Assessment of Meteorological Drought Trends in a Selected Coastal Basin Area in Poland—A Case Study

Abstract

The aim of this study is to investigate the patterns and trends of drought occurrence in the northern part of Poland on the example of the Łeba river basin in the years 1956–2015. The study of meteorological drought was conducted on the basis of the Standardized Precipitation Index (SPI) on the scale of 1, 3, 6, 9, and 12 months. Annual precipitation totals did not show significant changes in the analyzed period, except for the station in Wejherowo, which is characterized by a significant increasing trend. The analysis of the long term of the variability average annual air temperature showed its statistically significant increase in the analyzed area at the rate of about 0.2 °C per decade. During the analyzed period, 14 to 84 meteorological droughts were identified, with durations ranging from 200 to 300 months. As the period of accumulating values of SPI, the number of droughts decreased, while their total duration increased. Most droughts were mild in nature, while extreme droughts accounted for between 5.2% and 10.7% of the duration. Drought intensification was shown only for SPI-1 in February and March in Wierzchucino station. On the other hand, a decreasing trend in SPI values was noted during longer periods of accumulation (SPI-6, 9, and 12).

1. Introduction

Drought is generally defined as water scarcity in a given period in a given area. Droughts can occur in any climatic conditions around the world, having a negative impact on the environment, economy (especially agriculture and food supply), health, and quality of life [1]. Taking into account the duration and extent of drought and its effects, successive stages of its development are distinguished: meteorological drought, soil (agricultural), hydrological, and socio-economic drought. Meteorological drought is defined as a period with anomalous precipitation deficiency, which is generally the main factor initiating drought [2]. The beginning of drought is usually associated with a long-term lack of precipitation or its deficiency, and high temperature [3,4,5]. Precipitation deficits are related to mechanisms at various spatial scales, including the occurrence of various types of atmospheric circulations in the global perspective [6]. An important role in the occurrence of precipitation deficiency may also be played by moisture cycle on a regional and local scale, related to the interaction of the atmosphere and the ground. Drought builds up gradually, its effects are cumulative and may persist for a long time after its end. Drought can occur in different time scales, from droughts lasting several weeks and months, characterized by a sudden onset and rapid intensification of effects, to long-term periods of precipitation deficiency [7,8]. Although drought typically cannot be characterized by a single universal definition or directly measured by a single variable [8], precipitation deficiency is the first and essential stage of drought development. Droughts have a major impact on society based on their overall impact on water availability for competing economic and environmental sectors [9]. The challenge of allocating water to meet all competing needs is even more difficult during a prolonged drought [10]. Drought information can be used by water resource managers. Accordingly, many indicators and methods have been developed to identify and determine the intensity of droughts based on the occurrence of precipitation deficiency [11,12]. One of the frequently used indicators for assessing precipitation deficiency is the Standardized Precipitation Index (SPI), developed in the 1990s by McKee, Doesken, and Kleist [13,14]. This indicator is used to quantify the precipitation deficit in different time scales and different climatic conditions [15,16,17,18,19]. Since 2009, it has been recommended by the World Meteorological Organization (WMO) as the basic index of meteorological drought monitoring [16]. It is used in Europe [20], the USA [21], and Australia [22]. The variability of meteorological drought since the mid-twentieth century using the SPI was studied on a global scale [23,24], in different regions of the world, e.g., Iran [25], Africa [26,27,28], China [29,30,31,32], Bangladesh [33], Korea [34], as well as for the entire European continent [35,36,37] and its different regions, such as eastern and central Europe [38], the Carpathian Mountains [39], the Czech Land [40]. Taking into account global warming, future changes in the frequency and intensity of droughts in the coming decades are increasingly being studied, taking into account various scenarios of climate change [41,42,43]. In Poland, the SPI index began to be used in 2000 as an indicator of drought monitoring in areas with fertile agricultural soils [44]. Since then, it has been frequently used in research on the phenomenon of drought in other regions of Poland [45,46,47,48,49,50,51,52]. Meteorological droughts occurring in Poland using the SPI index were also considered in terms of the impact of atmospheric circulation [53], as well as in terms of the impact of future climate change [54,55]. The impact of meteorological drought on other types of droughts as well as on water resources and agricultural production is often analyzed [56,57]. Identification of droughts in Poland in the last millennium was carried out by [58]. Evidence of changes in the drought phenomenon on a global scale is supplemented on each continent by the results of research on a regional scale [2]. Therefore, it is important to provide as much information as possible from various regions, including single, relatively small catchments. At a smaller spatial scale, the impact of changes in land use on the water cycle is likely to be revealed more quickly. The previous works concerned mainly catchment areas located in central Poland [59] or mountainous areas [60,61]. There is a lack of works that analyze the course of meteorological droughts in river basins located in the zone of the Polish Baltic coast. This study is therefore the first to comprehensively document the spatial and temporal differentiation of meteorological drought characteristics in the coastal catchment, which is characterized by specific physical, geographical, and climatic conditions. The aim of this study is to investigate the patterns and trends of drought occurrence in the northern part of Poland on the example of the Łeba river basin in the years 1956–2015. The study of meteorological drought based on SPI in the scale of 1, 3, 6, 9, and 12 months in the Łeba river basin allowed to document the course of short, medium, and long-term events.

2. Materials and Methods

2.1. Study Area

The study area is the Łeba River basin located in northern Poland. According to the hydrographic division of Poland [62], the total area of the Łeba catchment area is 1764 km², and the length of the Łeba River is 126.7 km (

Table 1. Characteristic parameters of the Łeba River Basin.

Parameter	Unit	Value
River length	km	126.7
Basin area	km2	1764.0
River network density	km·km2	2.2
Lake surface	km2	86.1
Maximum basin height	meters at sea level	268.6
Minimum basin height	meters at sea level	0.0

). The Łeba River is one of the largest coastal rivers that flows directly into the Baltic Sea. The basin has the diversified lay of the terrain of a young glacial character. It is characterized by significant terrain differences, diversified geological structure, and other features typical of the Pleistocene accumulation areas, which makes it similar to the neighboring drainage basins. Taking into account the morphological conditions, it can therefore be considered representative of the region of the immediate catchment area of the South Baltic. The highest areas are located in the southern part of the catchment (reaching 270 m above sea level), from where the area gradually decreases towards the Reda-Łeba ice-marginal valley, bordering it with a clear edge (Figure 1). The bottom of the Reda-Łeba ice-marginal valley lies at an altitude of 45–15 m above sea level and descends to the west. Going further north, the surface of the basin rises, reaching over 100 m above sea level in the strip of moraine plateaus of the Baltic Coast [63]. In the further part, the catchment area decreases to a height of 1–5 m above sea level, while in the spit zone it reaches a height of 15–30 m above sea level, with a culmination of 42 m above sea level. In Łącka Góra, the catchment is characterized by a dense hydrographic network and the presence of many lakes. Currently, there are 6 lakes with an area of over 1 km² in the Łeba catchment area. The largest of them are Łebsko (71.4 km²) and Sarbsko (6.1 km²), which are coastal lakes located in the northern part of the catchment. The most important tributaries of the Łeba include the Dębnica, Mirachowska Struga, Kisewska Struga, Okalica, Pogorzelica, and Chełst (Figure 1). The catchment is mainly composed of boulder clays and sands and gravels of fluvioglacial origin. In land depressions, especially river valleys and the proglacial valley of Reda-Łeba, there are silts, alluvial soils, and bog peats. In terms of land use, agricultural areas dominate, consisting of arable land covering 31.6% of the catchment area and meadows (14.6%). A significant part of the catchment area is covered by the above-mentioned forests (approx. 38%), concentrated in the middle section of the river course near Lębork, where mixed and coniferous species predominate. The northern, coastal part of the Łeba catchment is protected by the Słowiński National Park (SPN) established in 1967. The onshore part of the SPN covers 215.7 km², which is 12.2% of the Łeba catchment area. Due to its location and greater distance from the influence of the oceanic masses, the Łeba catchment area reflects the climatic characteristics of the region of the central and eastern coast of the South Baltic Sea.

The Łeba River is a river with a large water resource and it is characterized by the stability of the outflow throughout the year. The resources expressed as the average annual river flow (Cecenowo station) in the years 1956–2015 were at the level of 11.72 m³·s⁻¹, while the unit outflow reached an average of 10.5 dm³·s⁻¹ and changed from 8.6 dm³·s⁻¹ in a year 2014 to 13.9 dm³·s⁻¹ in 1981 (Figure 2). Low values of average annual flows were also observed in 1964–1966, 1969, 1992, 2003, and 2006. The even distribution of runoff is the result of the dominant share of underground supply in the total runoff. According to the authors [64], this supply for the Łeba River is over 77% of the total outflow. Small amplitudes of flow variability of the Łeba River are reflected by the outflow irregularity index equal to 10.5.

2.2. Data

The study used meteorological and hydrological data from gauging stations located in the Łeba catchment area and its vicinity (Figure 1). Meteorological data are daily precipitation totals for 7 meteorological stations (

Table 2. Location of meteorological stations and basic characteristics of annual precipitation sums.

Station	Longitude	Latitude	Height above Sea Level	Average Annual Precipitation in Years 1956–2015 [mm]	Standard Deviation of the Annual Precipitation in Years 1956–2015 [mm]
Wierzchucino	18°00′23″ E	54°47′11″ N	8	671.1	138.2
Łeba	17°32′05″ E	54°45′13″ N	2	661.7	131.1
Żelazno	17°51′23″ E	54°43′26″ N	95	810.0	146.4
Wejherowo	18°16′13″ E	54°35′54″ N	22	664.8	142.0
Słupsk	17°02′08″ E	54°27′48″ N	17	803.9	151.8
Lebork	17°45′25″ E	54°33′11″ N	39	697.6	113.7
Kościerzyna	17°57′43″ E	54°07′43″ N	190	658.3	120.4

) and daily air temperature values for 3 stations (Łeba, Lębork, and Kościerzyna). For hydrological characteristics, daily flow values of the Łeba River at the Cecenowo hydrological station were used. On the basis of daily data, monthly and annual precipitation totals were calculated, as well as monthly and annual mean air temperature and flows. The data from 1955 to 2015 were made available by the Institute of Meteorology and Water Management National Research Institute.

2.3. Research Methods

The standardized precipitation index (SPI) was used to assess meteorological drought. The SPI is determined on the basis of multi-year precipitation sequences over a specified period of time, which are normalized using a transforming function and then standardized [65]. The best quality of information is obtained when the indicator is determined on the basis of at least 30-year data series, with 50–60 years (or more) being optimal and preferred [66]. The SPI can be determined in time scales from 1 to 48 months or longer, which makes it possible to use it in the study of various stages of drought [17]. SPI has its limitations. First of all, it uses only one meteorological parameter, which is precipitation. The choice of the reference period has a large impact on the assessment of drought, especially in the context of climate change. Therefore, 60 years of data series were included in this study. The uncertainty of the SPI calculations has a large impact on the categorization of droughts, but does not affect the assessment of the temporal characteristics of drought [67]. In this study the SPI was determined on the basis of a series of monthly precipitation sums at each measuring station, in various time scales, i.e., 1, 3, 6, 9, and 12 months. Among the various methods of determining the SPI, the one chosen consisted of transforming a number of monthly precipitations sums into a distribution consistent with or close to normal by means of a transforming function:

$$\mathrm{f}\left(\mathrm{P}\right)=\sqrt[3]{\mathrm{P}}$$

(1)

where

P—monthly precipitation (mm);

f(P)—transformed precipitation sum.

The fact indicated in the literature was used that the empirical frequency distributions of monthly precipitation totals most often correspond to the gamma distribution in shape and that for a random variable X with a gamma distribution, the variable Z = ∛x has an approximately normal distribution and is the recommended transformation to the normal distribution in the case of a series of sums monthly rainfall [45,68]. After the transformation, the series was standardized according to the equation, determining the SPI index:

$$\mathrm{S}\mathrm{P}\mathrm{I}=\frac{\mathrm{f}\left(\mathrm{P}\right)-\mathsf{\mu }}{\mathsf{\delta }} \mathrm{i}\mathrm{f} \mathrm{P}\ge 0$$

(2)

where

SPI—Standardized Precipitation Index;

μ—average of the transformed precipitation sequence [mm];

δ—standard deviation of the transformed precipitation sequence [mm].

The SPI was determined for each month in the five time scales mentioned. The types of droughts were adopted in accordance with the severity classification of droughts presented in

Table 3. Classification of droughts based on the Standardized Precipitation Index [13,14].

SPI Value Range	Drought Category
0 to −0.99	mild
−1.00 to −1.49	moderate
−1.50 to −1.99	severe
≤−2.00	extreme

.

In order to establish patterns of drought change, this study considered drought components such as drought duration (D), drought frequency (F), and drought intensity (I) [69]. These parameters were used to detect the potential impact of climate change on drought characteristics over the past 60 years. The commonly known Mann–Kendall test was used to examine trends in the time series of precipitation, air temperature, and SPI [59,70,71,72,73,74]. The linear trend coefficient were determined with MS Excel using the least squares method.

3. Results

3.1. Precipitation and Air Temperature in the Years 1956–2015

The spatial variability of mean annual precipitation sums in the Łeba catchment area is shown in Figure 3. The average long-term precipitation sums in the analyzed period ranged from 661.7 mm in Łeba to 810 mm in Żelazno. The values of the 10th percentile of annual precipitation sum, adopted as the criterion of the extreme phenomenon [75], ranged from 479 mm in Łeba to 623 mm in Żelazno (Figure 4). The lowest values of annual sums in the research period at three stations occurred in 1959 (Figure 5, Figure S1): in Wierzchucino (330 mm), Słupsk (410 mm), and Łeba (522 mm). At the other stations, the lowest precipitation totals occurred in 1964, and their values ranged from 287 mm in Wejherowo to 492 mm in Żelazno. The largest sums of precipitation ranged from 960 mm in 1998 in Kościerzyna to 1261 mm in 1981 in Słupsk. The annual precipitation sums at the Lębork station are characterized by the greatest regularity, as evidenced by the smallest value of the standard deviation at this station (113.7 mm) (Table 2). In turn, the most varied annual precipitation falls in Słupsk (the standard deviation equals 151.8 mm). Annual totals of atmospheric precipitation did not show significant changes in the analyzed period, but only an upward tendency (

Table 4. Linear trend coefficients (mm per decade) of monthly precipitation sums in the Łeba basin, 1956–2015.

Station	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII	Year
Łeba	−0.6	−1.9	1.5	−2.9	1.6	0.8	−3.8	−0.5	2.6	1.0	1.0	−0.1	−1.3
Wierzchucino	3.1	1.5	3.3	−0.6	3.1	1.6	−2.4	0.5	5.4	1.0	3.2	5.0	24.7
Żelazno	1.9	0.6	2.5	−2.1	2.1	0.6	−4.9	−3.2	1.7	−0.8	0.2	1.6	0.1
Wejherowo	3.3	−0.1	4.3	−1.6	2.8	1.4	2.0	0.3	2.6	1.0	0.8	3.1	28.7
Lębork	1.7	−1.2	2.7	−1.4	1.7	1.6	−2.3	−3.0	3.2	1.4	2.1	2.8	9.2
Słupsk	3.9	0.3	2.9	−2.7	0.5	2.7	−3.4	−2.1	1.8	0.0	0.6	2.9	7.4
Kościerzyna	4.4	0.1	3.9	−1.0	1.4	0.6	0.8	−2.9	−0.6	0.7	0.8	3.1	11.3

Note: Statistically significant (at 0.05 level) values are in bold.

). The exception is the series from the station in Wejherowo, which is characterized by a significant growing trend, by almost 29 mm per 10 years.

In the year course, the lowest precipitation amounts fall in February, March, and April and range from 33 to 42 mm per month. The largest monthly totals, from 79 mm to 95 mm, occur in July and August. Taking into account the long-term course of monthly precipitation totals, a downward trend can be observed at all analyzed stations in April and at four stations in July and August, and at one station in October (Table 4). In most cases, however, these changes are not statistically significant. A statistically significant increase in monthly precipitation sums can be observed in January in Kościerzyna, in March in Wierzchucino, Wejherowo, and Kościerzyna (by 3.3 mm, 4.3 mm, and 3.9 mm per decade, respectively), and in Wierzchucino in December (by 5 mm per decade). In the remaining cases, no statistically significant changes were found, only tendency to grow.

Thermal conditions in the analyzed area are different depending on the location of the station. The average annual temperature in Łeba and Lębork was 7.8 °C in Łeba and 7.9 °C in Lębork, while at the highest station (Kościerzyna) it was 7.0 °C. The warmest month at all stations is July, in which the average monthly air temperature was the highest in Lębork (17.3 °C), at the highest situated station it was 16.8 °C, and at the station located close to the seashore (Łeba) it was amounted to 16.9 °C, the cooling effect of the waters of the Baltic Sea is observed there. The highest monthly temperature in the analyzed period was recorded in July 2006: 20.5 °C in Łeba, 20.8 °C in Kościerzyna, and 21.4 °C in Lębork. In the coldest month, January, the average long-term air temperature was −0.6 °C in Łeba, −1.0 °C in Lębork and −2.5 °C in Kościerzyna. Extremely low monthly average values were recorded in January 1987, which amounted to −7.7 °C in Łeba, −9.1 °C in Lębork, and −10.8 °C in Kościerzyna. The influence of the sea and the height of the station above sea level on thermal conditions in winter is clearly visible. The analysis of the variability of the long-term average annual air temperature showed its statistically significant increase in the analyzed area (

Table 5. Linear trend coefficients (mm per decade) of monthly mean air temperature in the Łeba Basin, 1956–2015.

Station	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII	Year
Łeba	0.27	0.37	0.31	0.43	0.37	0.13	0.29	0.33	0.16	0.01	0.18	0.22	0.26
Lębork	0.27	0.41	0.30	0.40	0.26	−0.01	0.32	0.33	0.13	−0.04	0.15	0.24	0.23
Kościerzyna	0.26	0.38	0.31	0.42	0.28	−0.01	0.34	0.35	0.09	−0.09	0.13	0.22	0.22

Note: Statistically significant (at 0.05 level) values are in bold.

, Figure 6). The rate of change is 0.3 °C per decade in Łeba (Figure 6) and 0.2 °C per decade in Lębork and Kościerzyna (Figure 6). Monthly mean air temperature in the analyzed period increases in all months except for October. In the period from March to August, this increase is statistically significant in most cases. The fastest rate of growth, by more than 0.4 °C at all three stations, is the April temperature. It should be noted that at the same time at all stations the sum of precipitation in this month shows a downward trend. There is also a clear warming in May, July, and August, with the fastest rate in May in Łeba (by almost 0.4 °C per decade).

3.2. Meteorological Drought

In the years 1956–2015, from 14 to 87 meteorological droughts were identified (

Table 6. Meteorological drought parameters in 1956–2015.

Station	Total Number Events	Number of Months with Drought (<−1.0)	Longest Drought (Month)	The Maximum Intensity of Drought
SPI-1
Lębork	84	205	11 (March 1964–January 1965)	−3.96 (June 1992)
Łeba	83	219	8 (March 1964–October 1964)	−3.09 (June 1992)
Kościerzyna	75	203	8 (March 1964–October 1964)	−3.70 (June 1992)
Słupsk	87	203	8 (March 1964–October 1964)	−3.68 (June 1992)
Wejherowo	85	199	6 (May 1964–_October 1964)	−4.17 (June 1992)
Wierzchucino	69	219	10 (February 1987–November 1987)	−3.93 (August 1977)
Żelazno	79	243	9 (December 2000–August 2001)	−3.11 (October 1996)
SPI-3
Lębork	48	262	19 (December 1963–June1965)	−3.10 (July 1992)
Łeba	40	256	19 (December 1978–June 1980)	−2.51 (November 1982)
Kościerzyna	53	272	17 (February 1964–June 1965)	−2.92 (July 1992)
Słupsk	49	254	22 (February 1975–November 1976)	−3.04 (July 1969)
Wejherowo	39	251	21 (November 1958–July 1980)	−3.19 (July 1969)
Wierzchucino	45	263	18 (December 1963–May 1965)	−3.45 (July 1969)
Żelazno	45	270	32 (October 1962–May 1965)	−3.29 (July 1969)
SPI-6
Lębork	31	287	19 (December 1963–June 1965 19 (June 2005–December 2006)	−2.95 (August 1964)
Łeba	23	257	23 (February 2014–December 2015)	−2.72 (March 1996)
Kościerzyna	25	298	28 (March 1964–June 1966)	−2.93 (April 1996)
Słupsk	28	284	24 (April 1975–March 1977)	−2.36 (May 1964)
Wejherowo	24	292	32 (December 1991–March 1994)	−2.55 (June 1957)
Wierzchucino	24	230	21 (July 1968–March 1970)	−3.0 (June 1959)
Żelazno	29	263	33 (October 1962–October 1965)	−2.94 (July 1963)
SPI-9
Lębork	19	264	33 (October 1962–June 1965)	−3.20 (August 1964)
Łeba	17	249	27 (July 1995–September 1997)	−2.86 (May 1980)
Kościerzyna	22	309	32 (July 1991–February 1994)	−2.78 (August 1964)
Słupsk	22	306	31 (June 2013–December 2015)	−2.64 (August 1964)
Wejherowo	17	283	34 (November 1959–August 1961)	−2.74 (July 1964)
Wierzchucino	18	264	38 (July 1975–August 1978)	−3.42 (July 1969)
Żelazno	23	295	39 (April 1962–June 1965)	−3.01 (July 1969)
SPI-12
Lębork	15	251	49 (July 1961–July 1965)	−2.98 (November 1964)
Łeba	15	259	35 (October 1975–August 1978)	−2.39 (January 1960; August 2015)
Kościerzyna	17	332	46 (March 1963–December 1966) 46 (July 1975–April 1979)	−2.43 (November 1964)
Słupsk	20	299	28 (July 2013–October 2015)	−2.16 (April 1960, October 1969)
Wejherowo	15	320	97 (January 1956–January 1964)	−2.47 (October 1969)
Wierzchucino	14	235	34 (October 1975–July 1978)	−3.30 (September 1959)
Żelazno	17	286	36 (July 1962–June 1965)	−2.67 (October 1964; November 1964)

). Most droughts were recorded in short, 1- and 3-month accumulation periods, while the least droughts were recorded in longer accumulation periods, i.e., 9 and 12 months. For SPI-1, the longest droughts lasted from 6 (Wejherowo) to 11 months (Lębork) and for most stations they occurred from March to October 1964. The maximum intensity of drought was recorded at most stations in June 1992. With the increase in the accumulation period, the number of months with the longest drought increased, while the maximum intensity of drought decreased. For SPI-3, the longest droughts lasted from 17 to 32 months, for SPI-6 from 19 to 33 months and for SPI-9 from 27 to 39 months. The greatest diversity was observed in the case of the longest droughts for SPI-12, which, depending on the station, lasted from 28 to 97 months. Particularly noteworthy in this regard is the Wejherowo station, for which the longest drought lasting 97 months was recorded. This drought lasted twice as long as in the other stations. Figure 7 shows the course of the SPI values in different time scales at the Łeba and Lębork meteorological stations located in the Łeba River basin in the years 1956–2015. The years in which the maximum intensity of meteorological drought or the longest lasting meteorological drought were identified coincide with the years in which the lowest values of average annual flows of the Łeba River in the Cecenowo section were recorded (Figure 2). This is particularly evident in the case of short accumulation periods of the SPI-1, 3 and 6 months in the years 1964–1966, 1969, and 1992. Low average annual flows of the Łeba River in 2004, 2006, and 2014 did not coincide with intensive or long-term meteorological droughts recorded at the analyzed meteorological stations. The course of the SPI values on the remaining stations is presented in Figure S2.

3.3. Types of Droughts and Their Spatial Distribution

The duration of months with droughts was different at individual stations and, depending on the accumulation period, represented from 28% (SPI-1) to 44% of the time from the period of the analyzed multi-year period (SPI-12) (Figure 8). Noteworthy is the duration of droughts at the Łeba and Lębork station, where droughts accounted for 36 to 38% of the time from the given multi-year period for SPI-3, 6, 9, and 12. At the remaining stations, the cumulative duration of droughts was longer and accounted for 38 to 44% of all months in the whole multi-year period. The droughts recorded for SPI-1 were characterized by a much shorter duration, i.e., between 28% and 34%. The presented graphical distribution indicates a distinct zonality in the course of drought duration in the analyzed multiannual period, which may be related to the location of the Łeba and Lębork stations at low altitudes, in the depression of the Reda-Łeba proglacial valley.

Detailed analysis of the type of droughts according to the adopted classification (Table 3) indicates that the analyzed multi-year period was dominated by mild droughts (SPI from 0 < −0.99), which accounted for 48.4% to 55.2% of drought time at the Łeba station and from 43.4% to 57.1% of time at the Lębork station (Figure 8). Shorter droughts were characterized by moderate and severe droughts. At Lębork station these droughts lasted from 23.9 to 28.8% of the time, while at Łeba from 18.8 to 28.8%. Extreme droughts accounted for from 5.2% to 9.6% of the duration at the Łeba station and from 6.9% to 10.7% at the Lębork station. The duration of droughts at the other stations is shown in Figure S3.

3.4. Analysis of Meteorological Drought Trends

For the multi-year period 1956–2015, an analysis of SPI value trends was carried out in various accumulation periods at all the discussed meteorological stations (

Table 7. Linear trend coefficients of the SPI values for the years 1956–2015.

SPI-1
Station	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
Łeba	−0.003	−0.007	0.007	−0.010	0.006	0.002	−0.005	0.000	0.006	0.003	0.002	0.002
Wierzchucino	−0.007	−0.016	−0.016	−0.004	0.001	−0.012	−0.005	0.002	0.011	0.000	0.010	−0.001
Żelazno	0.000	−0.012	−0.009	−0.002	−0.001	−0.014	−0.011	−0.002	0.012	−0.002	0.006	0.000
Wejherowo	0.009	0.000	0.018	−0.008	0.010	0.004	0.002	0.000	0.006	0.003	0.005	0.012
Lębork	0.004	−0.005	0.011	−0.006	0.006	0.004	−0.002	−0.006	0.007	0.004	0.005	0.010
Słupsk	0.009	0.001	0.010	−0.006	0.003	0.005	−0.005	−0.004	0.006	0.001	0.001	0.010
Kościerzyna	0.013	0.001	0.018	−0.008	0.006	0.000	0.004	−0.005	−0.002	0.001	0.004	0.013
SPI-3
Station	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
Łeba	-0.002	-0.007	-0.003	-0.008	0.002	-0.002	-0.001	-0.003	-0.001	0.004	0.007	0.005
Wierzchucino	0.022	0.020	0.020	0.013	0.015	0.008	0.005	0.001	0.005	0.009	0.013	0.017
Żelazno	0.006	0.008	0.011	0.004	0.007	0.002	0.000	−0.006	−0.005	−0.002	0.002	0.003
Wejherowo	0.012	0.011	0.015	0.007	0.013	0.005	0.009	0.005	0.007	0.007	0.006	0.010
Lębork	0.009	0.005	0.007	0.001	0.008	0.003	0.001	−0.004	−0.003	0.002	0.010	0.012
Słupsk	0.010	0.011	0.012	0.002	0.003	0.002	0.001	−0.003	−0.004	0.000	0.004	0.007
Kościerzyna	0.013	0.013	0.018	0.008	0.011	0.001	0.004	−0.001	−0.003	−0.004	0.001	0.011
SPI-6
Station	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
Łeba	0.005	0.002	0.000	−0.007	−0.004	−0.002	−0.005	−0.003	−0.002	0.002	0.001	0.001
Wierzchucino	0.020	0.024	0.023	0.024	0.023	0.019	0.010	0.008	0.008	0.009	0.009	0.012
Żelazno	0.004	0.006	0.008	0.006	0.009	0.008	0.001	−0.004	−0.004	−0.002	−0.004	−0.003
Wejherowo	0.012	0.011	0.015	0.012	0.015	0.013	0.011	0.011	0.008	0.009	0.007	0.009
Lębork	0.008	0.011	0.011	0.007	0.008	0.008	0.002	−0.001	−0.001	0.002	0.003	0.005
Słupsk	0.009	0.010	0.011	0.009	0.010	0.010	0.002	−0.002	−0.003	0.000	0.000	0.000
Kościerzyna	0.006	0.009	0.017	0.014	0.015	0.012	0.007	0.004	−0.002	0.000	−0.001	0.003
SPI-9
Station	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
Łeba	0.004	0.001	0.002	0.002	0.002	0.000	−0.006	−0.006	−0.002	−0.001	0.001	0.000
Wierzchucino	0.018	0.018	0.019	0.023	0.028	0.024	0.019	0.015	0.014	0.012	0.013	0.014
Żelazno	0.003	0.002	0.004	0.005	0.008	0.007	0.003	0.000	0.000	−0.001	−0.002	−0.002
Wejherowo	0.015	0.012	0.015	0.013	0.015	0.014	0.014	0.014	0.013	0.010	0.010	0.009
Lębork	0.008	0.005	0.007	0.008	0.013	0.011	0.006	0.002	0.003	0.003	0.006	0.005
Słupsk	0.008	0.007	0.008	0.009	0.010	0.010	0.007	0.005	0.004	0.001	0.001	0.001
Kościerzyna	0.007	0.005	0.009	0.008	0.013	0.013	0.012	0.009	0.007	0.003	0.003	0.003
SPI-12
Station	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
Łeba	0.002	0.001	0.002	0.002	0.001	0.001	0.000	−0.002	−0.002	−0.002	−0.001	−0.001
Wierzchucino	0.020	0.020	0.020	0.020	0.020	0.021	0.021	0.021	0.020	0.019	0.017	0.018
Żelazno	0.004	0.003	0.004	0.004	0.004	0.004	0.003	0.001	0.001	0.001	0.001	0.001
Wejherowo	0.015	0.015	0.015	0.016	0.015	0.015	0.015	0.015	0.015	0.014	0.013	0.013
Lębork	0.008	0.007	0.007	0.008	0.007	0.008	0.007	0.007	0.007	0.007	0.007	0.008
Słupsk	0.008	0.007	0.007	0.008	0.007	0.008	0.007	0.006	0.005	0.005	0.005	0.006
Kościerzyna	0.010	0.009	0.009	0.009	0.008	0.008	0.008	0.008	0.009	0.008	0.008	0.010

Note: Statistically significant (at 0.05 level) values are in bold.

). The analysis of long-term SPI trends using the MK test showed that statistically significant changes in drought were recorded in selected months in Wierzchucino, Wejherowo, and Kościerzyna. The increasing drought trend was recorded only for SPI-1 in February and March in Wierzchucino. The downward trend in drought was recorded in longer periods of accumulation (SPI-3, 6, 9, and 12).

3.5. Discussion

In this study, meteorological droughts were identified on the basis of the SPI index in various accumulation periods, i.e., 1, 3, 6, 9, and 12 months. Meteorological droughts are the effect of low precipitation sums in individual months of the analyzed multi-year period. The average annual precipitation in the study area was 20 to 170 mm higher than the average annual rainfall in Poland, determined on the basis of the values from 52 stations in the years 1951–2018, which was 640 mm [76]. Annual precipitation trends have not satisfactorily explained the variability of precipitation throughout the year. Studies conducted in various regions of Poland [4,50] and Central Europe showed no clear changes in annual precipitation sums. Statistically significant changes were rarely reported, e.g., only at less than 20% of over 400 analyzed stations [77]. Annual precipitation totals did not show significant changes in the analyzed period, except for the station in Wejherowo, which is characterized by a significant upward trend. This trend was 10 mm greater than the increase in annual precipitation totals that had been found at some stations in Poland since 1951 [76]. On the other hand, more clear changes were observed in the case of air temperature, which is characterized by a systematic increase by over 0.2 °C per decade, which confirms the previously reported changes in thermal conditions in Poland [78]. On the basis of the SPI values in various time scales, meteorological droughts were identified, which were characterized by high variability, duration, and intensity at the considered meteorological stations. As in the case of other catchments located in lowland areas in Poland [69,79], it was observed that with the increase in the accumulation period of the SPI value, the number of droughts decreased, while their total duration increased. This is due to the accumulation of months in which there was a complete lack of precipitation or their amount was below the long-term average value for a given month. In the analyzed period, the driest years were 1964–1965, 1969, 1982, 1989, 1992, 2003, and 2015. These droughts also occurred in other regions of Poland and Europe [36,52,80,81]. The conducted analysis reveals a high internal variability of the occurrence of meteorological drought in the discussed area. A statistically significant increasing trend in drought was recorded only in February and March for SPI-1, while for SPI-3 a decreasing trend was already recorded at the Wierzchucino station. Therefore, the meteorological drought at the Wierzchucino station was short-lived. Other studies conducted in the Polish Carpathians showed no significant changes in SPI-1 and SPI-3, and changes in SPI-6 were insignificant there [50]. In turn, an inconsistent direction of changes in the SPI-3 value on the Polish coast of the Baltic Sea in spring and summer was demonstrated [52]. On the part of the coast located to the west of the catchment studied here, a decreasing trend of SPI-3 values was found, indicating the intensification of short-term droughts. A positive trend was found for SPI-12 in all months at the station in Wierzchucino and from January to September in Wejherowo, which means that the risk of drought intensification in these regions is decreasing year by year. However, taking into account the positive trend of the average monthly air temperature from March to May and in July and August, it can be assumed that the drought will most likely intensify, and as a consequence may lead to hydrological drought. The presented analyses indicate a significant diversification of the course of meteorological droughts, which may be related to the location of the station in relation to the Baltic Sea and the terrain. The catchment area of the Łeba River presents a unique area located by the Baltic Sea. The Łeba river basin is located in close proximity to the Baltic Sea, and its impact range is estimated at 20–30 km inland [82,83]. This impact, most intense in the coastal zone, is mainly manifested by a decrease in the amplitude of air temperature and an increase in air humidity, which reflects the course of thermal and pluvial continentalism indicators. Studies of the variability of climatic conditions conducted indicate that in the last 65 years, only the temperature of the sea surface and the air temperature have changed in the area of the coastal belt of Poland [84]. Therefore, as suggested by [6], who studied the precipitation regime in the eastern part of the Baltic Sea, in such cases one cannot talk about the variability of trends, but about the variability of low-frequency precipitation. Research conducted by [85] showed that in the long-term context, for Central Europe, the period after 2007 is the driest and warmest since 1881, and the drought was forced by the prevailing large-scale atmospheric circulation, characterized by an increase in the frequency of atmospheric blocking over the North Sea and Central Europe. According to [86], in addition to the well-known influence of the North Atlantic Oscillation, other low-frequency modes of internal variability, such as the Atlantic multidecadal variability, have been found to have a profound effect on the climate of the Baltic Sea region. Further research into many other variables of the Earth system is therefore warranted to establish convincing predictions of climate change for the future.

4. Conclusions

The paper analyzes trends in the occurrence of meteorological drought in the northern part of Poland on the example of the Łeba river basin in the years 1956–2015. The study was based on the analysis of average annual and monthly precipitation sums and the standardized precipitation index (SPI) on a scale of 1, 3, 6, 9, and 12 months. The conducted analysis showed that precipitation varies even in a relatively small area. Annual precipitation trends have not satisfactorily explained the variability of precipitation throughout the year. However, the demonstrated increase in air temperature is clear, which may intensify evaporation and consequently contribute to water shortage. Meteorological droughts, which were identified in the Łeba catchment over the period of 60 years, were characterized by high variability, duration, and intensity. Over a longer period of accumulation of the SPI, the number of droughts decreased, while their total duration increased. Significant variation in the course of meteorological droughts, which may be related to the location of the station in relation to the Baltic Sea and the terrain. The obtained results do not clearly confirm the upward trend of drought in the Łeba catchment area, but there are clear symptoms of changes that should be combined with other meteorological and hydrological variables. Changes in precipitation and air temperature are important in shaping water resources in the Łeba river basin, which requires further research in this area. Despite the limitations of the SPI index, which does not take into account other elements of the water balance than precipitation, its use allowed us to identify and assess meteorological drought as the first stage of drought.