Records on drought for historical reconstruction of climate can be found in many different historical sources from Poland. Their number has significantly increased since the mid-15th century, which is why the mid-15th century was adopted as the initial chronological boundary for the reconstruction of the number and intensity of droughts in the Polish territory using documentary evidence. Below we describe the types of historical sources used to reconstruct droughts in Poland.

Records of droughts in the Polish territory are most often found in narrative sources – chronicles, yearbooks, memoirs, diaries, and travel accounts. The information included in these sources has a varying degree of accuracy. Often only one account concerning drought appeared, e.g. “magna siccitas”. In many of the records, however, more detailed descriptions of the course of droughts and accompanying phenomena were given. In the ancient sources droughts were described above all when their manifestations were very clear and when they had an impact on economic and social life. Another group of sources used by us is daily records that have the character of meteorological observations. Sometimes they were prepared by scholars, such as professors Marcin Biem (ca. 1470–1540) and Michał of Wiślica (1499–1575) of the Jagiellonian University, who conducted such observations in Kraków from 1499 to 1531 and from 1534 to 1551 (Limanówka, 2001), or townsmen with scientific ambitions such as Gottfried Reyger (1704–1788), who began his observations in Gdańsk in 1721 as a 17-year-old man and continued them later, among others as a member of the Naturforschende Gesellschaft in Gdańsk until 1786 (Filipiak et al., 2019). Sometimes daily observations were conducted by amateurs; the best examples of which are the records of the Polish nobleman from the eastern territories of the Polish–Lithuanian Commonwealth, Jan Antoni Chrapowicki, which were conducted for the years 1656–1685 (Nowosad et al., 2007). Sources of this kind are nonetheless relatively rare.

The correspondence, the manuscript press (“written newspapers”), and printed press were also used in the reconstruction of droughts. In the case of written newspapers, these are often records similar to those that appear in chronicles. They were drawn up on a regular basis, which increases their credibility. They provided news from the region, as well as information coming from other countries, e.g. from Lviv, from which a news writer in 1698 wrote: “w tych krajach chaniebnie [! – emphasis added] susze wielkie, dla których na zimę bardzo mało siano, bo nie podobna lemieszem ukroić ziemię” [in English “in these countries shamefully there are great droughts, for which reason we sowed very little for the winter, because you cannot cut the land with the ploughshare”] (Maliszewski, 2018). Other sources that turned out to be useful for the implementation of our project were official files (e.g. protocols from meetings of the regional dietines and the Parliament (Sejm), treasury registers, and inspection reports) and documenting activities undertaken, e.g. in connection with droughts and fires. They reported requests for financial support in connection with drought, tax exemption requests, etc. In economic files one can find explanations for low harvests, which occurred for example due to drought. There are a few sources concerning religious behaviours in which, for example, the organisation of prayers asking for rain or describing the end of a drought were described. When such accounts appeared, it can be assumed that the drought must have been severe for people and the environment.

In addition to the above-mentioned historical sources collected during the queries in Polish, Lithuanian, Ukrainian, and German archives, the authors used several published collections (of varied quality) of historical sources concerning the climate research in the period from the 10th century to the end of the 18th century for Poland, central Europe, or selected regions of central Europe. They include the period from the 10th century to the end of the 16th century (Girguś et al., 1965), the Middle Ages (Malewicz, 1980), 1450–1586 (Walawender, 1935), the years 1648–1696 (Namaczyńska, 1937), and 1772–1848 (Szewczuk, 1939). In the last 20 years, two databases containing over 10 000 weather records were made available in universities in Toruń and Wrocław as part of cooperation between climatologists and historians. They have been used many times to study Poland's climate in historical times (Wójcik et al., 2000; Przybylak et al., 2001, 2004, 2005, 2010; Przybylak, 2011, 2016); they have also contributed to widening the scope of this research.

To sum up, for the purpose of this research over 200 accounts referring directly to droughts and prolonged shortages of rainfall were used, along with a few hundred more descriptions from everyday weather observations, the use and critical elaboration of which allowed periods of drought to be indicated. The state of the preservation of sources for particular periods and for individual regions is uneven. Most of them describe droughts in Silesia, Pomerania, and Lesser Poland. A large number of entries refer to droughts affecting the whole territory of Poland. In the case of Silesia, the distribution of sources is fairly even for the whole period; in the case of other regions their number increases with successive ages. The only exception is the first half of the 17th century, in which the number of preserved records is definitely smaller. To some extent, this was affected by the losses in the state of preserved sources that occurred during the Swedish invasion of Polish territories in 1655–1660. Many sources from the first half of the 17th century were then destroyed as a result of military actions.

The accuracy scale of the collected information is variable. Some accounts provide quite precise information concerning the duration of the drought, even to the accuracy of 1 d, while others are definitely more general – they only indicate the existence of a drought in a given year. It very often occurs that one drought is described in several, or sometimes even several dozen, independent sources, which confirms its high intensity.

To assess the credibility of individual records, it was necessary to conduct a critical source analysis, in which it turned out that sometimes even short accounts provided very important and reliable information, while other records with a similar structure proved to be wrong due to the fact that, for example, the year of the occurrence of the drought was changed (e.g. by 1 year) when the information was being copied from another, earlier source. The sources containing daily records, as in the case of the memoirs of J. A. Chrapowicki (1988) or Reyger (1770, 1788), required a different treatment. It was possible to count the days with precipitation and without precipitation along with a very precise indication of the duration of the droughts.

We used 22 chronologies (17 oak chronologies, 5 pine chronologies, and 1 fir chronology) from different locations in Poland to detect pointer years (Table 1, Fig. 1). Table 1 presents a list of them, including also time coverage and sources. As shown in Table 1, the longest chronology available to us covers the years 996–1986, and it was constructed for western Pomerania (site 5). For Upper Silesia (sites 16 and 18) and Lesser Poland (sites 21 and 22), the pointer years were detected by Opała and Mendecki (2014) and Opała (2015) and by Szychowska-Krąpiec (2010), respectively (Table 1, Fig. 1).

The collected historical sources informing about droughts were evaluated according to a three-level scale, taking into account, first of all, signalled manifestations and consequences of the drought and its duration. The droughts were divided into “extreme”, “severe”, and “moderate”.

Extreme droughts (index -3) constituted periods of no rainfall or very scarce rainfall that were long-lasting – they lasted at least one season (2–3 months or more). The principle that extreme droughts should be recorded in sources from two regions or more was adopted; even in view of the absence of sources this allows us to assume that these were droughts of an exceptional nature, having been noted by many writers. Such an extreme drought of 1473 was described, among others, in the Annals by Jan Długosz and, for example, in the local chronicle of Wrocław by Nicolaus Pol. When the source information indicated an extreme drought but at the same time there appeared information, for example, about the high level of water or floods, which may have indicated heavier precipitation especially in the summer season, it was concluded that no extreme drought had taken place. In agricultural terms, extreme droughts contributed to much earlier cereal harvests; they often seriously threatened the growth and size of yield, as was mentioned in the sources. Descriptions of extreme droughts usually contain several permanent elements: severe water shortages, fires, the destruction of crops; sometimes there also appeared records about the fact that people did not remember a similar drought having occurred in their lifetime. These droughts caused water reservoirs – ponds and lakes – to dry up completely. Sometimes, and probably in an exaggerated way, sources reported the drying up of smaller rivers.

During extreme droughts, there were frequent records of persistent, very low water levels in the largest rivers – the Odra and the Wisła (Table 3). The result was a lack of water for people and animals, halting the work of water mills in whole provinces. The consequences of drought were underlined – particularly a lack of food and high prices. Numerous fires broke out in cities, villages, and forests. The sources used such phrases as “estas ferventissima et siccitas inaudita” [very hot and incredible summer drought], “sidere solari plus solito effervescente et nullas dante pluvias” [extraordinary sun heat and continuous drought], “unaufhörlich trockene Witterung” [unbelievably dry weather] and “alle Bäche vertrockneten” [all streams dried up] and the like, underlining the extreme nature of the drought. Superlative adjectives were very often used.

Severe droughts that lasted almost the whole season but no longer (up to about 2–3 months) were marked with the index -2. When they fell in the spring period of plant growth, they influenced the quality of the harvest. It was frequently reported that crops had dried up in fields, on hillslopes especially exposed to the sun, and with less-humid soils than in the valleys. Those droughts made it difficult for people and animals to obtain water; sometimes they prevented the work of some mills on the rivers, but they did not paralyse grain milling in the entire province. Droughts were incidentally related to forests and meadows. Efforts were made to focus on those descriptions in which at least two of the phenomena described above appeared. There was no requirement to describe such droughts in more sources. Examples of descriptions of severe droughts in different historical sources are presented in Table 4.

Moderate droughts (marked with the index -1) were ones whose appearance was noticeable by people; however, they lasted for a relatively short period of time and affected crops to a limited extent. This group also includes records that seem incidental, are not confirmed in other sources, or may indicate a small range of drought, yet they were significant enough to be recorded in the sources (Table 5). There is no record of consequences (including economic ones). In the description of the drought, a superlative adjective is not used. There appear such expressions as “dürrer Sommer” [dry summer]. In other sources, in reference to the same period of time, there may be records that indicate, for example, rain instead of drought.

Therefore, relatively long periods of 50 years were adopted to assess long-term (secular) frequencies. It should also be added that most probably in the oldest sources from the 15th–17th centuries, primarily droughts of considerable intensity were recorded (i.e. droughts referred to by us as severe and extreme), while those of a smaller scale (moderate) were omitted. This is due to the nature of the sources at the time and the relatively modest number of such records. Therefore, it can be assumed that droughts of -1, and probably in some part also droughts of -2 may be underestimated from the perspective of historical sources. The sources for the 18th century are definitely more precise. In the 18th century, the duration of the drought as well as its territorial range can often be very precisely determined, though not always.

We hypothesised that narrow tree rings are linked with drought. The limited access to water during the vegetation season leads to a water deficit in trees and as a consequence the cambium activity decreases and produces fewer cells, which is positively correlated with tree-ring widths (Liang et al., 2013). De-trending of the chronology was done with R program (R Core Team, 2019) and dplR package (Bunn, 2008) using the smoothing spline option, which reflects trends in the chronology better than other options. The “n-year spline” was fixed at two-thirds the wavelength of n years (Cook et al., 1990). The residual version of the chronology was built by pre-whitening, performed by fitting an autoregressive model to the data with AIC model selection (Bunn, 2008). At first the relationships between tree growth and precipitation were checked. We analysed the effect of climate monthly precipitation and temperature on tree-ring widths using the treeclim package (Zang and Biondi, 2015). Analysis of climate growth relationships for monthly data for Toruń revealed that precipitation during the vegetation season plays a significant role for both pine and oak. For example, a significant positive correlation was observed for June and July for pine, while for oak a positive correlation was observed for the previous August and current June and a negative correlation for August (Fig. 2). For each site, the climate growth relationships were tested against monthly precipitation and temperature data starting from 1951, and they cover a maximum time span depending on the length of the chronology (Table 6). Because the time span was too short (for example for site 2 when chronology covers the years 1951–1986) for some extended analysis going back to previous months, the common period from previous October to current September was taken into account. The sum of monthly precipitation was also included; the months were selected dependably on the significant correlation of the single months and period selected by daily data analysis.

Next we used daily data for Toruń and tree-ring chronologies of pine and oak representing region III. Daily data show more precisely the period of the year which influences tree growth. According to previous findings, the climate growth relationships are comparable at different sites in Poland (Zielski et al., 2010), so we used the relationships between daily data and sites 11 and 13 as a model for the rest of our study sites. The reason for this generalisation was also the limited access to daily data. A period of 90 d with the 1-year lag for the years 1947–2015 was used to find the significant relationships between the daily precipitation data and indexes of tree rings. For this purpose, we used the dendroTools package (Jevšenak and Levanič, 2018). The optimal window of days was revealed to be from 6 May to 3 August for pine with maximal correlation coefficient 0.435 and from 21 April to 19 July for oak with maximal correlation coefficient 0.305. The sums of daily precipitation for these periods were summed and correlated with indexed growth in years of growth reduction (narrow rings) and growth recovery (wide rings). The correlation coefficient is 0.79 (p<0.05) for pine and 0.65 (p<0.05) for oak. Next, the same summed daily precipitations for the selected periods were correlated with the remaining tree-ring indexes (after exclusion of wide- and narrow-ring indexes). The correlation coefficients are 0.40 for pine and 0.16 for oak.

To determine the pointer years we used the dplR package (Bunn, 2008). The minimum absolute relative radial growth variation, above which the growth change from year t-1 to t is considered significant, was 10. Any year in which more than 95 % of trees per site displayed significant relative radial growth variations above 10 was qualified as “extreme reduction”; “great reduction” was determined as between 85 %–95 % of trees; and “moderate reduction” was between 75 % and 85 %.

As shown in Table 2, for the analysis of droughts in the instrumental period, eight long-term series of monthly totals of precipitation have been used. All these precipitation series were checked for completeness. The few data gaps in the analysed series were completed using homogenised precipitation series from the nearest stations. For this purpose, a simple method of constant quotients was utilised (Pruchnicki, 1987). However, due to the lack of available reference series, such a procedure was not used to fill data for the period 1880–1884 for Orzysz. Homogenisation of all the used precipitation series was checked using the AnClim software (Štěpánek et al., 2009).

On the basis of the completed series of atmospheric precipitation, the possibility of obtaining a synthetic precipitation index for the whole country was tested. A similar method was adopted in Brázdil et al. (2007) to determine drought indices in the Czech Republic for the period 1881–2006. In Poland, Kożuchowski (1985) presented a 100-year series of average areal annual atmospheric precipitation for 1881–1980 (his Table 3) calculated from data from 12 meteorological stations using precipitation regression equations relative to altitude above sea level. Miętus (1996), in turn, presented mean areal precipitation for the coastal area. For the analysis, we took 30-year moving correlation coefficients (r) for monthly totals of precipitation counted for the period 1901–2000. All correlation coefficients were statistically significant (p<0.05) with values varying from 0.46 to 0.71 (see Table 7, upper part). Only the Kraków series had a significantly lower value of r (the highest value of 0.33 described the relationship between Kraków and Wrocław). For annual precipitation totals in the period 1951–2000, Kożuchowski and Żmudzka (2003) obtained only slightly higher values of correlation coefficients, varying from 0.6 to 0.8. Unsatisfactory results of r, particularly related to the series for Kraków, suggested that we should not construct monthly precipitation series for the whole of Poland. It seems that the number of long-term precipitation series is probably relatively too small for a country of such an area (312 679 km2). Further analysis was thus carried out on regions delimited by a landscape criterion, though this excludes mountains, whose atmospheric precipitation is spatially and temporally far more variable (Kożuchowski, 1985).

The aim of the analysis of instrumental series was to calculate the number, length, and category of droughts in the area of Poland since 1722, i.e. for almost 300 years. The Standardised Precipitation Index (SPI: McKee et al., 1993) was calculated from monthly precipitation totals to explore the occurrence of droughts in the analysed locations (Table 2). This index is one of the simplest methods used to identify meteorological droughts, since it uses only monthly totals of precipitation and is therefore widely used in the literature. Osuch et al. (2016) state that the SPI is used for both research and operational purposes in over 60 countries. The SPI is also most popularly used in Poland (e.g. Łabędzki, 2007; Kalbarczyk, 2010; Bąk et al., 2012; Bartczak et al., 2014; Osuch et al., 2016; Bąk and Kubiak-Wójcicka, 2017). What is more, the SPI is used also by two institutes mentioned in Sect. 1 (IMGW-PIB and the Institute of Technology and Life Sciences, ITP) and also by the Institute of Soil Science and Plant Cultivation, which is responsible for agricultural drought monitoring in Poland (for more details see Łabędzki and Bąk, 2014). Hence our decision to also use this index in our work.

The program SPI Generator (National Drought Mitigation Center, University of Nebraska) was used to perform this analysis. SPI was initially calculated for 1-, 3-, 6-, 12-, and 24-month timescales. Further analysis was, however, done using SPI calculated only for 1-, 3-, and 24-month timescales. All of them represent meteorological droughts, from short-term to long-term droughts. The last two (SPI3 and SPI24) can also be used as a good proxy for agricultural and hydrological droughts, respectively. For climate conditions in Poland, it was shown that there exists a strong spatial relationship of SPI values (Table 7, lower part). Significant empirical relations were also found between SPI and pure agricultural and hydrological indices. Łabędzki et al. (2008) found high correlation coefficients (|r|>0.7) between SPI and some agricultural indices such as crop drought index (CDI), water deficit (N), and relative duration of soil moisture deficit (tdef). On the other hand, a much weaker relation (r<0.5) was found between SPI24 and hydrological droughts estimated based on SWI-24 (24-month Standardised Water-level Index) for the Wisła river in Toruń by Bąk and Kubiak-Wójcicka (2017). According to them, this relation was reduced by the influence of external factors (the hydropower plant in Włocławek located in middle part of the river, major groundwater basin), and climate factors appearing in the upper and middle parts of the river basin.

To identify droughts (dry months), the criterion proposed by McKee et al. (1993) and modified for Polish climate conditions by Łabędzki (2007) was used. Droughts were divided into three categories based on SPI values: moderate (-0.50 to -1.49), severe (-1.50 to -1.99), and extreme (≤-2.00) droughts. Methods that identify multi-month droughts using the SPI calculated for different, rigidly defined numbers of consecutive months (3, 6, 12 or 24) simplify the analysis, especially in terms of drought duration and calculating the cumulative intensity of the whole phenomenon. Therefore, in this work, we have adopted the following criteria to identify droughts and determine their duration. Firstly, instances of an SPI1 value within any of the above ranges for only a single month were considered irrelevant. Secondly, a drought was considered to be at least two consecutive months during which the SPI1 value was ≤-0.50. Thus identified, a drought was determined both in terms of duration and by category. Thirdly, drought category was determined by the dry month of lowest SPI1 value. A drought was thus considered extreme if the SPI1 value for at least one of the drought months was ≤-2.00. If the SPI1 of the driest month within a particular instance of drought was between -1.50 and -1.99, the drought was determined to be severe. The remaining droughts were qualified as moderate. The number of droughts was determined for years and for climatological seasons. A drought's final month determined its season.

Drought is a widely occurring phenomenon, but its frequency is extremely limited within particular long-term periods. For this reason, it was decided to group numbers of droughts into longer periods. For a fuller comparison of drought occurrence identified on the basis of dendrochronological data (narrow rings), we used instrumental data to calculate the number and duration of droughts within 10-year periods, starting from the slightly shorter period 1722–1730, through full decades, to the 5-year period 2011–2015. Next, we also summed the number of droughts by a 50-year period, also determining seasons in this case, just as we did when analysing the documentary data.

For the purpose of comparison of SPI1 values (meteorological droughts) against historical indices (-1, -2, and -3), the following assumptions were established: the index -1 was attributed to SPI1 values ranging from -0.50 to -1.49; -2 for the range -1.50 to -1.99; and -3 for SPI1 ≤-2.00. Frequency of occurrence of meteorological droughts for the instrumental period was calculated for standard meteorological seasons (December–February, March–May, etc.) as well as for May–July. This allowed for comparison of the occurrence of droughts against their statistics available in documentary evidence (seasons) and dendrochronological data (May–July). The last period was added because for this time a significant influence of precipitation on tree-ring widths in Poland was found (see Sect. 3.2). It was revealed that most of the growth reduction (negative pointer years) was related to the occurrence of drought. Thus, years with extreme, great, and moderate tree-growth reductions can roughly, and with a large probability, indicate the occurrence of extreme, severe, and moderate droughts, respectively. In the case of documentary data such droughts were described using indices -3, -2, and -1.

As mentioned in Sect. 3.1, information about drought in historical times is rather heavily underestimated, in particular in the case of moderate droughts; therefore, documentary data that identified droughts of categories -2 and -3 have frequently been used for the purpose of comparison against other sources. Such an approach also increases the probability that identified droughts occurred in large parts of Poland. In addition, to be sure that they were caused only by climate, the assumption of their occurrence in a minimum of two geographical regions was usually also utilised. On the other hand, for comparison of droughts delimited using dendrochronological and instrumental data, all categories of them were used.

The number of months Ni in each class of drought intensity (moderate, severe, and extreme) was computed for the 1-, 3-, and 24-month timescales. Then the number of droughts per century was calculated according to the following formula proposed by Łabędzki (2007): Ni,100=Nii⋅n⋅100, where Ni,100 is the number of droughts for a timescale i in 100 years, Ni is the number of months with droughts for a timescale i in the n-year set, i is the timescale (1, 3, 24 months), and n is the number of years in the particular study data set.

It seems that droughts were not very frequent in Poland. In particular regions (including droughts presented in sources as nationwide, and therefore also noticeable in individual regions) in total from 33 to 71 droughts were recorded between 1451 and the end of the 18th century (Fig. 3). Most of those were recorded in Pomerania and Silesia and the least were recorded in Greater Poland, Masuria, and Mazovia (Figs. 3 and 4). This is undoubtedly not a reflection of the frequency of droughts in individual regions, but a consequence of the sources preserved for each region. Without a shadow of doubt, the richest and most accurate sources come from two regions: Pomerania (especially from big cities like Gdańsk, Toruń, and Elbląg) and Silesia. It very often happens that one drought is described in many sources from the region; moreover, it is confirmed by records referring to the entire territory of Poland. A drought described in this way can be analysed more accurately. The sources from Greater Poland, Mazovia, and Masuria are definitely poorer. Consequently, it is probable that the number of droughts in these regions was actually higher and close to the number of droughts in Silesia or Pomerania.

Information that refers to the same year and comes from different regions confirms a larger territorial range of drought. This does not mean, however, in cases where such information was preserved only for one of the regions, other areas were not affected by drought. This lack of reports may have resulted from the lack of appropriate sources and not from the fact that there was no drought in a given region. These numbers undoubtedly depend on the surviving sources and reflect part of the actual state of affairs. In order to partially compensate for these source deficiencies, it was assumed that the records referring to drought in the whole country refer simultaneously to each of the six identified regions.

We also calculated the frequency of all droughts occurring (Fig. 5). In the chronological order in the periods of 50 years, the number of extreme droughts (-3) never exceeded five; in the first half of the 16th century only the drought of 1540 was recognised as such, while in the first half of the 17th century extreme droughts were completely absent (Fig. 5). It seems that extreme droughts, whose total number in the period 1451–1800 was 17, were regularly recorded in sources, and this information is quite reliable.

The number of severe droughts (-2) was usually between four and nine in particular periods of 50 years. Many more droughts belonging to this category were recorded in the second half of the 17th century and in the second half of the 18th century; their numbers were respectively 14 and 13 (Fig. 5).

However, the total frequency of extreme (-3) and severe (-2) droughts amounted to 80 and ranged from 4 to 12 in particular 50-year periods, except for the second half of the 17th century and the second half of the 18th century, when there occurred as many as 18 and 16 droughts, respectively (Fig. 5). The increase in the number of identified droughts in the second half of the 17th century was certainly due to the availability of detailed weather records from the period 1656–1685 taken from the memoirs of Jan Antoni Chrapowicki (Nowosad et al., 2007). However, the minimum number of droughts (only four) took place in the first half of the 17th century (Fig. 5), for which, in turn, we recorded significant losses in the sources.

The number of moderate droughts (-1) varied in all 50-year periods from 3 to 6, except for the second half of the 18th century, when there were as many as 11 droughts belonging to this category recorded (Fig. 5). A larger number of such droughts starting from the beginning of the 18th century undoubtedly results from regional sources being more accurate. In this century, many historical sources were created; they now allow for a fairly accurate reconstruction of the weather conditions, including the appearance of smaller droughts and prolonged shortages of rainfall.

Spring (31) and summer (37) droughts prevailed among the recorded droughts. Also, droughts in spring–summer were often mentioned (22) but much less frequently in summer and autumn (4). Rare were droughts that occurred only in autumn (4). Winter droughts were reported only in three different years. In the case of many reports mentioning “a drought occurring this year” it is difficult to decide what the time of its occurrence was.

Nevertheless, the findings should be treated with some caution. The specificity of the chronicle's narrative was that weather phenomena were recorded in the case of their extreme rare character or because of their consequences for human existence. Droughts undoubtedly posed a serious threat to crops during periods of plant growth – above all in spring and summer. In the case of winters, the lack of snowfall could hardly be perceived as a manifestation of drought.

Twenty-two local chronologies of trees (pine, oak, and fir) from Poland were taken into account for detecting negative pointer years, showing narrow rings. In a year in which we have narrow rings at more than one site, we count this pointer year as a “multiple observation” year, whereas in a year with only one observation, at one site, we call it a year “without multiple observation”. In total, 758 pointer years with multiple observations were detected and 432 years without multiple observations. There are 237 multiple observation years of extreme reduction, 122 of great reduction, 252 of moderate reduction, and 147 negative pointer years from the literature (Opała and Mendecki, 2014; Opała, 2015; Szychowska-Krąpiec, 2010) (Fig. 6). The number of pointer years in selected 50-year periods varies (Fig. 7). At least 30 pointer years were noted within the years 1401–1450 and within each of the 50-year intervals from 1701 to 1950. The evidently smallest number of negative pointer years occurred in the first 150 years (Fig. 7). In the years 996–1000, drought did not occur, and therefore this period was omitted in Figs. 6 and 7. However, the small number of pointer years from 996 to 1200 may be related to the low number of samples. This period is called as Medieval Climate Anomaly and reconstruction for northern-central Europe revealed considerably drier conditions for these years (Scharnweber et al., 2019). The number of chronologies varies and depends on region. More chronologies in the last 300 years result from existing old trees. It also led to the detection of more pointer years. According to Neuwirth et al. (2007), during extreme climatic conditions trees react in the same way, but during years of less pronounced weather conditions regional differences in growth reactions increase. Narrow rings observed in the same year in trees from different regions suggests extreme climatic conditions.

Instrumental observations of precipitation in Poland are among the longest-standing observations in the world (Filipiak, 2007). As shown in Table 2, they are available since 1722. In Fig. 8 we present the SPI calculated for eight sites in Poland for 1-, 3-, and 24-month timescales. The values of SPI3 and SPI24 were filtered by 10-element and 30-element low-pass Gauss filters, respectively, in order to more clearly distinguish long-term dry periods. The analysis of Fig. 8 reveals that the occurrence of droughts in different areas of Poland shows both similarities and discrepancies. It is very clear that in northern and central Poland, a long-term (24-month duration, red line) and extreme drought occurred at the threshold of the 1850s/60s. Almost 100 years later (at the threshold of the 1940s/50s) such a strong drought was present across the entire area of Poland (Fig. 8). Except for Kraków, and also Gdańsk in the last few years, severe droughts have not been observable at the turn of the 21st century. In Silesia, a very dry period occurred for almost the entire first half of the 19th century, and then significantly less severe droughts occurred only in the 1950s and 1990s. For the 18th century, we mainly have information for Gdańsk. Figure 8 shows that dry periods (moderate droughts) occurred here only at the threshold of the 1750s/60s and in the mid-1770s. The most extreme droughts in different parts of Poland occurred in different times. For example, in Gdańsk at the threshold of the 1910s/20s, in Koszalin and Orzysz-Mikołajki in the 1850s, in Toruń in the 1910s, in Poznań in the 1980s, and in Kraków in the 1980s and 1990s (Fig. 8).

Trend coefficients calculated for three types of SPI (SPI1, SPI3, and SPI24) are very small and not statistically significant in all study regions. This means that the long-term frequency of droughts in Poland has been stable for the last two or three centuries.

The number of moderate, severe, extreme, and all-category droughts (see Sect. 3 for definitions) in 10-year periods calculated from the Polish instrumental series (oriented from north to south) in the period 1722–2015 is presented in Fig. 9. In the period 1876–2015, for which complete series of SPI are available for all study sites, the number of all-category droughts (Fig. 9d) varies mainly in the ranges 3–4 and 8–12 per decade. Below the lower threshold of this range we must mention the occurrence of only two droughts in the decade 2001–2010 in Warszawa. On the other hand, this range of frequency was exceeded in only three decades. The greatest 10-year number of all-category droughts (14) in the study period was noted in Gdańsk in the decade 1881–1890. In another two decades (1951–1960 and 1991–2000) 13 droughts occurred in Toruń and Kraków, respectively (Fig. 9d). Two decades 1851–1860 and 1861–1870 were very dry in Poland, in particular in its northern and western parts, and the number of droughts varied between 6 and 10 per decade. For pre-1850, the information about drought occurrence is significantly sparser, but it can be stated that in both areas for which data exist (Silesia and Masovia) the number of droughts in the first half of the 19th century (8–14 per decade) was higher than in the rest of the study period. The contrast is particularly great for Silesia (see also Fig. 8). The number of droughts occurring in the 18th century varied from 4 to 8–9 per decade and was typical as in the rest of the study period (Fig. 9).

In line with expectations, moderate droughts evidently dominate, usually with a frequency of 2–8 per decade (Fig. 9a), then severe (Fig. 9b) and extreme (Fig. 9c) droughts with typical frequencies not being much different at 1–4 and 1–3 per decade, respectively. In terms of these drought characteristics (Fig. 9), as with the characteristics described by SPI1, SPI3, and SPI24 (see Fig. 8), no long-term trends are observable in Poland for the last two or three centuries.

For comparison against the number of droughts delimited using documentary evidence, 50-year frequencies of the three categories of droughts were calculated for climatological seasons (Fig. 10). It comes as little surprise that the frequency of all-category droughts was greatest in winter. Other seasons show more or less similar frequencies. In winter, droughts evidently dominated in the study period in the second half of the 19th century, this is particularly well seen in the case of severe droughts, and slightly less so for moderate droughts, which were also quite frequent in the first half of the 20th century. Extreme droughts in winter do not show any significant changes over time, but it should be emphasised here that they were slightly more frequent in 1951–2000 than in 1851–1900. Moreover, in addition to winter droughts, it should be pointed out that the deficiency in precipitation during this season is usually connected to temporarily increasing continentality of climate conditions which are related to the advection of very cold and dry polar continental air masses from the east, sometimes even with the mixture of very cold arctic air masses. During such conditions deep soil frost increases, which does not allow the water infiltration into deeper layers. Thus, almost all melting snow is transformed into spring surface run-off volume and only a negligible part of this volume is transformed into groundwater. Such conditions may lead to the occurrence of very dry spells in spring. In spring, moderate droughts prevailed still in the period 1851–1950 (usually 4–6 cases), with a greater frequency in the first 50-year period. Both severe and extreme droughts were most frequent (usually 1–3 cases) in 1851–1900 and in particular in 1951–2000 (Fig. 10). In summer there is a clear change in the time pattern of drought occurrence: drought frequency rises in the 20th century (except severe droughts) and in the case of moderate droughts particularly in its second half. Frequency of extreme droughts is evidently higher in the 20th century compared to the pre-1900 period. In autumn, moderate droughts do not show great changes in the last two centuries, while severe and extreme droughts were most frequent in the 20th century (Fig. 10).

The frequencies of droughts per century calculated for different durations (2, 3 months, etc.) are shown in Fig. 11. The greatest number of all-category droughts occurred in Gdańsk (165) and in Żagań/Wrocław (155), while the smallest was in Kraków (104). In line with expectations, moderate droughts clearly dominate (55–75). The number of severe and extreme droughts is more or less comparable, most often ranging between 25 and 40. Both of these two categories of droughts were most frequent in the coastal part of Poland and least frequent in Lesser Poland (Fig. 11). Most droughts lasted 2 months (about 60 %–70 %) and then 3–4 months (10 %–20 %). The frequency of droughts of 5-or-more months was less than 10 %. The longest droughts had durations of 7–8 months and occurred in Gdańsk from January to July of 1771, in Wrocław from March to September of 1805, in Poznań from May to November of 1874, in Toruń from March to September of 1900, and in Wrocław (again) from August 1953 to March of 1954 (8 months).

Łabędzki (2007) proposed a simple formula to calculate the frequency of occurrence of dry months and droughts per century based on SPI values (see Methods section). Using his formula we calculated frequencies of dry months using SPI1, short-term droughts (SPI3), and long-term droughts (SPI24), including three categories of them (see Fig. 12). Analysis of this figure shows that the number of dry months in Poland usually ranges around 350 per century (100-year period (from 342 in Orzysz/Mikołajki to 366 in Poznań). The number of short-term droughts (SPI3) for Poland as a whole is comparable and usually ranges around 120 per century (from 119 in Koszalin to 127 in Wrocław and Kraków), while the frequency of long-term droughts (SPI24) is 15–16 per century. The short-term droughts distinguished here using SPI3 are most comparable to droughts delimited using the method proposed in the paper. Ratios of frequencies between moderate, severe, and extreme droughts are generally similar in both methods (Figs. 11 and 12), although in the Łabędzki method there is a greater domination of moderate droughts over the other two categories. Severe droughts are also clearly more numerous than extreme droughts (Fig. 12), which is not so clearly visible in drought frequencies calculated using our method (Fig. 11).

Based on detailed analysis of all documentary evidence gathered for the period 1451–1800 we distinguished 17 megadroughts (also referred to in the paper as “extreme droughts”, index -3) in Poland (see Fig. 5). Six of them – the most severe (Fig. 13) – have been chosen for more detailed presentation here. The main features of each megadrought are described (e.g. time of occurrence, duration, geographical area, consequences for nature, socio-economic impact).