The paper presents characteristic precipitation amounts in months of the period 1986–2015. For each month, the average value (x), minimum (Min), maximum (Max), standard deviation (δ), and coefficient of variation (v) were calculated. Monthly precipitation totals for Western Polesie are highly variable (Table 2). The presented data show that the highest average monthly precipitation occurred in July and amounted to 77.4 mm, while the lowest average precipitation occurred in January – 23.8 mm. The lowest total monthly precipitation was recorded in January 2000 and amounted to 1.1 mm, and the highest in August 2006 which amounted to 262 mm. The greatest variation in precipitation was determined using standard deviation in July – 54.1, and coefficient of variation in August – 76,2%. The smallest standard deviation was recorded in March – 14.3, and the lowest variability was found in November – 49.3% (Table 2). Annual precipitation ranged from 347 mm in 2003 to 746 mm in 2014 (Fig. 2). The average annual precipitation in the study area was 540 mm and was 100 mm lower than the average annual precipitation in Poland, which was 640 mm. Studies conducted in Europe did not show any significant changes in annual precipitation [Ziernicka-Wojtaszek & Kopcińska, 2020]. In Western Polesie, the total annual precipitation ranged from 510 mm in Terespol to 580 mm in Bezek. The increase in precipitation towards the south is due to the increase in terrain altitude from 133 to 197 m above sea level.
Table 2
Characteristics of monthly precipitation
Month | I | II | III | IV | V | VI | VII | VIII | IX | X | XI | XII |
x | 23.8 | 25.6 | 28.4 | 35.5 | 62.9 | 60.7 | 77.4 | 60.6 | 52.3 | 38.2 | 33.8 | 30.5 |
δ | 16.1 | 15.7 | 14.3 | 19.7 | 33.4 | 39.6 | 54.1 | 46.2 | 31.3 | 27.7 | 16.6 | 17.4 |
Min | 1.1 | 2.6 | 7.4 | 5.9 | 19.6 | 7.1 | 7.5 | 3.8 | 6.8 | 6.7 | 1.9 | 3.5 |
Max | 62.7 | 67.1 | 74.9 | 84.1 | 174 | 172 | 242.4 | 262 | 141 | 122.8 | 72.5 | 82.2 |
v | 67.7 | 61.5 | 50.4 | 55.4 | 53.0 | 65.2 | 69.9 | 76.2 | 59.9 | 72.4 | 49.3 | 57.3 |
Table 3
Duration of individual dry and wet periods
Temporal (monts) | SPI-3 | SPI-6 | SPI-12 |
Dry periods |
3 | 12 | 8 | 5 |
6 | 2 | 4 | 5 |
12 | 1 | 2 | 2 |
Wet periods |
3 | 8 | 5 | 4 |
6 | 5 | 4 | 4 |
12 | 1 | 2 | 3 |
In Poland, the lowest precipitation occurs in the Greater Poland Lake District, where it amounts to 490 mm per year and increases towards the north and south [Ziernicka-Wojtaszek & Kopcińska, 2020]. Based on the hierarchy of small retention needs, Western Polesie was classified as an area requiring an increase in water resources [Kowalczak et al., 1997]. Annual rainfall totals show multiannual precipitation variability but do not provide insight into annual variability. In the analyzed period, precipitation shows a statistically insignificant increasing tendency. The IPCC report [Stocker, 2014] shows that although the amount of precipitation does not change, the frequency of precipitation decreases and its intensity increases. In the analyzed area, this is particularly visible in August (Table 2), when the lowest precipitation was 3.8 mm and the highest 262 mm.
The SPI results for 1, 3, 6, and 12 months are presented in Fig. 3. These charts are used to indicate the temporal variability of the SPI series in the Western Polesie. SPI time series are free from a linear trend. All four series of SPI results show cyclical behavior: periods of intense drought (negative SPI values) alternating with wet periods (positive values). Based on the series of results, we can identify four main periods of intense drought during 1986–2020. They took place in the years 1990–1993, 2002–2006, 2007–2008 and 2019–2020 respectively. The dry period lasted from one to three years, followed by a wet period. Meteorological droughts were followed by more severe agricultural droughts. Catastrophic agricultural droughts in Poland occurred in 1992, 2003, 2009, and 2019 [Łabędzki, 2007; Łabędzki & Bąk, 2014; Ziernicka-Wojtaszek, 2021]. Three wet periods were identified, lasting between two and three years. Intense wet periods occurred in 1997–1999, 2009–2011, 2013-14. The consequence of heavy precipitation was the occurrence of disastrous floods in 1997 and 2010 [Kundzewicz et al., 1999; Svetlana et al., 2015; Bryndal et al., 2017]. In agriculture, the effects of drought that occur after a period of precipitation deficiency are important for assessment. Agricultural drought is most often a consequence of meteorological drought. For this reason, periods longer than 1 month (3-, 6- and 12-months) used for long-term precipitation anomalies are also taken into account. Agricultural drought is preceded by 3–6 months of precipitation deficits and negative effects of precipitation deficits (soil moisture, groundwater depth, crops) [Łabędzki, 2007; Grzywna, 2017; Radzka et al., 2019; Oleszczuk et al., 2022]. It was observed that as the accumulation period of the SPI index value increased, the number of droughts decreased and their total duration increased (Table 3).
The longest meteorological drought lasted from 3 months for SPI-1 to 49 months for SPI-12. For SPI-6 and SPI-12, the longest drought began in July 2002 and ended in July 2006. However, based on the analysis of changes in SPI-1 and SPI-3, it was noted that this drought began in December 2001, when the indicators reached the value of -2.15. This phenomenon was caused by several short meteorological droughts in 2002–2006, because in January and September 2003, the SPI-3 value exceeded − 2.0. During this period, droughts were much more intense than in previous years. One of the reasons was the high air temperatures recorded in Poland in 2001. In the analysis period, an increase in the number of hot days (Tmax ≥ 30°C) was recorded during the intensive vegetation period (from May to September), which was particularly visible in 2006 [Graczyk et al., 2017; Kaszewski, 2020]. The second cause of long-term drought was low annual precipitation, which in 2002–2005 ranged from 350 to 440 mm. In the case of SPI-6 and SPI-12, the second-longest drought lasted from August 2007 to November 2008. One of the main causes of this drought was very low precipitation totals in spring and winter 2007. In the period February-April 2007 and October 2007-February 2008, monthly precipitation totals did not exceed 15 mm. The lowest precipitation for many years was recorded in April and December 2007, 9 and 5 mm, respectively. The total annual precipitation in 2007 was also very low and amounted to 411 mm. In 2007 and 2008, the second cause of drought was high air temperatures. The average annual temperature was then 9.5oC and was 1.5oC higher than the multiyear average [Kulik et al. 2016]. In the case of SPI-6 and SPI-12, the tree-longest drought lasted from July 2019 to May 2020. One of the main causes of this drought was very low precipitation totals in spring and summer 2020. In February, April, and July 2019, monthly precipitation totals did not exceed 15 mm. The total annual precipitation in 2019 was also very low and amounted to 427 mm. Moreover, in 2019, June was the warmest month in the entire multi-year period with a temperature deviation of 4.8oC [Bulletin, 2019]. In the period 1990–1993, for SPI-3 and SPI-6, four series of droughts lasting approximately 6 months were recorded. Meteorological droughts in the years 1989–1994 were also found in the river catchments of northern Poland [Bąk & Kubiak-Wójcicka, 2017]. In the years 1990–1992, catastrophic droughts covered over half of Poland [Łabędzki, 2007]. Meteorological droughts in 1990–1993 and 2002–2005 also occurred in other European countries [Spinoni et al., 2015; Trnka et al., 2016; Hanel et al., 2018; Spinoni et al., 2019; Hänsel et al., 2019]. In turn, drought in 2007–2008 occurred in northern Poland [Kubicz and Bąk, 2019; Kubiak-Wójcicka et al., 2023]. The same periods were confirmed in studies using the standard precipitation evapotranspiration index (SPEI) [Somorowska, 2016].
There is a lot of information in the literature about meteorological drought, but it is difficult to find information about meteorological floods [de Oliveira-Júnior et al., 2021]. The information presented in references most often concerns wet years and the Growing Season. However, these studies most often analyses only the total annual precipitation over multiyear [Caloiero et al., 2018; Gajić-Čapka et al., 2015; Ziernicka-Wojtaszek & Kopcińska, 2020]. In turn, other studies analyses the amount of precipitation during the Growing Season against the background of evapotranspiration [Tomczyk & Szyga-Pluta, 2019]. Various indicators of extreme precipitation are also used to characterise periods of the meteorological wet period (maximum monthly precipitation, number of days with precipitation over 10 mm) [Pińskwar et al., 2019; Twardosz & Cebulska, 2020].
The longest meteorological wet period lasted from 5 months for SPI-1 to 37 months for SPI-12. For SPI-6 and SPI-12, the longest excess wet period began in July 2009 and ended in June 2012. This phenomenon was caused by several short periods of meteorological moisture in the years 2009–2012, in July 2009 and 2011, and in May 2010 the SPI-1 value exceeded 2.0. One of the reasons was the high precipitation recorded in Poland in 2009–2011. The highest annual precipitation in Western Polesie was recorded in 2010 and amounted to 740 mm, i.e. 200 mm higher than the multiyear average (Fig. 2). The main cause of the hydrological flood in May 2010 was very high precipitation, which amounted to over 150 mm per month. The 2010 flood was also noticed in the Czech Republic and France [Gaume et al., 2016; Alfieri & Thielen, 2015]. In the case of SPI-12, the second longest wet period lasted from July 1997 to January 2000. However, based on the analysis of changes in SPI-1 and SPI-3, it was noticed that the excess moisture began in May 1997, when the indicators exceeded the value of 1.5. One of the main reasons for this condition was very high precipitation amounts in May and July 1997, 122 and 208 mm, respectively. The sum of annual precipitation in 1997 was also very high and amounted to 700 mm (Fig. 2). Another cause of this drought was low air temperatures because the average annual temperature in 1996 was 6.3oC and was 1.2oC higher than the multiyear average [Kaszewski 2020]. The occurrence of a hydrological flood in 1997 was also confirmed in Germany [Becker & Grünewald, 2003]. The last period of meteorological humidity recorded for SPI-12 lasted from April 2013 to May 2015. The main cause of excess water during this period was heavy precipitation in May and June 2013, which exceeded 110 mm per month. Extreme precipitation was also recorded in May and August 2014, with amounts of 173 and 108 mm, respectively. Moreover, in 2014, the highest precipitation in multiyear was recorded, which was 746 mm. The 2014 flood also occurred in southern Poland [Twardosz & Cebulska, 2020].
The study also compared the frequency of dry and wet periods in the 20th century (1986–2000) and the 21st century (2001–2020). The frequency of dry periods over 35 years was 27.2%, of which in the 20th and 21st centuries it was 22.2 and 32.2%, respectively. Extremely dry periods for SPI-1 occurred with a frequency of 2 and 8. The frequency of wet periods was 31.4%, of which in the 20th and 21st centuries they were 24.4 and 38.3%, respectively. Extremely wet periods for SPI-1 occurred with a frequency of 1 and 8. The presented data show that in the 21st Century, there has been a statistically significant increase in the frequency of dry and wet periods. The phenomena of dry and wet periods appeared with greater intensity and longer duration. Moreover, the occurrence of dry and wet periods was characterized by very high temporal variability. For example, in 2011, after an extremely wet July (SPI-1 was 2.24), an extremely dry September occurred (SPI-1 was − 2.14). The greatest variation in wet conditions was found in August, in 2006 and 2015 SPI-1 was 2.68 and – 2.40, respectively. The rapid changes in the amount of precipitation are one of the premises of climate change.