**1. Introduction**

Drought is a phenomenon that negatively affects many economic sectors. Depending on the duration, effects and intensity, drought can be classified into four types: meteorological, agricultural, hydrological and socio-economic [1]. According to IPCC [2], mountains are among the areas most endangered by climate change, and droughts have been observed and are predicted in various mountain ranges, for example, in the Alps [3]. The pan-European study showed that in the 1990s and 2000s, drought hotspots were identified in the Mediterranean and Carpathian Regions; in the latter, drought severity and duration were highest in Hungary and Slovakia [4]. In the period 1961–2010, the worst droughts occurred in 1990, 2000, and 2003; less intense or prolonged droughts took place in 1964, 1970, 1973/74, 1983, 1987, 1992, and 2007 [5]. The Carpathians are located in Central Europe where the future drought risk is estimated to be relatively high with projected increases in hydrological, agricultural and ecological droughts at mid-century warming levels of 2 ◦C or above, regardless of greenhouse gas emission scenarios [6]. The Carpathian region includes the Carpathian Mts. and the Pannonian Basin. Studies concerning droughts in the whole region were based on gridded data (e.g., [5,7,8]) and showed that the region's

**Citation:** Bokwa, A.; Klimek, M.; Krzaklewski, P.; Kukułka, W. Drought Trends in the Polish Carpathian Mts. in the Years 1991–2020. *Atmosphere* **2021**, *12*, 1259. https://doi.org/ 10.3390/atmos12101259

Academic Editors: Alexander V. Chernokulsky, Andrzej Walega and Agnieszka Ziernicka-Wojtaszek

Received: 28 July 2021 Accepted: 20 September 2021 Published: 27 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

lowlands are much more endangered than the mountains. National-level studies confirmed the results for the lowlands and revealed the complexity of the issue in the mountains. In the lowlands of Hungary, Serbia, Romania and Slovakia, droughts have already caused significant agricultural yield losses in recent decades ([9–11]). Studies for the Carpathian Mts. are more limited and encompass only a few mountain ranges. For the part of the Western Carpathians located in Slovakia, in the period 1951–2007 precipitation exceeded potential evapotranspiration [9,12]. Those results have been confirmed for the upper Hron region [13]. In a study for Romania, with data for the period 1961–2010, it was shown that drought may affect areas with both low and high precipitation averages, and can occur in mountains or lowlands. Large-scale atmospheric circulation is the major drought driver in Romania in winter, while thermodynamic factors (such as air temperature and humidity levels) are the major drivers in summer. The Carpathian mountain chain itself is the second regional factor influencing drought spatial variability, triggering differences between intra-Carpathian and extra-Carpathian regions in wintertime [14]. The Tatra Mts are the highest range in the Carpathians, located at the border between Poland and Slovakia. For the Slovakian part, data from the period 1961–2010 were used to show that the occurrence of drought has a cyclic pattern over an approximately 30-year period. The core areas of the biosphere reserve of the Tatra National Park inhabited by unique species (altitudes over 1500 m a.s.l.) are in the relatively "drought-safe altitudinal zone". Unfortunately, ecosystems at lower altitudes (up to 900 m a.s.l.) could be impacted by drought due to a lower precipitation surplus. The occurrence of drought episodes was influenced by the precipitation shadow of the Tatra Mts range and of the surrounding mountains situated to their north and northwest. Thus the occurrence of drought is more likely in the south and southeastern regions of the mountains than on the windward north or northeastern parts. In addition, another drought-prone area was indicated in the Western Tatra Mts. This area is influenced by the Oravsk'e Beskydy and Oravsk'a Magura ranges located to the northwest [15]. A study concerning the Polish part of the Tatra Mts was compared with a study for the Ukrainian Carpathians for the period 1984–2015, concerning the occurrence of dry months. At least one extremely dry month at each meteorological station was detected, but only in November 2011 was an extreme drought at all the stations observed. That was a month with precipitation less than 5% of the long-term average at specific meteorological stations [16]. In Poland, atmospheric drought is observed most often from April to September [17], and it affects mainly the lowlands where agriculture is concentrated [18,19]. The Carpathians receive much more precipitation than the rest of the country (with the exception of the Sudeten Mts where precipitation is comparable), but long periods without precipitation have been observed more frequently during the last few decades both on the mountain foreland [20] and on the mountain ranges [21,22]. Such a tendency combined with increased runoff and decreased retention (due to human activity) is a huge disadvantage for water resource management, and for the functioning of ecosystems. Additionally, an increase in hydrological drought risk has been observed in the Carpathians over the period 1901–2002 [23]. Agricultural drought has been studied for Poland for the period 1961–2010, but mountain areas were excluded from that research. However, the study showed that foreland areas were relatively little endangered by agricultural drought [24].

In spite of receiving the highest annual precipitation totals, in comparison to the rest of the country, the Polish Carpathians are affected by current climate change too, and drought has to be considered one of the potential new threats to the region. Therefore, this paper is aimed to show whether atmospheric drought risk varies in the vertical climatic profile of the Polish Carpathians and whether the eastern part of the mountain chain is more endangered by drought than the western part due to more continental climatic conditions. This aspect of the present climate has not yet been studied for that part of the Carpathians in contrast to some other parts. The present paper shows variability and trends in atmospheric drought occurrence in the most recent 30-year period. The middle of the 1980s represents a turning point for all the climatic variables in the Carpathian region [25] and shows the beginnings of presently observed climate change. The data from the period 1991–2020 represent the current long-term climatic period characterized by ecosystems entering a new state of dynamic balance. The Polish Carpathians are not an important agricultural region in the country; the main economic sector developed there is tourism and the properties of the natural environment are one of the most important factors in its development. The Carpathians, apart from offering picturesque landscapes, are a European biodiversity hotspot, with rich flora and endemic plants, and including the most extensive primeval forests across the whole of Europe; there are many different bird species and it is home to the largest communities of carnivores and predators such as bears and wolves [5,26]. Beech is the main species of the *Dentario glandulosae-Fagetum* community, a typical element of the Carpathian environment. Therefore, forest drought indices have also been used in the present study to estimate trends in atmospheric conditions favorable for beech. This is one of the indicators of the long-term impact of the drought trend on the natural environment of the Carpathians.

#### **2. Study Area**

The Polish Carpathian Mts. are part of the huge Carpathian mountain chain in Central Europe. It is divided into the Southern Carpathians (located in Romania), the Eastern Carpathians (Ukraine, Slovakia, Hungary, Poland), and the Western Carpathians (Poland, Slovakia, Czech Republic, Hungary, Austria). The relief of the Carpathians varies from undulating foothills to typical alpine landscapes in the Tatra, Rodna, Fagaras, and Retezat mountains [27]. The highest peaks can be found in the Tatra Mts, in Slovakia: Gerlach, 2655 m a.s.l.; and in the Fagaras Mts, in Romania: Moldoveanu, 2543 m a.s.l. As much as 88% of the Carpathian area located in Poland belongs to the Western Carpathians [28]. The climatic and hydrological conditions of the Polish Carpathians have been the subject of many studies, for example, [29–35], but rarely in the context of atmospheric drought as it is the region with the highest precipitation on a national scale. Most of the climatic parameters show increasing climate continentality from the west toward the east; for example, the mean annual air temperature range increases by 0.49 ◦C per degree of longitude on convex landforms, and by 0.35 ◦C on concave ones [31]. Even more important are changes in climatic conditions with altitude which are visible as vertical climate-vegetation zones [29]. The location of zonal boundaries (i.e., altitudes where a certain mean annual air temperature is found) depends on the scale of a particular mountain range, slopes aspect, the main geomorphological features and the prevailing direction of air mass advection [36]. According to the original vertical zone pattern [29], the study area contains six zones from "cold" with a mean annual air temperature from −4 to −2 ◦C, to "moderately warm" with a mean temperature from 6 to 8 ◦C. However, one of the effects of global warming is a shift in zonal boundaries [33] and this will be discussed further in the present study. The vertical climate-vegetation zone that occupies the largest area in the Polish Carpathians is that of deciduous forest, with its dominating *Dentario glandulosae-Fagetum* plant community [37]. European beech (*Fagus sylvatica*) is the main species of that community. The deciduous forest zone is located in the mountain foothills and on medium-height mountain ranges which are areas of intensive anthropopressure due to tourist activity. Therefore, a potential impact of drought on beech forest conditions will be presented later in the paper.

#### **3. Materials and Methods**

The data used in the present study come from 12 meteorological stations located in the Polish Carpathians (Figure 1 and Table 1). The stations represent the highest parts of the Carpathians, that is, the peaks of the Tatra Mts (Kasprowy Wierch) and the neighboring basin (Zakopane), the Beskidy ranges which are medium-height mountains (Limanowa, Nowy S ˛acz, Krynica, Lesko and Koma ´ncza), the foothills (Gaik-Brzezowa, Łazy and Bielsko-Biała), and the foreland (Kraków and Katowice). The stations in Gaik-Brzezowa and Łazy belong to the Institute of Geography and Spatial Management, Jagiellonian University, Kraków, while the others are administered by the Institute of Meteorology and Water Management—National Research Institute. The spatial distribution of the stations

allows drought occurrence to be studied in the Polish Carpathians both vertically and from west to east. %clearpage

**Figure 1.** Location of the meteorological stations used in the study numbers as in Table 1.

**Table 1.** Coordinates and altitude of the meteorological stations used in the study numbers as in Figure 1. TPZ concept is explained in Section 4.1.


The data used come from the 30-year period 1991–2020 (while for Gaik-Brzezowa the data cover that of 1991–2019) and consist of mean monthly air temperatures and monthly precipitation totals. The choice of study period is linked to data availability and to the fact that since the 1990s, a significant change in climatic conditions has begun on both global and regional scales [38]. Therefore, the analyses presented show the contemporary situation and trends which are the effect of the present climate drivers. The data were used first to determine the variability of air temperature and precipitation in the study period and to distinguish areas with different air temperature and precipitation patterns.

Then the data were used to calculate indices widely used to determine the occurrence of drought (see also Appendix A):

1. SPI (Standardized Precipitation Index) [39]: it is based on the probability of precipitation which is the only input parameter. SPI was calculated for 1-, 3- and 6- monthly timescales for each station: SPI ≤−2.0—extreme drought, −1.99 < SPI ≤ −1.50—strong drought, −1.49 < SPI ≤ −1.00—moderate drought, −0.99 < SPI < 0.99 near normal conditions, 1.0 < SPI < 1.49 moderately wet, 1.5 < SPI < 1.99 very wet.


In order to estimate the impact of drought on beech, the following indices of forest drought were used and calculated for each year and station:


The SPI index was calculated with SPI Generator software [48], SPEI was calculated with the Package 'SPEI' software (https://cran.r-project.org/web/packages/SPEI/SPEI. pdf, accessed on 20 August 2021) and other indices were calculated with MS Excel, with the formulas listed in Appendix A and provided in the publications mentioned above.

In the calculation of SPI and RPI, only precipitation data are taken into consideration, while the Selianinov index and SPEI include also air temperature data. The forest drought indices are not only based on data for air temperature and precipitation but are calculated at various temporal resolutions, which allows different aspects of the phenomenon to be observed.

The series of air temperature, precipitation and index values were tested using regression analysis; linear trends were determined together with their equations, R<sup>2</sup> and *p* level with Statistica software (https://www.statsoft.pl/, accessed on 1 August 2021). Then the data series were additionally tested for the presence of trends by the Mann–Kendall test [49–51] using XLSTAT software (https://www.xlstat.com/en/, accessed on 5 August 2021). That software was also used to calculate standard errors for mean values. For each data series, the variability coefficient was calculated with MS Excel following the formula:

$$\text{Vc} = \text{(}\text{\textdegree (}\text{\textdegree m)} \times 100\text{\textdegree m)} \tag{1}$$

where:

Vc—variability coefficient (in %)

σ—standard deviation

m—mean value

The values of the coefficient calculated were interpreted in the following way:

<25%—low variability, 25–45%—mean variability, 46–100%—high variability, >100% very high variability.

The drought occurrence estimation was based on a comparison of the SPI, SPEI, RPI and Selianinov index outcomes, that is, the number of dry months defined as SPI ≤ −1.00, SPEI ≤ −0.8, RPI ≤ 75%, Selianinov index < 1.4. The frequency of dry months was presented for specific decades of the study period, for the whole year, the warm half-year (May–October) and the cold half-year (November–April). The forest drought indices can be calculated with only an annual resolution so the number of years with conditions unfavorable for beech in specific decades has been shown.

#### **4. Results**

All the indices used in the study are based on data concerning air temperature, precipitation or both. Therefore, the spatial and temporal variability of air temperature and precipitation is presented first in order to provide background for the analysis of drought indices.
