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Article

Spatial Regularities of Changes in the Duration of Low River Flows in Poland Under Climate Warming Conditions

by
Dariusz Wrzesiński
1,
Andrzej A. Marsz
2,
Anna Styszyńska
2,
Adam Edmund Perz
1,
Wiktoria Brzezińska
1 and
Leszek Sobkowiak
1,*
1
Department of Hydrology and Water Management, Faculty of Geographical and Geological Sciences, Adam Mickiewicz University, Bogumiła Krygowskiego Str. 10, 61-680 Poznań, Poland
2
Association of Polish Climatologists, Krakowskie Przedmieście Str. 30, 00-927 Warsaw, Poland
*
Author to whom correspondence should be addressed.
Water 2025, 17(2), 243; https://doi.org/10.3390/w17020243
Submission received: 25 November 2024 / Revised: 24 December 2024 / Accepted: 14 January 2025 / Published: 16 January 2025
(This article belongs to the Section Hydrology)

Abstract

:
On the basis of daily discharges recorded in 140 water gauges located on 96 Polish rivers, the long-term changes of runoff and the number of days with low flows (NDLF) in relation to selected meteorological variables were studied. The analyses were performed for the entire multi-annual period 1951–2020 and two sub-periods: 1951–1988 and 1988–2020 that are before and after climate change. The average values of these hydro-meteorological variables in the two sub-periods were then compared. It was found that after 1988, a statistically significant (p < 0.001) increase in the average air temperatures, ranging from 0.9 to over 1.3 °C, occurred. Similarly, statistically significant changes were determined for evaporation, which increased by about 10–25%. Precipitation did not show such changes—a statistically significant decrease in precipitation (by over 5%) was recorded only in the southern part of the Odra River basin, and in most stations, statistically insignificant increases were recorded. The most complex changes took place in river runoff. After 1988, in most gauges, a decrease in runoff by about 5–15% was detected; in some cases, these decreases were statistically significant. In the south-eastern part of the country, primarily in the catchments of the right tributaries of the Vistula River, an increase in runoff by about 5–10% was detected. However, only in the case of one gauge, these tendencies were statistically significant. Next, in order to determine spatial regularities in long-term changes in the NDLF, the cluster analysis method was used, and the gauges were grouped according to the values of 70 annual NDLF. This resulted in separating three relatively homogenous territorially groups of rivers, demonstrating a clear regional differentiation of NDLF. It was concluded that separation of these three groups of rivers in terms of different long-term changes in NDLF was mainly influenced by climatic conditions.

1. Introduction

In the period of progressive global warming, water circulation conditions are changing [1,2,3,4]. This concerns, among others, river regimes, including high- and low-water periods. In Europe, including Poland, this is primarily associated with changes in river water supply conditions [5,6] as a result of changing patterns of evaporation [7] and precipitation distribution in the yearly cycle [8,9], decreasing thickness and duration of snow cover [10], increasing rainfall intensity [11] and changing types of precipitation characteristic of a given season, primarily the occurrence of rain instead of snow in winter [12,13]. Another cause is the rising air temperature and longer and more severe heat waves [14], which increase evaporation and also affect the thermal regime of streams [15,16,17] and lakes [18,19].
In the context of changes in river regimes, both from the point of view of the natural environment and economic activity, hydrological droughts are particularly important, which are manifested by low flows in rivers. Raczyński and Dyer [20], using the trend decomposition method, showed that streamflow droughts were increasing in many parts of Poland, especially in the central, western, and southwestern regions. These results differ from the findings of earlier modeling [21,22], which indicated that future streamflow droughts were projected to be less severe [23]. Wrzesiński et al. [24] indicated that from 1951 to 1988, most of the analyzed gauges showed decreasing trends in the number of days with low flows, but this trend changed and increased from 1988 to 2020. This may be related to the observed climate change in Poland [25], reflected in meteorological data since 1988 [26]. Additionally, the occurrence of low flows on rivers in Poland may be associated with, among others, the temperature of surface waters and thermohaline circulation of the North Atlantic [27,28,29].
Until now, most studies on the hydrology of low flows on Polish rivers have treated the hydrological conditions after 1951 as stationary. While in the latest scientific literature the impact of climate change on river regimes in Poland has been investigated [30,31,32,33,34], there is still a lack of studies on the regional differentiation in the occurrence and course of low flows on Polish rivers.
The aim of this paper is to fill this research gap and present results of research on determining the multi-annual variability and the spatial differentiation in the number of days with low flows (NDLF) in Polish rivers in the context of the climate warming observed after 1988.

2. Materials and Methods

The paper analyzes changes in meteorological (air temperature, precipitation, and surface evaporation) and hydrological elements (runoff and NDLF) in the multi-annual period 1951–2020 and in two sub-periods, i.e., 1951–1988 and 1988–2020, which are before and after the change in climatic conditions in Poland. The year of division (1988) was selected based on previous studies that showed clear differences in the course of selected meteorological elements before and after that year [26]. The meteorological and hydrological data used in the study were obtained from the publicly available archives of the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB) in Warsaw, Poland (https://danepubliczne.imgw.pl, accessed on 10 October 2024). Analyzes of river runoff and NDLF were performed based on daily discharges from the multi-annual period 1951–2020, recorded in 140 water gauges located on 96 rivers in Poland (Figure 1; Table A1).
Surface evaporation (Ev, mm of water column equivalent) in a given month from the area of Poland was calculated with the help of the Ivanov formula (after [35]):
Ev = 0.0018 × (25 + t)2 × (100 − f),
where t is the average monthly temperature (°C) and f is the monthly relative humidity (%), in which arguments of function (1) are the area means of annual air temperature (TPL) and relative humidity (fPL) in Poland.
The method of estimating surface evaporation using the formula proposed by Ivanov performs satisfactorily in Poland (e.g., [36,37,38,39]). Annual surface evaporation was calculated as the sum of monthly evaporation. Using a series of annual Ev values, the approximate water balance in a given calendar year was estimated as the difference between annual precipitation (PPL) and annual evaporation (Ev) in Poland, without taking into account retention.
Daily discharge was used to calculate the number of days with flows below the 10th percentile (Q10) in the hydrological year (in Poland lasting from 1 November to 31 October) in three temporal variants: the entire multi-annual period 1951–2020 and two sub-periods, i.e., before (1951–1988) and after climate change (1988–2020). Then, the trends of their changes and statistical significance were examined.
In order to examine the occurrence of low flows in rivers in Poland, the threshold level method was applied. This is one of the most common methods used in low flows research [40,41]. In our study, we used the 10th percentile (Q10) from the sum flow curve with the lower values, which corresponds to the 90th percentile (Q90) from the sum flow curve with the higher values. The low-flow periods were characterized with the use of the number of days with low flows (NDLF) below a threshold value, which was adopted as the 0.1 (10%) percentile (Q10) from the set of daily flows in 1951–2020. In our study, low flow was defined as the period during which the flows were equal to or lower than the adopted threshold value. Based on the daily flow dataset from 1951 to 2020, we assumed the threshold value to be the 0.1 (10%) percentile (Q10) of the sum flow curve with the lower values, which corresponds to the 90th percentile (Q90) from the sum flow curve with the higher value.
In the next stage, the change trends in the number of days with flows below Q10 detected in the respective gauges and their statistical significances were determined. The differences in the duration of low flows below Q10 between the period after climate change (1988–2020) and the period before it (1951–1988) were also determined.
The average NDLF was calculated based on the sum of the number of low flow days over the entire multi-annual period, which was then divided by the number of years, in which they occurred.
In the next stage, the linear regression method was used to determine the long-term trends in the NDLF and their statistical significance at three levels: 0.05, 0.01, and 0.001. The critical values of the correlation coefficient for these significance levels for the entire multi-annual period 1951–2020 and the two sub-periods were derived from the statistical tables (Table 1).
In order to determine changes in the investigated hydro-meteorological elements during the warming period (1988–2020) relative to the period 1951–1988, the change index was computed (Equation (2)):
S X ¯ 1988 2020 X ¯ 1951 1988 = X ¯ 1988 2020 X ¯ 1951 1988 X ¯ 1951 1988 100 %
where S X ¯ 1988 2020 and   S X ¯ 1951 1988 are the average values of hydro-meteorological elements in the separated sub-periods of the multi-annual period 1951–2020.
The calculated index shows the percentage increase or decrease of the analyzed elements in the period after climate change (1988–2020) in relation to their values in the period preceding it (1951–1988). The differences in the annual values of hydro-meteorological elements between 1988–2020 and 1951–1988 were calculated. The statistical significance of these differences was tested using the t-test for independent samples. Each time, the hypothesis H0:μ1 = μ2 of equality of expected values was tested against H1:μ1 ≠ μ2. Rejection of the hypothesis allowed us to conclude on significant differences in the average maximum river runoff observed after and before climate change. The t-statistic has a Student’s t-distribution, with n1 + n2-2 degrees of freedom (Equation (3)):
T = X ¯ 1 X ¯ 2 S X ¯ 1 X ¯ 2   ,
where S X ¯ 1 X ¯ 2 is written as follows (Equation (4)):
S X ¯ 1 X ¯ 2 = n 1 1 · S 1 2 + n 2 1 · S 2 2 n 1 + n 2 2 · 1 n 1 + 1 n 2 ,
where n 1 and n 2     are sample sizes, S 1 2 and S 2 2 are variances of both samples, and X 1 ¯ and X 2 ¯ are mean values of both samples.
In further analysis, in order to detect spatial regularities of long-term changes in the NDLF, the cluster analysis method [42] was used to group the gauges according to the values of 70 annual NDLF. The grouping results were presented in the form of a dendrogram, which reflected the similarity structure of the studied set of gauges and was used to separate typological classes. In the study, the number of classes was determined based on the analysis of the dendrogram geometry and the linkage distance curve. In the procedure of verifying the optimal number of clusters, the GWZ distance criterion [43] and the “Mojena rule” [44] were used. These methods usually give satisfactory results of unambiguous division into the number of classes [45] and are recommended for use in cluster analyses by many experts (e.g., [46]).
The application of hierarchical cluster analysis, which is a key element in minimizing differences within clusters, resulted in distinguishing two groups of gauges, designated as A and B, respectively. Group B was additionally split into two sub-groups: B1 and B2. The mathematical and statistical calculations were performed with the help of Statistica 13 software licensed by Statsoft®.

3. Results

3.1. Changes in the Average Values of Hydro-Meteorological Elements in Poland in 1951–2020

The highest average air temperatures, exceeding 9 °C, were recorded in South-western Poland, with temperatures decreasing towards the east and in mountainous areas (Figure 2). Evaporation also showed a similar spatial distribution (Figure 2). The central part of Poland was characterized by relatively lowest annual precipitation totals, which increased towards the north and south; a similar relationship can be seen in the case of river runoff (Figure 2). Both the average annual air temperatures and evaporation showed strong increasing trends throughout the country, in most cases statistically significant at the level of p < 0.001 (Figure 2). The situation is different in the case of precipitation totals and river runoff—in most of the analyzed locations, the trends were statistically insignificant (p > 0.05); however, increasing trends of runoff (p < 0.01) were concluded in South-western Poland (Figure 2).
A comparison of the average values of the hydro-meteorological variables studied between the sub-periods of the analyzed multi-annual period, i.e., 1951–1988 and 1988–2020, was also performed, and the statistical significance of the obtained differences was examined (Figure 3). The average air temperatures increased after 1988 throughout Poland from 0.9 to over 1.3 °C (Figure 3A). Similar changes were determined for evaporation, which increased by about 10–25% (Figure 3C). For both air temperature and evaporation, the statistical significance of the changes was very high, mostly at the level of p < 0.001 (Figure 3A,C). Precipitation did not show such changes—a statistically significant decrease in precipitation totals (by over 5%) was recorded only in the southern part of the Odra River basin, and in most stations, statistically insignificant increases in values were recorded (Figure 3B). The most complex changes spatially occurred in the case of river runoff—most of Poland was characterized by a decrease in runoff after 1988 by about 5–15%, of which some changes were statistically significant (Figure 3D). In the south-eastern part of the country, primarily in the catchments of the right tributaries of the Vistula River, an increase in runoff by about 5–10% was recorded, but only in the case of one gauge, these changes were statistically significant (Figure 3D).
Taking into account the entire studied multi-annual period, statistically significant decreasing trends of NDLF in the eastern and southern parts of the Vistula River basin were clearly visible—such trends were characteristic of the whole Poland in the first sub-period (1951–1988) (Figure 4). After 1988, the situation changed, and an increase in NDLF (statistically significant in selected rivers) was observed in most stations (Figure 4). The exceptions are several rivers in Southern Poland, for which statistically significant decreasing trends of NDLF were noted.
Annual NDLF was correlated with average annual air temperature (Figure 5A), evaporation (Figure 5B), and precipitation totals (Figure 5C). In the case of air temperature, both positive (for most of the study area) and negative correlations (mainly for the right part of the Vistula River basin) can be determined (Figure 5A). Correlations of annual NDLF with evaporation were only positive and statistically significant over most of Poland; the exception is again the southern and eastern parts of the Vistula River basin, where the strength of correlation was lower and more trends did not show statistical significance (Figure 5B). The correlation of annual NDLF with precipitation totals was negative over the entire country and in many cases statistically significant at the level of p < 0.001 (Figure 5C).

3.2. Results of the Procedure for Grouping Water Gauges Based on the Number of Days with Low Flows

One hundred and forty series of the NDLF in a year were subjected to the agglomeration procedure performed with the use of Ward’s method. The Euclidean distance was used as a measure of taxonomic distance. The distance was calculated with the use of Equation (5):
ESS = j = 1 K ( x j x ¯ ) ( x j x ¯ ) ,
where K is the number of clusters, x j is the multivariate measurement associated with the jth item, and x ¯ is the mean of all items.
This type of procedure revealed the typological diversity of the analyzed set. The graphical results of this grouping are presented in Figure 6.
The number of individual taxonomic levels of grouping was different in individual clusters. The analysis showed that in the case of lower levels of grouping, the aggregation into a cluster was the result of the location of series of gauges at small distances from each other—either on the same stream or in catchments located close to each other. In some cases, in a given cluster at the 3rd, 4th, or 5th grouping level, there were single gauges located in distant areas, whose variance of the NDLF was similar to the variance in catchments located close to each other.
The highest taxonomic level of grouping revealed that there were two groups of gauges on Polish rivers differing in terms of the variance of the NDLF. The number of gauges in each of these groups was noticeably diversified: group A consisted of only about 25% of all analyzed gauges, while group B consisted of 75% of all analyzed gauges. It can be noticed that at a lower taxonomic level, group B was divided into two sub-groups that differed from each other, designated as B1 and B2, respectively (Figure 6).
The analysis of the differentiation at the highest taxonomic levels (ESS = 1600) revealed a large differentiation of the variance of the NDLF in gauges in Poland. This issue seemed important, so our further analysis will be devoted to it. The similarity of the characteristics of flow distributions in closely located gauges, either on the same stream or in two small catchments located close each other, was obvious and did not require further studies. For this reason, the differentiation of the NDLF at lower levels of grouping will not be addressed here.
The statistical characteristics of the NDLF in individual groups are presented in Table 2.
The average numbers of days with low flows per year in the multi-annual period 1951–2020 were very similar in all these groups (Table 2), and the differences between them were statistically insignificant. However, the analysis of the distributions of values in individual groups revealed that there were some differences between them, and these constituted an internal differentiation of the set, revealed in the agglomeration procedure. The values of the correlation coefficients between the series of average numbers of days with low flows between groups were highly significant, but of moderate strength (group A and sub-group B1: r = 0.56, p << 0.001; group A and sub-group B2, r = 0.47, p << 0.001). The correlation between sub-groups B1 and B2 was clearly stronger and also highly significant (r = 0.73, p << 0.001).
The distribution of correlation coefficients between the series of NDLF in a year explained that the basic framework for shaping the NDLF in a year in Poland was formed under the influence of common factors, which, however, showed quite clear variability in individual areas (Figure 7).
Such very similar statistical characteristics of the individual groups suggest that internal differences of the set are the reason for the separation of respective types, resulting either from regional differentiation or from differentiation of the NDLF courses in individual sub-sets.

3.3. Regional Differentiation in the Number of Days with Low Flows in Poland

In order to explain this issue and obtain a picture of the spatial differentiation of NDLF, individual gauges were plotted on the map (Figure 8), in accordance with their geographical coordinates and association with a specific group.
The NDLF in individual gauges classified into respective types (groups) formed relatively homogenous territorial groups spatially. This means that the detected individual types of the NDLF showed clear regional differentiation. In the border areas between two different groups, there were few cases of “overlapping” of gauges belonging to different groups (e.g., along the Bug River). Such a situation in border zones is typical, as physical-geographical boundaries are zones, not lines. There are few examples of gauges classified in the grouping procedure to a specific group, but located in area typical to another group, e.g., while located in the area belonging to group A, the Zwierzyniec gauge at the Tanew River was classified into sub-group B1, and located in the area belonging to sub-group B2, the Dobra gauge at the Biała River was designated to sub-group B1.
The distinguished group A covered relatively the smallest part of Poland. It included the south-eastern part of the country, and its borders were marked from the south and east by the state border, to the west by the Soła River and the Vistula River, and to the north by the course of the Bug River—from the state border to the Wyszków gauge. This area will be referred to as “South-Eastern (SE) Poland” hereinafter.
Group B covered the remaining part of the country, with sub-group B1 covering the Northern and North-eastern Poland that included the regions of Pomerania and Warmia, the lower Vistula River, and the Masurian Lake District. The southern border of the area covered by sub-group B1 ran from Western Pomerania through the area between the Warta and middle Vistula rivers to the Bug River and the state border. Sub-group B2 covered the area from the western borders of Poland to the Silesian Beskids, following the Soła River and then along the Vistula River to the area where the Pilica River flowed into the Vistula. The area covered by gauges belonging to sub-group B1 will be referred to as “Northern (N) Poland”, and the area covered by sub-group B2 will be referred to as “South-Western (SW) Poland”.

3.4. Formal Reasons for Spatial Differentiation of the Number of Days with Low Flows in Poland

A more detailed analysis of the patterns of average NDLF allowed us to indicate the basic reasons for splitting all gauges in Poland into two groups, representing separate types of low flow patterns.
The most important reason for the formation of two separate types in the NDLF set was differences in the nature of the variability of the course of the NDLF. The analysis of linear trends in each of the areas, but (i.e., in each group) carried out in each area separately for the years 1951–1988 and for the years 1988–2020, i.e., in the sub-periods before and after climate change, revealed the basic difference in their course between groups A and sub-groups B1 and B2 (Figure 9, Figure 10 and Figure 11).
For the gauges included in the first group (Figure 9), a dual time distribution of linear trends was typical in each of these sub-periods: in the first sub-period (1951–1988), the trend in the NDLF was negative and statistically significant (p = 0.012), while in the second sub-period (1988–2020), the trend was close to zero; there was a very weak and insignificant (p = 0.738) negative trend, and the hypothesis that it was different from zero could not be rejected.
Similarly, a dual trend distribution was also typical for the course of the NDLF for gauges included in the second group (Figure 10 and Figure 11). The formal feature that caused the “separation” of the set of stations of group B from group A in the agglomeration procedure was the different behavior of trends in the years 1988–2020 between these groups.
In the set of the NDLF of group B in the years 1988–2020, contrary to group A, there was not a very strong, but statistically significant (p = 0.047 (group A) and 0.048 (group B)) positive trend. Further differentiation of group B into sub-groups B1 and B2 in a greater extent was due to differences in the course of the NDLF in the sub-period 1951–1988 rather than in 1988–2020. This is clearly visible in the comparison of Figure 5 and Figure 6. As a result of the changes in trends in both periods, the average NDLF in individual areas changed (Table 3).
In group A, the average NDLF in the sub-period 1988–2020 decreased noticeably (by 13.36 days per year) compared to the average NDLF in the sub-period 1951–1988 (statistically significant decrease). In group B1, it did not change significantly (the difference of −2.02 days per year was statistically insignificant), and in group B2, it increased remarkably (by 16.54 days per year). Comparing the average NDLF values in the individual sub-periods (Table 2), it can be concluded that, between the sub-periods 1951–1988 and 1988–2020, there was a “swap” of the area with the highest NDLF from SE Poland (group A) to SW Poland (sub-group B2), with no significant response of NDLF to the climate changes in N Poland (sub-group B1).
In general, in the years 1951–1988, the NDLF showed a tendency to decrease irregularly throughout Poland. This tendency was stronger in Northern Poland and slightly weaker in the rest of the country. After 1988, the NDLF stopped decreasing in SE Poland and began to increase in the rest of the country, most rapidly in its western and south-western parts and in Central Poland.
Based on these findings, it can be concluded that the most important reason differentiating Poland in terms of the variability of the NDLF is the different response of individual catchment groups to the rapid climate change that occurred in 1987–1989 [29,47].

4. Discussion and Conclusions

The study revealed different directions of changes of low river flows in Poland in the multi-annual period 1951–2020, occurring under the influence of regionally differentiated climatic changes. Three groups of rivers were distinguished, different in terms of changes in NDLF. These rivers constitute relatively homogenous regions located in the south-eastern, northern, and south-western parts of the country.
It can be assumed that the separation of these three groups of rivers in terms of different long-term changes in NDLF is mainly influenced by climatic conditions. In the analyzed multi-annual period 1951–2020, rivers of group A were characterized by a significant decreasing trend in NDLF. This referred to the whole hydrological year and also the winter and summer half-years. Contrary to group A, rivers of sub-groups B1 and B2 showed a predominantly increasing trend in NDLF. In the years 1951–1988, NDLF on most of the studied rivers decreased significantly, especially in the summer half-year (May−October). On the other hand, in the years 1988–2020, NDLF of rivers of sub-groups B1 and B2 showed a clearly increasing trend. This is different in the case of rivers of group A; NDLF values of these rivers showed both increasing, but statistically insignificant, and decreasing trends.
In the south-eastern part of Poland (group A), the smallest changes (increases) in air temperature and evaporation were observed. These were the only rivers that stood out in the period of warming after 1988 with higher runoff. This explained why the NDLF of these rivers showed a decreasing trend. As a result of the change in annual runoff, the NDLF also changed significantly. In the period of warming after 1988, the increase in runoff concerned only the Carpathian tributaries of the Vistula River, which belonged to group A (Figure 3D). The NDLF correlations with air temperature were negative only in the case of rivers assigned to group A, and for the Carpathian tributaries, these correlations were statistically significant (Figure 5A). As a result, the NDLF correlations of these rivers with evaporation also clearly differed from those in the other regions of the country (Figure 5B). The correlations were positive but were the weakest and often statistically insignificant. On the other hand, the NDLF of most of the studied rivers was strongly negatively correlated with the precipitation totals, and the rivers of group A did not differ significantly from this regularity (Figure 5C). The distinction between sub-group B1 with a relatively smaller increase in NDLF and sub-group B2 with a clear increase in NDLF of more than 30 days was mainly driven by changes in pluvial conditions. In the south-western part of Poland, precipitation decreased in the period after 1988 (Figure 3B). Most of the rivers of group B2 were located there. This was also confirmed by stronger, statistically significant negative correlations of NDLF with precipitation over the entire multi-annual period 1951–2020 (Figure 12).
The NDLF values of rivers of groups A and B1 showed strong, but less statistically significant relationships with precipitation in the subsequent 20-year sub-periods of the multi-annual period 1951–2020. On the other hand, clearly stronger and statistically significant relationships between NDLF values of rivers of sub-groups B1 and B2 and evaporation were observed over the entire period 1951–2020.
Generally, major findings of this research are as follows:
  • In the multi-annual period 1951–2020, the average annual air temperature and evaporation showed strong increasing trends in Poland, in most cases statistically significant at the level of p < 0.001. In the case of precipitation totals and river runoff, the situation was different: in most locations, the trends were statistically insignificant (p > 0.05); however, increasing trends of runoff (p < 0.01) were concluded in south-western Poland.
  • After 1988, the average air temperature increased throughout Poland from 0.9 to over 1.3 °C. Similar changes were determined for evaporation, which increased by about 10–25%. For both air temperature and evaporation, the statistical significance of the changes was very high, mostly at p < 0.001. In turn, such changes were not detected in the case of precipitation, with a statistically significant decrease by over 5% recorded only in the southern part of the Odra River basin. In most stations, statistically insignificant increases in precipitation totals were recorded. The most complex changes spatially occurred in the case of river runoff, with most of Poland characterized by a decrease in runoff after 1988 by about 5–15%, of which some changes were statistically significant. In the south-eastern part of the country, an increase in runoff by about 5–10% was recorded, but only in the case of one gauge, these changes were statistically significant.
  • Statistically significant decreasing trends of NDLF in the eastern and southern parts of the Vistula River basin were concluded, and such trends were characteristic of the whole Poland in the first separated sub-period (1951–1988). After 1988, a statistically significant increase in NDLF was observed in most stations along selected rivers, with some exceptions, including rivers in Southern Poland, where statistically significant decreasing trends of NDLF were detected.
  • Annual NDLF was correlated with average annual air temperature, evaporation, and precipitation. In the case of air temperature, both positive (for most of the study area) and negative correlations (mainly for the right part of the Vistula River basin) were determined. Correlations with evaporation were positive and statistically significant over most of Poland; the southern and eastern parts of the Vistula River basin was an exception. The correlation of annual NDLF with precipitation totals was negative over the entire Poland and in many cases statistically significant (p < 0.001).
  • The grouping of water gauges based on the NDLF revealed its regional differentiation. This led to a separation of two groups of gauges forming relatively homogenous territorial groups spatially. The distinguished group A included the south-eastern part of Poland, while group B covered the remaining part of the country. Group B was additionally split into sub-group B1 covering Northern and North-eastern Poland and sub-group B2 covering the south-western part of the country. The most important reason for the formation of these separate types in the NDLF set was differences in the nature of the variability of the course of the NDLF. The analysis of linear trends in each of the areas, carried out in each area separately for the years 1951–1988 and for the years 1988–2020, i.e., in the sub-periods before and after climate change, revealed the basic difference in their course between group A and sub-groups B1 and B2.
The presented results for the whole analyzed period (1951–2020) are in line with findings of [22]. The authors, using the trend decomposition method, showed that duration time of streamflow droughts (in the period 1965–2020) decreased in the south-eastern part of the Vistula basin and increased in most of other river gauges in Poland.
The proposed method has some limitations. It concerns NDLF as only one parameter of low flows referring to the total time of occurrence of the phenomenon in a given year, while low flows can be described by parameters linked to factors, such as drought/low flow severity (like volume or minimal flow) or the number of episodes. In future research, the threshold method used to select low flow events could be reconsidered as the problem of the objectivity of the choice of thresholds values has been raised by [46].
Our research also focuses on changing climate conditions and their impact on low flows. The specific changes in precipitation and evaporation in 1951–1988 and 1988–2020 resulted in the formation of three spatially homogenous areas in Poland (marked as A, B1, and B2) differing in the course of the climatic water balance and trends in the course of NDLF over time. However, in some of the analyzed river gauges, the anthropogenic influence (such as land use changes, water management practices, or water intake) may play a key role in explaining the observed trends in low flow changes.
To sum up, low flows research and understanding of their changes under climate warming conditions are crucial for developing proper and responsible water management practices, taking into account their impact on aquatic ecosystems, biodiversity, and economic activity (especially agriculture). Our research results show that policymakers should consider developing proper water management and drought consequences mitigation strategies and plans focusing on different objectives for each identified region.

Author Contributions

Conceptualization, D.W., A.A.M. and A.S.; methodology, D.W., A.A.M. and A.S.; software, D.W., A.A.M., L.S., W.B. and A.S.; validation, D.W., A.A.M. and A.S.; formal analysis, D.W., A.A.M. and A.S.; investigation, D.W., A.A.M. and A.S.; resources, D.W.; data curation, D.W. and W.B.; writing—original draft preparation, D.W., A.A.M., A.S., L.S., A.E.P. and W.B.; writing—review and editing, L.S., A.E.P. and W.B.; visualization, D.W., A.S., W.B. and A.E.P.; supervision, D.W., L.S. and A.E.P.; project administration, D.W.; funding acquisition, A.E.P., L.S. and D.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was carried out under the internal research grant (internal faculty research grant) at the Faculty of Geographical and Geological Sciences of Adam Mickiewicz University in Poznań, Poland.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Basic hydrological characteristics of the analyzed rivers. Numbering was performed in accordance with Figure 1.
Table A1. Basic hydrological characteristics of the analyzed rivers. Numbering was performed in accordance with Figure 1.
No.RiverGaugeCatchment Area [km2]Runoff Depth [mm]NDLFTrend NDLF
R *
River Regime **
1OdraChałupki4666282.448.8−0.2184
2OdraRacibórz-Miedonia6744300.847.6−0.1544
3OdraŚcinawa29,584188.952.10.1684
4OdraCigacice40,106170.656.60.1822
5OdraPołęcko47,370167.157.80.1752
6OdraSłubice53,600174.259.80.1872
7OdraGozdowice109,729146.966.20.1612
8SuminaNędza94.4191.356.50.280 *4
9BiałaDobra35399.459.70.0164
10Nysa KłodzkaBystrzyca Kłodzka260466.745.60.2244
11Nysa KłodzkaKłodzko1084364.951.0−0.1504
12Nysa KłodzkaNysa3276278.643.6−0.1154
13Nysa KłodzkaSkorogoszcz4514249.852.00.1304
14Bystrzyca DusznickaSzalejów Dolny175378.154.70.0022
15ŚcinawkaTłumaczów256279.857.8−0.0232
16ŚcinawkaGorzuchów511274.850.50.1914
17Biała GłuchołaskaGłuchołazy283542.057.3−0.1084
18BystrzycaKrasków683205.946.90.2134
19PiławaMościsko291172.657.10.253 *4
20StrzegomkaŁażany356198.058.0−0.324 *4
21BaryczOsetno4579101.648.90.336 *3
22BóbrŻagań4254275.854.50.255 *4
23KamienicaBarcinek97.2389.755.00.0652
24KwisaNowogrodziec736306.952.70.261 *4
25WartaDziałoszyn4088185.864.60.293 *2
26WartaSieradz8140171.365.20.0372
27WartaPoznań25,126124.568.3−0.1402
28WartaSkwierzyna31,268123.470.7−0.0662
29WartaGorzów Wielkopolski52,186124.459.90.1192
30OleśnicaNiechmirów592126.760.90.321 *3
31NerDąbie1712181.644.5−0.0122
32ProsnaMirków1255126.046.40.296 *2
33ProsnaPiwonice2938119.255.40.0912
34ProsnaBogusław4304114.348.20.1422
35NiesóbKuźnica Skakawska246123.862.8−0.238 *3
36OłobokOłobok447110.760.00.0933
37MogilnicaKonojad66377.352.80.1093
38WełnaPruśce113092.565.20.1873
39FlintaRyczywół27674.153.40.2223
40SamaSzamotuły39583.458.00.277 *3
41NotećNowe Drezdenko15,970143.253.70.2192
42GwdaPiła4704180.049.40.1811
43DrawaDrawsko Pomorskie609209.276.5−0.1322
44InaGoleniów2163185.566.60.1742
45RegaTrzebiatów2628239.442.40.2152
46ParsętaTychówko896289.148.00.245 *2
47WieprzaStary Kraków1519327.053.5−0.283 *1
48SłupiaSłupsk1450338.535.1−0.0021
49ŁupawaSmołdzino805325.744.8−0.333 *1
50WisłaSkoczów297641.039.3−0.0494
51WisłaGoczałkowice738381.044.7−0.0665
52WisłaJawiszowice971426.144.00.1135
53WisłaBieruń Nowy1748381.849.9−0.351 *4
54WisłaJagodniki12,058334.451.40.1354
55WisłaSzczucin23,901307.444.6−0.1854
56WisłaSandomierz31,846284.849.8−0.1844
57WisłaZawichost50,732261.945.4−0.2004
58WisłaAnnopol51,518261.844.3−0.1514
59WisłaDęblin68,234227.946.4−0.1794
60WisłaToruń181,033167.544.4−0.0502
61WisłaTczew194,376167.345.4−0.1242
62SołaOświęcim1386473.145.00.1354
63SkawaSucha Beskidzka468507.943.6−0.0954
64SkawaWadowice836465.047.90.0614
65RabaProszówki1470356.654.3−0.247 *4
66DunajecKrościenko1580634.945.2−0.360 *5
67DunajecNowy Sącz4341472.844.9−0.466 *4
68PopradMuszyna1514365.945.4−0.2204
69PopradStary Sącz2071380.545.6−0.1304
70BiałaKoszyce Wielkie957290.547.8−0.254 *4
71NidaBrzegi2259177.156.70.1372
72NidaPińczów3352168.253.90.2182
73Czarna NidaTokarnia1216171.450.20.0912
74CzarnaPołaniec1354149.244.80.1753
75WisłokaŻółków581384.742.70.1314
76RopaKlęczany483406.967.1−0.569 *4
77BrzeźnicaBrzeźnica484212.553.8−0.0664
78KoprzywiankaKoprzywnica502127.145.9−0.0103
79SanLesko1614555.055.4−0.586 *4
80SanPrzemyśl3686444.939.0−0.484 *4
81SanJarosław7041312.748.8−0.328 *4
82SanRadomyśl16,824241.654.0−0.440 *4
83OsławaZagórz505509.038.2−0.1134
84WiarKrówniki789252.749.0−0.1624
85WiszniaNienowice1185178.647.70.0164
86WisłokKrosno596327.847.9−0.475 *4
87WisłokŻarnowa1427287.349.0−0.3844
88WisłokRzeszów2086263.143.4−0.0644
89WisłokTryńcza3516227.751.1−0.2284
90MleczkaGorliczyna529173.245.3−0.0764
91TanewHarasiuki2034189.753.9−0.0302
92KamiennaWąchock476192.744.00.252 *2
93WieprzZwierzyniec405160.166.3−0.238 *1
94WieprzKrasnystaw3001129.859.0−0.440 *2
95WieprzLubartów6364111.758.8−0.1962
96WieprzKośmin10,231113.254.5−0.295 *2
97BystrzycaSobianowice1265125.659.2−0.334 *2
98PilicaPrzedbórz2536185.956.60.430 *2
99PilicaNowe Miasto6717166.950.80.415 *2
100PilicaBiałobrzegi8664160.449.90.1392
101WolbórkaZawada616137.544.0−0.0642
102DrzewiczkaOdrzywół1004166.944.00.285 *2
103NarewNarew1978146.056.9−0.521 *3
104NarewSuraż3376139.562.2−0.387 *3
105NarewStrękowa Góra7181141.459.8−0.332 *3
106NarewWizna14,308148.463.6−0.248 *3
107NarewPiątnica-Łomża15,296150.965.4−0.240 *3
108NarewNowogród20,106152.458.0−0.275 *3
109NarewOstrołęka21,862155.855.5−0.2063
110NarewkaNarewka590156.956.30.1673
111SupraślFasty1817153.348.6−0.258 *2
112BiebrzaSztabin846171.562.10.0053
113BiebrzaDębowo2322167.356.7−0.2263
114BiebrzaOsowiec4365160.062.2−0.0513
115BiebrzaBurzyn6900158.061.1−0.1923
116BrzozówkaKarpowicze650154.152.0−0.0623
117PisaPtaki3562181.668.5−0.0581
118PisaDobrylas4061182.468.2−0.0162
119RozogaMyszyniec231155.249.0−0.0242
120OmulewKrukowo1265170.049.90.243 *2
121OrzycKrasnosielc1268142.249.1−0.0562
122BugWłodawa14,410120.572.8−0.375 *3
123BugFrankopol31,336118.168.8−0.394 *3
124BugWyszków39,119122.565.3−0.347 *3
125WłodawkaOkuninka576116.561.3−0.2043
126KrznaMalowa Góra3128108.152.0−0.420 *3
127NurzecBoćki556134.044.8−0.0923
128NurzecBrańsk1227126.845.20.1923
129LiwiecŁochów2466134.143.0−0.1493
130DrwęcaNowe Miasto Lubawskie2725188.968.9−0.1442
131DrwęcaBrodnica3526192.563.90.0932
132DrwęcaElgiszewo4959173.769.40.1552
133WelKuligi764206.356.70.1271
134BrdaTuchola2462248.640.50.1431
135WdaCzarna Woda940210.757.4−0.261 *1
136WierzycaBrody Pomorskie1544175.049.40.0242
137ŁynaSępopol3647211.743.70.1332
138GuberProsna1568171.648.8−0.0412
139GołdapaBanie Mazurskie548266.954.1−0.0743
140Czarna HańczaCzerwony Folwark454262.664.6−0.0332
Note: * R statistically significant at p < 0.05; ** types of river flow regimes: 1—nival poorly developed; 2—nival moderately developed; 3—nival strongly developed; 4—nival-pluvial; 5—pluvial-nival. Source: adapted from [48].

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Figure 1. Geographical positions of meteorological stations and water gauges in the study area. Numbering was performed in accordance with Table A1.
Figure 1. Geographical positions of meteorological stations and water gauges in the study area. Numbering was performed in accordance with Table A1.
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Figure 2. The average values of hydro-meteorological elements ((A) temperature; (B) precipitation; (C) evaporation; (D) runoff) and trends of their changes in the multi-annual period 1951–2020.
Figure 2. The average values of hydro-meteorological elements ((A) temperature; (B) precipitation; (C) evaporation; (D) runoff) and trends of their changes in the multi-annual period 1951–2020.
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Figure 3. Changes in the average values of hydro-meteorological elements ((A) temperature; (B) precipitation; (C) evaporation; (D) runoff) in the warming period after 1988 compared to in the period 1951–1988 and their statistical significance.
Figure 3. Changes in the average values of hydro-meteorological elements ((A) temperature; (B) precipitation; (C) evaporation; (D) runoff) in the warming period after 1988 compared to in the period 1951–1988 and their statistical significance.
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Figure 4. The average numbers of NDLF days and their trends in the multi-annual period 1951–2020 and the two sub-periods before and after climate warming.
Figure 4. The average numbers of NDLF days and their trends in the multi-annual period 1951–2020 and the two sub-periods before and after climate warming.
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Figure 5. Correlations between annual NDLF and air temperature (A), evaporation (B), and precipitation amount (C) and their statistical significance (p).
Figure 5. Correlations between annual NDLF and air temperature (A), evaporation (B), and precipitation amount (C) and their statistical significance (p).
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Figure 6. Dendrogram of water gauge grouping due to NDLF (Q = 0.1) in subsequent years of the multi-annual period 1951–2020 with a graph of the linkage distance curve. Numbering was performed in accordance with Appendix A.
Figure 6. Dendrogram of water gauge grouping due to NDLF (Q = 0.1) in subsequent years of the multi-annual period 1951–2020 with a graph of the linkage distance curve. Numbering was performed in accordance with Appendix A.
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Figure 7. The course of the number of days with low flows in a year in individual groups.
Figure 7. The course of the number of days with low flows in a year in individual groups.
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Figure 8. The geographical positions of water gauges in groups distinguished with the use of Ward’s hierarchical grouping method against the background of changes in the average NDLF in the hydrological year in the warming period after 1988 compared to in the period 1951–1988.
Figure 8. The geographical positions of water gauges in groups distinguished with the use of Ward’s hierarchical grouping method against the background of changes in the average NDLF in the hydrological year in the warming period after 1988 compared to in the period 1951–1988.
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Figure 9. The course of the area-averaged number of days with low flows per year (NDLF) in SE Poland (group A) with linear trends in the years 1951–1988 and 1988–2020 (days per year). Trend values and standard errors of their estimation (in brackets, following the trend value) are shown in frames.
Figure 9. The course of the area-averaged number of days with low flows per year (NDLF) in SE Poland (group A) with linear trends in the years 1951–1988 and 1988–2020 (days per year). Trend values and standard errors of their estimation (in brackets, following the trend value) are shown in frames.
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Figure 10. The course of the area-averaged number of days with low flows per year (NDLF) in N and NE Poland (group B1) with linear trends in the years 1951–1988 and 1988–2020 (days per year). Trend values and standard errors of their estimation (in brackets, following the trend value) are shown in frames.
Figure 10. The course of the area-averaged number of days with low flows per year (NDLF) in N and NE Poland (group B1) with linear trends in the years 1951–1988 and 1988–2020 (days per year). Trend values and standard errors of their estimation (in brackets, following the trend value) are shown in frames.
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Figure 11. The course of the area-averaged number of days with low flows per year (NDLF) in W and SW Poland (group B2) with linear trends in the years 1951–1988 and 1988–2020 (days per year). Trend values and standard errors of their estimation (in brackets, following the trend value) are shown in frames.
Figure 11. The course of the area-averaged number of days with low flows per year (NDLF) in W and SW Poland (group B2) with linear trends in the years 1951–1988 and 1988–2020 (days per year). Trend values and standard errors of their estimation (in brackets, following the trend value) are shown in frames.
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Figure 12. Changes of the correlation coefficients between NDLF and air temperature (T), precipitation (P), and evaporation (Ev) in subsequent 20-year periods of the multi-annual period 1951–2020 in three distinguished groups.
Figure 12. Changes of the correlation coefficients between NDLF and air temperature (T), precipitation (P), and evaporation (Ev) in subsequent 20-year periods of the multi-annual period 1951–2020 in three distinguished groups.
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Table 1. Limit values of correlation coefficients for various levels of statistical significance for the periods studied.
Table 1. Limit values of correlation coefficients for various levels of statistical significance for the periods studied.
Significance Level (p)1951–2020 (70 Years)1951–1988 (38 Years)1988–2020 (33 Years)
0.050.2320.2130.334
0.010.3020.4030.430
0.0010.3800.5010.532
Table 2. The average numbers of days with low flows per year (NDLF) in individual groups and statistical characteristics of their differentiation (1951–2020).
Table 2. The average numbers of days with low flows per year (NDLF) in individual groups and statistical characteristics of their differentiation (1951–2020).
GroupMeanBSSStandard Deviation (σ)MedianMin.Max.Lower QuartileUpper Quartile
A (SE Poland)36.073.4729.0530.610.42127.9713.4249.70
B1 (N Poland)35.984.1935.0521.830.15136.2810.7052.38
B2 (SW Poland)35.563.9737.2525.361.21124.638.9658.60
Note: mean—the average annual NDLF in a given group; BBS—standard error of a given mean (days); min. and max.—the minimum and maximum NDLF in a given group, respectively; lower quartile and upper quartile—values corresponding to 25 and 75% of the series of the NDLF, respectively; median—middle value of the set.
Table 3. Average numbers of days with low flows (NDLF) in the areas of occurrence of group A and sub-groups B1 and B2 in the years 1951–1988 (before climate change) and 1988–2020 (after climate change).
Table 3. Average numbers of days with low flows (NDLF) in the areas of occurrence of group A and sub-groups B1 and B2 in the years 1951–1988 (before climate change) and 1988–2020 (after climate change).
Group1951–19881988–2020
Average NDLFBSSAverage NDLFBSS
A (SE Poland)42.175.2928.813.86
B1 (N Poland)36.426.1034.405.66
B2 (SW Poland)27.554.3744.096.53
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Wrzesiński, D.; Marsz, A.A.; Styszyńska, A.; Perz, A.E.; Brzezińska, W.; Sobkowiak, L. Spatial Regularities of Changes in the Duration of Low River Flows in Poland Under Climate Warming Conditions. Water 2025, 17, 243. https://doi.org/10.3390/w17020243

AMA Style

Wrzesiński D, Marsz AA, Styszyńska A, Perz AE, Brzezińska W, Sobkowiak L. Spatial Regularities of Changes in the Duration of Low River Flows in Poland Under Climate Warming Conditions. Water. 2025; 17(2):243. https://doi.org/10.3390/w17020243

Chicago/Turabian Style

Wrzesiński, Dariusz, Andrzej A. Marsz, Anna Styszyńska, Adam Edmund Perz, Wiktoria Brzezińska, and Leszek Sobkowiak. 2025. "Spatial Regularities of Changes in the Duration of Low River Flows in Poland Under Climate Warming Conditions" Water 17, no. 2: 243. https://doi.org/10.3390/w17020243

APA Style

Wrzesiński, D., Marsz, A. A., Styszyńska, A., Perz, A. E., Brzezińska, W., & Sobkowiak, L. (2025). Spatial Regularities of Changes in the Duration of Low River Flows in Poland Under Climate Warming Conditions. Water, 17(2), 243. https://doi.org/10.3390/w17020243

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