Droughts in the area of Poland in recent centuries in the light of multi-proxy data

The history of drought occurrence in Poland in the last millennium is poorly known. To improve this knowledge we have conducted a comprehensive analysis using both proxy data (documentary and dendrochronological) and instrumental measurements of precipitation. The paper presents the main features of droughts in Poland in recent centuries, including their frequency of occurrence, coverage, duration, and intensity. The reconstructions of droughts based on all the mentioned sources of data covered the period 996-2015. Examples of megadroughts were also chosen using documentary evidence, and some of them were described. Various documentary sources have been used to identify droughts in the area of Poland in the period 1451-1800 and to estimate their intensity, spatial coverage, and duration. Twenty-two local chronologies of trees (pine, oak, and fir) from Poland were taken into account for detecting negative pointer years (exceptionally narrow rings). The delimitation of droughts based on instrumental data (eight long-term precipitation series) was conducted using two independent approaches (Standard Precipitation Index, SPI, calculated for 1-, 3-, and 24-month timescales, and a new method proposed by authors). For delimitation of droughts (dry months), the criteria used were those proposed by McKee et al. (1993) and modified for the climate conditions of Poland by Labedzki (2007). More than 100 droughts were found in documentary sources in the period 1451-1800, including 17 megadroughts. A greater than average number of droughts were observed in the second halves of the 17th century and the 18th century in particular. Dendrochronological data confirmed this general tendency in the mentioned period. Analysis of SPI (including its lowest values, i.e. droughts) showed that the long-term frequency of droughts in Poland has been stable in the last two or three centuries. Extreme and severe droughts were most frequent in the coastal part of Poland and in Silesia. Most droughts had a duration of 2 months (about 60 %-70 %) or 3-4 months (10 %-20 %). Frequencies of droughts with a duration of 5-or-more months were lower than 10 %. The frequency of droughts of all categories in Poland in the instrumental period 1722-2015 was greatest in winter, while in the documentary evidence (1451-1800) droughts in this season are rarely mentioned. The occurrence of negative pointer years (a good proxy for droughts) was compared with droughts delimited based on documentary and instrumental data. A good correspondence was found between the timing of occurrence of droughts identified using all three kinds of data (sources).