.704
MedAug	−108	−1.40	0.162
MedSep	−208	−2.71	0.007 *	−0.047
MedOct	−206	−2.68	0.007 *	−0.087
D1min	−294	−3.83	0.000 *	−0.026
D3min	−286	−3.73	0.000 *	−0.026
D7min	−266	−3.47	0.001 *	−0.026
D30min	−210	−2.73	0.006 *	−0.030
D90min	−172	−2.24	0.025 *	−0.034
D1max	−130	−1.69	0.092
D3max	−126	−1.63	0.102
D7max	−118	−1.53	0.126
D30max	−118	−1.53	0.126
D90max	−120	−1.56	0.120
BFI	−290	−3.78	0.000 *	−0.054

* Statistically significant trend at the level of 0.05 (Sen’s slope values were not calculated for non-significant trends).

). Moreover, a decrease in 1, 3, 7, 30, and 90 day minimum flows and BFI values occurred. No changes in the values of maximum flows were observed. Group 2 of the IHA indicators for the Cybina River, in particular, showed different behaviours than those of the Główna River. The IHA indicators from group 1 provided similar results for the Główna and Cybina rivers, with the exception of April and July.

4. Discussion

The adaptation of water conditions is one of the key factors shaping human activity, as reflected both in the history (e.g., development of the settlement network) and present times with highly advanced technologies [33,34]. It is equally important to ensure the appropriate amount of water for food production, i.e., the drainage of excess and irrigation of arable fields in water deficit periods. The implementation of these assumptions basically covers two directions of activity: meliorations and an increase in retention [35,36].

The results obtained in this paper show that, from a hydrological point of view, changes related to the construction of the Kowalskie Reservoir are beneficial. This is confirmed by earlier research conducted for a shorter period (1971–2012), in which Sojka et al. [24] evidence its impact on the frequency and duration of high and low impulses. The construction of the dam altered the components of the water balance and decreased the volume of water flow and, as a consequence, water outflow. The cycle of functioning of the reservoir is currently relatively simple, i.e., the normal water level is maintained throughout the year (87.00 m a.s.l.) and the maximum water level is maintained during the flood period (87.30 m a.s.l.). The simulated further increase in air temperature and, consequently, evaporation, will require the introduction of new guidelines for the functioning of the reservoir. Attention should be paid to the minimum amount of water in the Główna River, i.e., the amount that is desirable to flow for biological and social purposes. Pursuant to the water management plan, environmental flow below the dam was determined to be at a level of 0.04 m³·s⁻¹. In reference to a neighbouring river (without a reservoir), no changes in maximum flow values were observed. This suggests that the current water management conducted on the Kowalskie Reservoir fully meets its assumptions.

Similar statistics (Mann–Kendall test and Pettitt trend test) in reference to overland flow were analysed by Shi and Wang [37]. This allowed for the natural transformed period to be defined. It was finally evidenced that the annual flow was decreasing. Changes in the structure of the global outflow suggest the key importance of dams for the peak and medium values, as well as the variability of the flow. For the stations under the impact of the dam, the significantly decreasing trend was almost twice as high as the share of stations showing a significantly increasing trend [38]. In the case of the catchment of the Chenggou River, it was determined that the dam system can retain more than 50% of the flow capacity of the catchment [39]. The Xiaolangdi Dam is responsible for approximately 52% of outflow reduction both in a year and outside of the flood season and for approximately 32% of flow increase during the flood season [40]. The study by Luan et al. [41] regarding the separation of the impact of the check dam on outflow from climate and vegetation changes on a loess plateau showed that the accumulated value of outflow reduction rapidly grew with an increase in the retention capacities of the dams. The highest importance is obviously credited to reservoirs on large rivers, although, according to Lehner et al. [42], smaller reservoirs have a substantial effect on the spatial range of flow changes. It was determined that catchments with functioning dams show economic activity approximately 25 times higher than catchments with no infrastructure [43]. The magnitude of water retention in the Al Massira Reservoir (Morocco) is a factor determining the maintenance of agricultural activity and an important resource for the population of the region [44]. After the construction of irrigation channels in the Punjab state (Pakistan), the cultivation intensity and agricultural yields increased. Moreover, due to the accessibility of water, the crop structure shifted from wheat and fodder plants to vegetables [45].

In Poland, retention reservoirs also meet various functions. Among them, Rzętała [46] mentions water supply, fire protection, recreation and tourism, fish farming, energy production, inland transport, and the extraction of natural minerals. They are particularly important in regions with considerable recorded water deficits, including the study area. The construction of the Kowalski Reservoir several decades ago corresponds with the broader policy of the expansion of water retention possibilities in Poland. The assumptions are implemented on several tiers, including the construction and reconstruction of weirs [47,48], the damming of natural lakes [49,50], and the construction of dam reservoirs [51,52]. Governmental institutions are conducting intensive work on the development of a long-term programme of construction of 19 retention reservoirs of various sizes for more than USD 1 billion [53].

Water is becoming an increasingly important resource, which is understandable in the context of the observed climate change and the frequency of the occurrence of extreme situations (droughts, floods) negatively affecting the hydrosphere and, consequently, human life. The determination of changes in water circulation caused by hydraulic structures refers to the analysis of periods before and after their construction. For example, the analysis related to the launch of the Beni Haroun dam showed an increase in precipitation on an annual scale, whereas the annual air temperature in nearby meteorological stations decreased [54]. The analysis of the multiannual period of 1952–1983 revealed that the construction of the Włocławek dam on the Vistula River contributed to an increase in mean annual water temperature by 0.55 °C, whereas climate change resulted in a decrease of 0.26 °C [55]. In the case of the Jeziorsko dam (Warta), its construction contributed to an increase in minimum flow and a decrease in maximum flow [56]. It should also be emphasised that, without the reservoir, there would be higher flow in the period from January to June and lower flow in the period from July to October.

The methodology applied in this study refers to the aforementioned research, where, in the case of the Główna River, two characteristic periods with different hydrological parameters were evidenced. The range and seasonality of the changes reflect the transformation of the current quasi-natural features of water circulation, confirming the efficiency of the hydraulic structure in decreasing water outflow from the catchment. Moreover, the results constitute a model for other investments of this type in the future, both in the analysed region and in rivers with similar parameters. Making a decision about the construction of a reservoir dam requires detailed information regarding all consequences of its operation. Based on the example of Australia, McMahon and Petheram [57] conclude that, due to the high investment costs as well as environmental and social changes, the decision regarding the construction of a dam is more complex today than it was several decades ago. These premises are universal and concern practically any investment of this type irrespective of its location. On the other hand, according to Liu et al. [58], considering population growth, the construction of large dams can be considered one of the best solutions for meeting the demand for water, food, and energy supply.

The issue of changes in the components of the water balance therefore remain of elementary importance, and the reconstruction and analysis of changes in flow rate as conducted in the paper allow for a clear illustration of the new hydrological conditions. Climate change is an additional challenge to conducting water management. Data for the meteorological station in Poznań show that, from the moment of launching the reservoir, no statistically significant changes have been recorded in the course of annual precipitation totals, and an evident increase in air temperature has been observed. In the future, improvement in the functioning of the reservoir may involve, among other strategies, the expansion of the system of forecasting in real time the meteorological conditions in the upper part of the catchment. Considering extreme hydrological situations on a monthly scale, such knowledge is particularly necessary in March and September, when, based on the obtained results (Figure 4), the highest and lowest water flows were recorded in the Wierzenica profile.

5. Conclusions

Ensuring that the appropriate amount of water is evenly distributed in reference to biological and economic processes provides the basis for maintaining their continuity. In the context of global transformations, it requires substantial and effective human interference in the hydrosphere. One of the methods for stabilising water resources is the introduction of hydrotechnical infrastructure that permits the regulation of water deficits and excess in the catchment. The conducted analysis, referring to changes in water flow in the Główna River before and after the construction of the Kowalski Reservoir, evidenced the important role of the reservoir in its course (particularly stabilisation) in an area considered one of the most deficient in water in Poland. In the context of the construction of further reservoirs, it is important to investigate the current scope of changes in water circulation in reference to objects of this type that are already functioning in the environment in consideration of the period before and after the construction of the dam. The following specific conclusions were drawn on the basis of the conducted studies:

• Monthly median flow and 1, 3, 7, 30, and 90 day low and high flows in the post-dam period (1984–2021) were lower than in the pre-dam period (1955–1983).
• In the years 1955–1983 (pre-dam period), there were no significant trends in monthly flow magnitude, annual extreme flow magnitude and duration and base flow conditions, annual extreme flow timing, high and low pulse frequency and duration, or rate and frequency of flow changes.
• During the period of 1984–2021 (post-dam period) there were decreasing trends in median flow for December, June, September, and October. In addition, strong decreasing trends in 1, 3, 7, 30, and 90 day maximum flows were observed.
• A different situation occurred on the Cybina River, where the strongest decreasing trends were observed for minimum flow between 1984 and 2021.
• The Kowalskie Reservoir dam had the strongest impact on median flow for December and January, 7 day and 30 day minimum flows, and the rise rate and fall rate.