Seasonal and Regional Patterns of Streamflow Droughts in Poland: A 50-Year Perspective

Abstract

Hydrological drought in Central Europe is becoming an increasingly serious threat to agriculture, industry, and people due to climate change and the rising frequency and intensity of extreme weather events. The main aim of the paper was to assess the spatial variability of streamflow drought in Poland. The spatial analysis was conducted using daily streamflow series from 340 gauging stations for the period 1973–2022. Hydrological drought was defined as a period with a streamflow lower than Q_90%. The results show that, on average, hydrological droughts occur 52 times per year at a given gauging station. Drought duration and volume depend on the gauge elevation. At higher-altitude stations, shorter and smaller-volume droughts are most commonly observed. The longest droughts are recorded in Northern Poland, particularly in the Lakeland regions, which is a serious problem mainly for the agriculture sector. Hydrological droughts in Poland most frequently begin in summer and end in late summer or early autumn. Analyses showed that hydrological drought has a strong spatial distribution, and it is possible to identify five main regions with homogeneous drought duration and volume. Trend analysis of the annual number of low-flow days indicates no statistically significant trend at 46% of stations, while 54% exhibit statistically significant increases, with marked regional variability. The highest number of stations with statistically significant decreasing trends occurs in the Southern and Eastern Baltic Lake District and in the Central Poland Lowlands and Highlands with Polesie. The study highlights the necessity of enhancing water retention, particularly in the central, lowland regions of Poland.

1. Introduction

Drought is a natural hazard, but one of the most complex hydroclimatic hazards, since it is very difficult to quantify its severity and to assess its effects, given the large number of systems affected [1]. As mentioned in the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC), drought is defined as “a period of abnormally dry weather long enough to cause a serious hydrological imbalance” [2]. This report noted an increase in meteorological, agricultural, and hydrological droughts in dry regions [3]. The Special Report on Global Warming of 1.5 °C assessed that limiting global warming to 1.5 °C is expected to substantially reduce the probability of extreme droughts, precipitation deficits, and risks associated with water availability (i.e., water stress) in some regions [4].

The defining of drought and measuring its severity makes it difficult to determine the beginning and end of a drought and the spatial extent of the event [5,6]. It is also challenging to define because of the complexity of the spatial and temporal drought propagation processes [7].

The most commonly used definition of drought is the traditional one, which states that it is a process consisting of the following stages: (a) atmospheric (metrological), which is related to deficits in precipitation combined with increases in evapotranspiration; (b) agriculture (or soil), which occurs when there is insufficient water in the ground for plant growth and vegetation, which can lead to the destruction of crops and increase the probability of forest fires; and (c) hydrological drought, which is associated with the reduction of the amount of water stored in the soil and the reduction of the flow in rivers, reducing the water supply for drinking water, irrigation, industrial needs, and hydro-power production, causing hampered navigation [5,8,9,10]. Many authors additionally distinguish the fourth phase of drought development—socio-economic drought [6,11].

This paper contains analyses of hydrological drought, which is a period when low water levels and streamflow persist in rivers, caused by long-lasting lack of precipitation, intensive evaporation (summer droughts), or long-lasting frost (winter droughts) [6,12,13]. Drought indices describing hydrological droughts use a variety of variables related to surface and groundwater amounts, such as, for example, the standardized streamflow index SSI [14] and the standardized runoff index SRI [15], both calculated with streamflow Q or the standardized groundwater level index SGI [16] based on groundwater levels. Since some snow is trapped as snow in winter, some indices describing hydrological drought use information on snow cover, e.g., the surface water supply index SWSI [17], based on, among other things, the snow water equivalent and the standardized snowmelt; the rain index SMRI [18], based on snow storage and P; and the standardized snow water equivalent index SWEI [19], based on SWE.

Since the variables needed to determine the above indicators are often difficult to access, the classical approach using streamflow drought is often used to characterize hydrological droughts [20].

Additional assumptions are sometimes used when defining streamflow drought episodes. Since a short, several-day period with a discharge lower than the critical discharge may not be significant as a period of water shortage, some authors assume a minimal duration of streamflow drought events [21]. Additionally, Fleig et al. [22] and Tallaksen et al. [23] reported that, if the time period between adjacent streamflow drought events is very short, they have the same cause and can be combined into one.

Hydrological drought in Poland and Central Europe is becoming an increasingly serious threat due to climate change and the rising frequency and intensity of extreme weather events. Studies indicate a systematic increase in air temperature and precipitation variability, leading to a reduction in water resources in rivers and water reservoirs [24].

The intensification of drought in Central Europe has been driven by a long-term slowdown of the Atlantic Meridional Overturning Circulation (AMOC) [25] and high-pressure anomalies [26]. A similar relationship between the intensification of atmospheric drought and atmospheric circulation associated with high-pressure systems over Southern Poland was described by Wałęga et al. [27]. Tokarczyk and Szalińska [28] report that, depending on the characteristics of the catchment, the recurrence interval of dry and extremely dry conditions affecting hydrological drought risk is approximately once every eight years. The effects of climate change may contribute to more frequent droughts in the winter and early spring seasons, when snowmelt may decrease. As reported by Somorowska [29], as a result of warming during the cold season, snow-dominated areas in Poland could shrink from covering 86% to only 30% of the country. Climate change exacerbates the impacts of hydrological drought, particularly concerning the ecological status of rivers, including reduced transport of nutrients and sediments, which negatively affects the nutrient balance for aquatic organisms [30]. The dynamics of drought phenomena are further complicated by their complex nature, particularly in mountainous regions, due to geological structure, land use, and precipitation patterns [31].

Unfortunately, there is a lack of research on the spatial variability of hydrological drought across the entire country based on long-term measurement series. Most studies have focused only on selected regions or individual catchments [32,33,34,35] and have primarily relied on drought indices, such as the Standardized Precipitation Index SPI or the Standardized Runoff Index SRI, rather than on threshold values for low flows. While these indices are valuable tools for drought monitoring, incorporating information on drought duration and low-flow deficits not only illustrates the scale of the drought threat but also provides critical insights for river management authorities regarding potential water deficits and the risk of failing to meet water demands for populations, agriculture, industry, and ecosystems.

The main aim of the paper was to assess the spatial variability of streamflow drought in Poland. To achieve this goal, the basic parameters of streamflow drought were determined in 340 water gauge cross-sections located throughout Poland, such as the number of streamflow drought episodes, the average D_mean, the median D_med, and the sum D_sum drought duration and the average V_mean, the median V_med, and the sum V_sum drought volume, as well as the longest drought duration D_max in a given cross-section and the drought with the largest volume V_max (at a given cross-section). The spatial distributions of those drought characteristics can be used as maps indicating drought risk areas in the studied region. The paper also determined in which months in Poland hydrological droughts most frequently begin and end, as well as the months in which the longest droughts start and finish. Additionally, trends in the annual number of hydrological drought days across Poland were analyzed against the background of physiogeographic regions. The work is a continuation and, above all, an extension of the article by Baran-Gurgul [36], in which the spatial variability of hydrological droughts in the Polish Carpathians was analyzed. At the end of the study, using the cluster analysis method, the regional occurrence of hydrological droughts was examined with regard to the simultaneous occurrence of streamflow drought at water gauge cross-sections in Poland.

2. Study Area and Data

Poland lies at the physical center of the European continent, approximately between latitudes 49° and 55° N and longitudes 14° and 24° E. The area of the country is inclined from southeast towards northwest [37]. According to Solon et al. [38], Poland spreads within six basic physiographic regions (from the north): Central European Lowland (with Coastlands, Lakelands, Lowlands), Eastern Baltic–Belarus Lowland, Sudety Mts with Sudety Foreland, Polish Uplands, Subcarpathians, Carpathians, and Ukrainian Uplands (Figure 1).

Streamflow droughts are primarily the result of a long-term lack of precipitation. Other meteorological factors that may influence the reduction of the flow in the river are those that influence the amount of evaporation: solar radiation, wind speed, and air temperature [39].

The climate in Poland, in particular precipitation and temperature, depends on the altitude. In general, as altitude increases, the temperature decreases, and the climate becomes more humid.

In the Coastal Areas, Lowlands, and Subcarpathia, the average area temperature is above 8 °C, while in the Lake Districts and Highlands, it is slightly cooler (below 8 °C) [40]. The coldest areas in Poland are the mountains: the Sudetes and the coldest region of the country, the Carpathians, with an average annual temperature on Kasprowy Wierch of 0.8 °C. The average annual area precipitation in Poland is slightly above 600 mm, exceeding 700 mm in the Outer Western Carpathians and the Sudetes, and 1000 mm in the Central Western Carpathians. The lowest values of average area precipitation occur in Central Poland: in the area of the Central Polish Lowlands and Polesie (around 550 mm). The highest sunshine duration in Poland is in the central–eastern part of the country, while the lowest is in the mountains in the south of the country.

The Lowlands are located in North and Central Poland, whereas the mountains and the Highlands are in the south. Lowland terrain dominates in Poland—about 75% of its area is located below 200 m.a.s.l.

For this reason, most of the gauges (246 out of 340) were located in the lowlands, of which one is below sea level (Tczew on the Vistula at a height of −0.496 m.a.s.l.). Eighty-five gauges were highland cross-sections (located at the altitudes between 200 and 500 m.a.s.l.), whereas the remaining nine are located in the mountains.

The data utilized in this paper was obtain from the Institute of Meteorology and Water Management—National Research Institute (IMWM-NRI) in form of daily flows (which based on water stage measured once a day at 6 UTC) series from the period between 1 November 1973 and 31 October 2022 (50 hydrological years) at 340 gauging cross-sections located within the area of Poland (Figure 1). Each series included 18,262 daily flows. Data are available at: https://danepubliczne.imgw.pl, accessed on 1 July 2024.

3. Methods

In this article, streamflow drought was defined using the POT method (Peak Over Threshold) [9,41], with Q_90% as the threshold level, and a drought event starts when the daily discharge falls below the Q_90% and ends when the discharge rises above that threshold and remains above it for ≥3 consecutive days (inter-event time criterion, ITC). Short exceedances (<5 days) are therefore treated as within-event breaks, and the deficit is cumulated over the entire continuous period below Q_90%. The methodology for preparing Drought Effect Counteracting Plans in Poland [42] considers three threshold flows (Q_70%, Q_90%, and Q_95%), where drought, defined by the Q_90% level, is referred to as deep drought (indicating an emergency state). For defining hydrological drought, Tallaksen and van Lanen [9] recommend using threshold flows from Q_70% to Q_95%. In this work, the Q_90% flow threshold is used to link with the actual preparation of the updated version of the drought risk assessment in Poland. The Q_90% flow roughly corresponds to the mean flow calculated from the minimum annual flows in a multiannual period [43]. Exceedance probability, which is often used in Polish legal regulations, is the basis for calculating, among others, the fees for water services and consumption, the minimum required flow, and the ecological flow [44]. Furthermore, the Q_90% flow is often used in determining environmental flow characteristics [45]. Therefore, the identified patterns of drought occurrence will also relate to the environmental conditions associated with ensuring the minimum biodiversity of aquatic organisms.

Based on a multi-annual daily flow series Q_t at a given gauging cross-section, the time series t_s,i and t_e,i were determined, which indicate the start t_beg and the end t_end of the drought (i = 1, 2, …), thus helping define different drought characteristics:

-

the duration D_i of drought i:

$$D_i=t_{end,i}-t_{beg,i}+1\left[\mathrm{day}\right]$$

(1)

-

the volume of drought V_i (shortage or deficit volume):

$$V_t=\sum _{t=t_{beg,i}}^{t_{end,i}}\left(Q_g-Q_t\right)/Q_m[\mathrm{day}]$$

(2)

where Q_t is the daily flow at time t, Q_g—threshold flow, and Q_m—mean daily flow over a multi-annual period.

Measuring the volume of a drought in days (2) shows the number of days needed to fill up a drought with the mean flow at a given cross-section, which allows for the comparison of volumes at various gauging cross-sections. This approach has already been used in the works of Baran-Gurgul [36,46].

In this paper, it is assumed that the minimum duration of a drought cannot be shorter than two days, and if two droughts occurred at a distance of less than 3 days, they were combined into one episode (considered to have a single cause for the occurrence of these droughts).

After determining hydrological flow in all analyzed water gauge cross-sections, calculated were the number of streamflow drought episodes, the average D_mean and median D_med drought duration, the average V_mean and median V_med drought volume, the longest drought duration D_max in a given cross-section, and the drought with the largest volume V_max (at a given cross-section). The calculation results were plotted on appropriate maps. Such spatial distributions can be used as maps that indicate drought risk areas in the studied region. The paper also presents information on which months in Poland hydrological droughts most frequently begin and end, as well as the months in which the longest droughts start and finish.

Spearman’s monotonic correlation coefficient was used to check the dependence of streamflow drought parameters on altitude.

From the perspective of assessing the impact of climatic and anthropogenic changes on low river flows, the temporal variability of X_t—defined as the annual total number of days experiencing hydrological drought—was analyzed. The null hypothesis was verified, that there is no trend in X_t over time t, against the alternative hypothesis proposing a decreasing trend. To evaluate this, the Mann–Kendall test with the Hamed and Rao correction for autocorrelation was used [47,48,49]. Given the multiple Mann–Kendall tests performed simultaneously, it was necessary to control the false positive rate. Therefore, the Bonferroni correction [50] was applied to maintain the overall probability of at least one false positive below the chosen significance level. For those cross-sections where a trend was detected, the slope of the regression line was estimated using the Theil–Sen estimator [51,52].

This article also applies cluster analysis to examine regional drought patterns in terms of the simultaneous occurrence of hydrological drought at gauging stations in Poland. Following Stahl [8], the binary Euclidean distance was used as a measure of cluster separation, and hierarchical clustering was performed using Ward’s method, which minimizes distances between clusters. The variables used for clustering in this study include a series of binary daily flow deficit indicators (where D_I = 1 indicates the occurrence of hydrological drought, and D_I = 0 indicates no drought) at measurement cross-sections in Poland. To determine the optimal number of clusters, the elbow method and dendrogram analysis were employed.

All statistical calculations were performed using the GNU R version 4.5.1 (Great Square Root) software package [53]. For all the tests considered in the paper, the significance level α = 0.05 was assumed.

4. Results

4.1. The Number of Streamflow Droughts

As previously mentioned, in this article, streamflow drought is defined as a period of time with a flow lower than the threshold flow equal to the probability flow Q_90% read from the flow duration curve. These flows range from 0.05 m³/s (at the Kuźnica Sulikowska cross-section on the Mitręga River) to 507 m³/s (at the Tczew cross-section on the Vistula River) (Figure 2a). The Q_t = Q_90% flows are highly, positively, and statistically significantly correlated with the catchment areas enclosed by these gauging cross-sections (Spearman’s correlation coefficient is 0.901) (Figure 2b). The distribution of Q_90% is asymmetric, with a mean Q_t of 18.16 m³/s differing from the median Q_t of 1.92 m³/s. The Q_g flow also depends on the elevation of the gauging station. Q_g decreases with elevation (Spearman’s monotonic correlation coefficient is −0.336), and this relationship is statistically significant at the significance level α = 0.05 (Figure 2c).

Based on the adopted threshold flows across all cross-sections in Poland, hydrological droughts were identified. The distribution of the number of streamflow drought events is right-skewed, with an average of 52 low-flow events observed at a given cross-section (Figure 3a). The number of streamflow drought events increases with the elevation of the zero-gauge elevation (r_s = 0.581) and slightly (though still statistically significantly) decreases with the increasing catchment area—Spearman’s correlation coefficient r_s = −0.161 (Figure 3b,c).

4.2. Duration and Volume of Streamflow Droughts

Assuming the threshold flow Q_90%, hydrological droughts in Poland between 1973 and 2022 had an average regional duration D_mean of 32.2 days (with a median regional duration D_med of 29.9 days) (Figure 4). The total duration of droughts over the study period ranged from 817 to 1929 days (with an average of 1564 days) at individual gauging cross-sections. Both the mean regional duration and the total duration of droughts significantly decrease with an increasing zero-gauge elevation (Spearman’s correlation coefficients are −0.538 and −0.291, respectively).

Another important parameter characterizing a hydrological drought is its volume. The average areal drought volume amounts to 2.25 days (with an average areal median of 2.03 days), while the total drought volume over the multi-year period at gauging cross-sections ranges from 38.3 days to 178.7 days (with a mean of 106.6 days) (Figure 5). Both the mean drought volume V_mean and the total volume V_sum show a significant decrease with the elevation of the gauge (Spearman’s rank correlation coefficients r_s are −0.581 and −0.467, respectively).

The distributions of the total values of the analyzed drought parameters show that D_sum and V_sum are close to symmetric, whereas the distributions of the mean durations D_mean and the mean volumes V_mean are right-skewed. The vast majority of gauging cross-sections (288 out of 340) are characterized by droughts, with an average duration of less than 30 days and an average volume of less than 2 days, whereas only four cases exhibit extremely long mean drought durations (exceeding 50 days) and very large mean volumes (greater than 3.5 days).

The longest average hydrological droughts and those with the largest mean volumes (but with the lowest D_max and V_max) are observed in the northern part of Poland—primarily in the lowest-lying Lakeland zone (Figure 6). Moving southward, droughts become shorter and their volumes smaller. However, two exceptions to the general trend of ‘the higher the elevation, the shorter and smaller the droughts’ can be noted. The first exception is the Coastal areas in the northwestern corner of the country—despite being low-lying, these regions experience shorter and less voluminous droughts on average compared to the Lakelands. The second exception involves the mountainous areas (the Sudetes in the southwest and the Carpathians in the south), where droughts are longer and have larger volumes than those observed in the uplands and the Subcarpathian region.

4.3. Drought Start and End Time

Hydrological droughts in Poland during the studied multi-year period most commonly began in summer (June, July, and August) and ended at the turn of late summer and early autumn (mainly in August and September) (Figure 7a).

The most critical droughts in terms of water shortages for agriculture, municipal services, and industry were the longest-lasting ones. In Poland, these typically began in June and ended in October, November, or December (Figure 7b). At some cross-sections located in the highest mountain areas (in the Tatra Mountains, Southern Poland), these longest droughts were predominantly autumn–winter events—usually starting in October and ending in January, February, or March (Figure 8).

4.4. Regionalization of Streamflow Droughts

The application of cluster analysis to determine homogeneously uniform areas resulted in the agglomeration of the studied stations into clusters. As illustrated in Figure 9, the elbow method indicates that the optimal number of clusters is five, as further increases in cluster number result in only marginal reductions in the within-cluster sum of squares (WSS). The locations of the stations forming the individual clusters, identified using Ward’s method, are shown in Figure 10. The analysis revealed the existence of five distinct clusters within the dataset. Cluster 1 (blue color in Figure 10) includes 66 gauging stations, located in the upper Vistula catchment area, in the region of the Carpathian Foothills and the Sandomierz Basin. Stations forming clusters 2 and 5 are located in the mountains: 11 stations are in the Tatra Mountains (cluster 5, red color), while 42 stations are in the Sudetes and their foothills (cluster 2, green color). Stations located in other areas of Poland belong to cluster 3 (yellow color, Lowlands and Uplands, 141 stations) and cluster 4 (orange color, Coastal and Lakeland areas, 80 stations).

The first cluster is characterized by the shortest average drought duration, which is below 14 days, and the smallest drought volume. It includes catchments located in the southeastern part of Poland, which are right-bank tributaries of the Vistula River. A characteristic feature of the rivers in this cluster is that the catchments are situated at altitudes exceeding 300 m above sea level, meaning they are submontane and upland areas. Many gauging stations in this cluster are located on large Vistula tributaries, such as the San and Dunajec rivers. The second cluster is characterized by the shortest total drought duration and relatively small drought volume. It is located in the Sudetes, in the southwestern part of Poland. In this cluster, the average drought duration is nearly 18 days, with an average volume of around 0.9 days. The third cluster, the largest in terms of the number of stations, includes the central–eastern part of Poland and, sporadically, the western part, primarily in the Warta River basin. Compared to the previous two clusters, the drought duration is notably longer. In this cluster, the total drought duration is the highest, exceeding 3000 days. Additionally, this cluster also records the largest total drought volume, amounting to more than 180 days. The catchments in this cluster are characterized by relatively small topographical variation, with an average elevation of about 150 m above sea level. The fourth cluster primarily includes the northern part of Poland, including catchments in the coastal areas and regions around the Great Masurian Lakes in the northeastern part of the country. This cluster is characterized by the longest average drought duration and the largest average drought volume. A clear correlation between drought duration and the location of gauging stations can be observed—these catchments are situated at the lowest altitudes, below 100 m above sea level. The last, fifth cluster comprises the smallest number of catchments, located in the Tatra Mountains. These catchments are characterized by the highest elevation, with an average altitude exceeding 500 m above sea level. At the same time, a reduction in total drought duration and volume is observed, reaching levels similar to those of the second cluster. It can thus be concluded that there are certain similarities in the processes that lead to the formation of droughts in high-mountain catchments belonging to the second and fifth clusters.

4.5. Temporal Variability of the Annual Sum of Days with Hydrological Drought

No statistically significant trend in the number of hydrological drought days was found in 46% of the gauging cross-sections. In most cross-sections (174 out of 340), regardless of their location, river size, or elevation, the statistically significant trends detected (marked with colored triangles on the map, Figure 11a) indicate an increase in the number of hydrological drought days in Poland. The Theil–Sen slope values range from 0.05 to 1.76 days per year, with an average of 0.7 days per year.

A decreasing trend in drought days (blue and green triangles on Figure 11a) was observed in 10 cross-sections—mainly located in the Dunajec (six cross-sections) and Ropa (two cross-sections) river basins within the Carpathian Mountains region, as well as one cross-section in the Southern and Eastern Baltic Coastlands region and one in the Central Poland Lowlands and Highlands with Polesie region. The highest number of increasing trends of drought days was observed in the Southern and Eastern Baltic Lake District and Central Poland Lowlands and Highlands with Polesie. The lowest number of cross-sections with increasing trends were observed in south part of Poland (Carpathian Mts, Sudety Mts.) and in north part (Southern and Eastern Baltic Coastlands)

5. Discussion

Spatio-temporal variability analysis of hydrological droughts in Poland reveals significant differences in parameters such as frequency, duration, and volume of droughts, depending on the geographical location and the altitude above sea level. According to the presented results, the number of droughts increases with the elevation of the catchment area, suggesting that higher areas are more susceptible to drought occurrences. This is consistent with observations that, in mountainous and foothill areas, due to steep terrain, precipitation runoff quickly occurs on the surface, limiting infiltration into the ground and groundwater recharge [54]. As a result, during dry periods, river flows rapidly decrease, leading to more frequent droughts.

It was observed that both the mean drought duration D_mean and the total drought duration D_sum decrease with increasing catchment altitude. A similar relationship was observed for drought volumes V_mean and V_sum, indicating that, in higher catchments, droughts are shorter and have smaller volumes. This can be explained by the quicker response of mountain rivers to precipitation and the shorter concentration time of runoff, which results in shorter periods of low flows [55]. Considering that agriculture is less intensive in higher catchments in Poland, frequent droughts present a problem for ensuring the required water supply for the population, especially in smaller catchments [56].

The longest and largest droughts are observed in the northern part of Poland, particularly in the Lakeland areas. This is due to the flat terrain and the presence of numerous lakes and wetlands, which perform a retention function, delaying water runoff and extending periods of low flows. In these regions, prolonged droughts pose a challenge to the agricultural sector [57], which is highly sensitive to water deficits and influenced by unfavorable soil conditions, such as sandy soils with low retention capacity. In contrast, in mountainous areas like the Sudetes and the Carpathians, despite the higher elevation, droughts are longer and have larger volumes compared to the uplands and the Subcarpathian region. This is associated with high forest cover, combined with recent investments in retention restoration and runoff attenuation in forested areas [58].

Streamflow droughts in Poland most commonly begin in summer (June–August) and end at the turn of late summer and early autumn (August–September). The longest droughts start in June and end in October, November, or December. In mountain catchments, especially in the Tatra Mountains, the longest droughts are autumn–winter events, starting in October and ending in January, February, or March. This seasonality is related to the distribution of precipitation and air temperature, which influences evaporation and water retention in the catchments. As noted by Hejduk et al. [13], the total annual drought volumes and the number of drought days significantly increase, indicating a substantial and relatively lasting seasonal change in the river discharge structure during low-flow conditions in the summer. Furthermore, this seasonality is determined by substrate characteristics, such as soil permeability [59].

Cluster analysis using Ward’s method allowed for the identification of five homogenous regions in terms of hydrological drought parameters in Poland. The results of this regionalization point to significant temporal and volumetric variability of droughts, which can be linked to the topographic, geological, and hydrological characteristics of the respective areas. The first cluster, encompassing the Foothills and Uplands (Carpathians and Sandomierz Basin), is characterized by the shortest average drought duration. This is influenced by the area’s elevation (300–500 m a.s.l.), which promotes rapid runoff of precipitation and shorter periods of low flow. The steep slopes limit infiltration and enhance surface runoff [60]. Somewhat different characteristics were found in clusters representing mountainous regions, such as the Sudetes and the Sudeten Foreland, as well as the Tatra Mountains. In these areas, the shortest total drought duration and relatively low drought volume were observed. Factors influencing these results may include topography that favors rapid surface runoff, low retention capacity of the substrate, and variable weather conditions. As reported by Nadudvari et al. [61], precipitation patterns—and, consequently, the hydrological regime of rivers in this region—are influenced by the North Atlantic Oscillation (NAO), as well as by intensive hydrotechnical infrastructure. Moreover, the markedly longer and more voluminous stream-flow droughts documented in the Sudetes and Outer Carpathians can be explained by the interplay of lithology-controlled groundwater storage, spring–stream connectivity, and land-surface processes, rather than by forest cover and recent retention projects alone. Crystalline and metamorphic rocks in the Sudetes and flysch sand and mud stones in the Carpathians form fracture or perched aquifers whose dynamic storage is shallow and rapidly depleted; base-flow studies and spring hydrographs show that the seven-day minima decline within weeks of a rainfall deficit in such settings [62]. Weekly monitoring in the headwaters of the Dunajec further reveals near-synchronous (0–2 week lag) drops in high-discharge fissure–spring yield and river discharge during severe droughts, underscoring the absence of a long groundwater buffer [31]. By contrast, many Polish uplands underlain by thick loess mantles, karstic Upper-Jurassic limestones, or Quaternary sand/gravel aquifers possess larger, slowly draining groundwater reserves that prolong recession times and shorten drought episodes despite comparable meteorological forcing [63]. High forest cover (>60%) in the Sudetes and Western Carpathians intensifies evapotranspiration and interception losses, hastening soil-moisture draw-down, while recent small retention structures moderate peak flows but add little long-term storage because the controlling aquifers remain thin and coarse-fractured. Consequently, the lithology–groundwater framework is the primary driver of the observed mountain–upland contrast in hydrological drought characteristics, with vegetation and retention works acting as secondary modifiers.

Despite high annual precipitation in cluster 2 (exceeding 700 mm), Sudeten rivers experience frequent but low-volume droughts ($\overline{\mathrm{D}}$ ≈ 18 days; $\overline{\mathrm{V}}$ ≈ 0.9 days). Crystalline and metamorphic bedrock hosts shallow fissure aquifers; spring hydrographs fall within two weeks of rainfall deficit, offering little base-flow support [64,65]. Dense forest cover (exceeding 60%) augments interception loss and transpiration, accelerating soil-water draw-down. The Nysa cascade (Otmuchów–Nysa reservoirs) is operated mainly for flood control; summer drawdowns often exacerbate downstream low flows. Blocking highs over the Bohemian Massif commonly divert Atlantic moisture, synchronizing meteorological and hydrological drought peaks [64].

The largest group includes cluster 3, encompassing catchments mainly in the Warta River basin and parts of Central Poland. This region exhibited the longest cumulative drought duration (over 3000 days) and the highest total volume (over 180 days). The terrain is highly diversified, generally undulating with distinct land elevations. The geological structure plays a role in the persistence of droughts, as the area is composed of sedimentary rocks (sandstones, conglomerates, limestones, marls, etc.) [66] that influence slow changes in the hydrological regime and the progression of droughts. Thick glacio-fluvial aquifers supply base flow, but once depleted, they recharge slowly, prolonging deficits. The region also has the lowest mean annual precipitation in Poland (approximately 550 mm) and the highest sunshine hours. Major abstractions for agriculture and industry, together with the operation of large reservoirs (Jeziorsko on the Warta, Sulejów on the Pilica), intensify summer low flows [67]. Channelization and drainage works have further accelerated runoff and reduced groundwater–river connectivity. The longest average drought durations and greatest drought volumes occurred in the Lakeland and Coastal regions. These features are caused by low flow dynamics due to gentle slopes and high permeability of the ground. This area consists mainly of plains and hills with elevations up to 100 m a.s.l., formed of moraine material. The terrain is shaped by ongoing processes such as wind, river, and sea activity, as well as post-glacial geomorphology. An additional factor influencing drought duration is the presence of lakes. The dense lake network attenuates peak flows but sustains only modest outflow during rain-free periods, thereby prolonging low-flow conditions. Persistent coastal winds and high sunshine enhance open-water and soil evaporation. Historic peatland drainage and polderisation (e.g., Notec valley) have lowered water tables by up to 1.5 m, accelerating early-summer flow decline. Canal transfers for navigation locally mitigate deficits but reduce flows elsewhere [68].

Regarding the temporal dynamics of the annual number of low-flow days, the analysis indicates no statistically significant trend at 46% of gauging stations, while 54% exhibit statistically significant increases. This pattern suggests that recent changes in Poland’s low-flow regime are driven primarily by rising evapotranspiration and a more negative precipitation–evapotranspiration (P–ET) balance rather than by systematic increases in annual precipitation. Polish studies indicate that following the circulation shift around 1987–1989, air temperature and sunshine increased while relative humidity declined—without a clear upward trend in annual rainfall—leading to more frequent low-flow days, particularly in Central and Western Poland [69]. Since 1988, stronger westerlies and more frequent positive NAO phases have favored anticyclonic conditions over Western and Central Poland, which helps to explain widespread positive trends there and weaker or even negative trends on some southern and eastern tributaries of the Vistula that receive more orographic precipitation. In the mountain regions (the Carpathians and the Sudetes), and locally in Central Poland, decreasing trends in low-flow days may reflect flow regulation by large reservoirs [70] and a changing role of snow and spring recharge [71]. Across the Central Lowlands and Uplands, positive trends are most numerous and pronounced, consistent with declining SPEI and a negative P–ET balance since the late 1980s. In regions with dense lake networks and historically drained valleys (melioration, polders), lakes and wetlands dampen peaks but lengthen recessions, favoring longer low-flow periods, especially under summer–autumn rainfall deficits and high open-water evaporation. Long-term lake-level records show highly heterogeneous trends and a strong local anthropogenic signal (drainage, regulation), which can obscure climate signals—consistent with the coexistence of “no-trend” and positive-trend gauges. Episodic coastal rainfall deficits (e.g., 2020) reinforce this picture [72].

In summary, the key findings of the study indicate the existence of five homogenous hydrological regions in Poland, differing in terms of drought duration and volume. It was also demonstrated that terrain elevation and slope gradient significantly influence drought characteristics. Hydrological droughts in Poland exhibit strong temporal variability, related to precipitation distribution and air temperature. The longest droughts occur in summer and autumn, while in the Tatra Mountains, they also extend into winter. The research findings have important implications for water resource management. The presented drought diagnostics highlight two priority zones for water-resource conservation, namely (i) the Central Lowlands and Uplands, where irrigation demand and reservoir draw-down prolong summer deficits, and (ii) the Coastal and Lakeland zone, where historic peatland drainage has lowered groundwater tables by up to 1.5 m. To prevent drought, the following action lines are recommended: (1) small-scale retention and re-wetting, (2) dynamic environmental-flow and abstraction rules, (3) demand-side efficiency and re-use, and (4) early-warning, forecast-triggered operation

All of these actions should be reflected in Poland’s water management plans and strategies to address the uncertainty surrounding future water resource availability.

6. Conclusions

The spatial analysis of hydrological drought variability in Poland was conducted based on data from 340 gauging stations over the multi-year period 1973–2022. The study focused on determining the frequency, duration, and volume of water deficit, as well as the seasonality of droughts. To identify hydrological drought episodes, the peak over threshold (POT) method was applied using the Q_90% threshold flow. The number of drought events, their average and maximum duration, deficit volume, and the months of onset and termination were calculated. A cluster analysis using Ward’s method enabled the delineation of regions with similar drought characteristics. The results revealed the significant spatial and temporal variability of hydrological droughts in Poland, driven by geographical and climatic factors. The average drought duration was 32.2 days. However, notable regional differences were observed. In mountainous areas, such as the Carpathians and Sudetes, droughts were generally shorter, while in lowland regions—especially in the lakelands—their duration was significantly longer. The longest drought episodes, with a cumulative duration of up to 1929 days (averaging 68 days annually), occurred in the central and northern parts of the country. These lowland areas were found to be the most vulnerable to prolonged and severe droughts, highlighting the need for measures aimed at increasing water retention.

Seasonal analysis showed that hydrological droughts most often began in summer (June–August) and ended in late summer or early autumn (August–September). The longest events typically started in June and lasted until late autumn or winter (October–December). In the Tatra Mountains, autumn–winter droughts were observed, which are related to limited snowmelt during that time.

Cluster analysis identified five homogeneous regions, each with distinct hydrological drought patterns, influenced by geological structure, topography, and climatic conditions. The trend analyses of the annual number of days with low flows indicate no statistically significant trend at 46% of gauging stations, while 54% exhibit statistically significant increases with strong regional variability. The highest number of stations with a statistically significant decreasing trend was observed in the Southern and Eastern Baltic Lake District and Central Poland Lowlands and Highlands with Polesie.

The findings are of significant relevance for water resource management, particularly in the context of climate change and the increasing frequency of extreme weather events. Future work will focus on modeling potential future drought scenarios and assessing their impact on the ecological status of rivers, agriculture, and drinking water supply.