Observed maximum extreme precipitation
Table 2 presents maximum values of extreme indices related to the precipitation totals over intervals of 24 h, 5 days and 1 month for the whole research period and their dates of occurrence. Most maximum daily precipitation records occurred in 2011 (five records), then in 2001 (four records) and in 2010 (three records). In 2001 and 2010, major floods were recorded in Poland. Comparison of two intervals: 1961–1990 and 1991–2015, shows that records at 20 stations occurred before 1991 (colder period) and at 27 stations after 1990 (warmer period, what was noted globally—cf. IPCC 2013 and nationally—cf. Kożuchowski and Żmudzka 2001; Graczyk et al. 2017; bold in Table 2). The highest value in this dataset occurred at a mountainous station, Kasprowy Wierch (232 mm on 30 June 1973), while at a lower located station—162.7 mm in Bielsko-Biała (16 May 2010). However, the highest maximum daily precipitation record in Poland, 300 mm, was observed at Hala Gąsienicowa (1520 m a.s.l.) in the Tatra Mts. on 30 June 1973. Even higher 24-h precipitation totals were observed in neighbour countries, near to Polish border during large floods: 312 mm from 12 August 2002, 6 a.m. to 13 August 2002, 6 a.m. in Zinnwald-Georgenfeld (Saxony, Germany), but this record observed from 12 August 2002, 3 a.m. to 13 August 2002, 3 a.m. reached 352.7 mm. A little less, 345.1 mm, was recorded on 30 July 1897 at the Nová Louka station (780 m a.s.l.) in the Jizerské Mts., Czech Republic, while also at Śnieżka, 24-h precipitation total was very high—239 mm (Cebulak 1992; Munzar and Ondraček 2010; Kundzewicz 2011).
Maximum 5-day precipitation total in the studied period was observed most often in July 1997 (five records), then in July 1996 (four records) and three cases during June and July 1980 and during August 2006. The highest precipitation totals were at two high mountain stations: Kasprowy Wierch (388.7 mm on 22–26 July 1980) and Śnieżka (350 mm on 31 July–4 August 1977). There were 19 maximum 5-day precipitation totals recorded before 1991 and 27 after 1990 (bold).
Maximum monthly precipitation sums in the studied interval occurred most often in July 1997 (month with a dramatic flood) and July 2011 (six records each), and then five records in July 2001 (flood month) and in August 2006 (very wet August after very dry July). The highest value of maximum monthly precipitation sum in this dataset was observed in the flood month—July 2001 at Kasprowy Wierch (654.1 mm) and 511.5 mm at a lower-situated station Bielsko-Biała (in May 2010; also flood month). Also, there were less record values of maximum monthly precipitation total before 1991 (18) and more after 1990 (28; bold).
Spatial changes in extreme precipitation
Figure 2a presents percentage change of daily maximum precipitation for the winter half-year and the summer half-year for the interval 1991–2015, related to 1961–1990. Maximum daily precipitation for winter half-year decreases in the west of Poland and increases in the south (except for Kasprowy Wierch and Zakopane, with negative changes) and north, with highest increases, above + 25%, in Elbląg. However, only increase in Kołobrzeg is statistically significant.
Decreases of the seasonal 24-h precipitation for the warm period, presented in Fig. 2b, are lower than for the cold period (the largest decrease is for Śnieżka − 28%, and this change is statistically significant). Daily maximum precipitation increases in the east-southern part of Poland and on the coast; the most for the stations: Świnoujście (+ 28%, statistically significant change), Racibórz (+ 23%), Włodawa (+ 22%), Poznań (+ 19%, also statistically significant) and Jelenia Góra (+ 17%).
Figure 3 shows changes of precipitation total for a longer period, that is maximum 5-day and monthly precipitation total. The maximum 5-day precipitation totals increase (see Fig. 3a) for 35 stations out of 46, the most for Włodawa, Nowy Sącz, Świnoujście, Poznań and Mława (all changes above + 15%), what is visible especially in southern and eastern parts of Poland and on the coast. The highest decrease, statistically significant, has been noted at Śnieżka (− 21%).
Maximum monthly precipitation sum exhibits similar spatial patterns of changes as maximum 5-day precipitation total (Fig. 3b), but there are more statistically significant changes. This index decreases only for 10 stations, and these changes are small, with the largest one, statistically significant, for Śnieżka (− 9%). For other stations, one can observe increases over a large area of Poland, with the highest ones in Włodawa (+ 23%; statistically significant), Słubice (+ 22%; statistically significant), Lublin (+ 21%; statistically significant), as well as in Świnoujście, Tarnów, Szczecin and Siedlce (in all four cases, increases are statistically significant) and Lesko (all changes above + 15%).
Changes in the number of days with intense precipitation (equal to or greater than 10 mm) and in the number of days with very intense precipitation (equal to or greater than 20 mm) are presented in Fig. 4. Number of days with daily precipitation equal to or greater than 10 mm (Fig. 4a) increases especially in the north-western part of Poland with three statistically significant changes (Kołobrzeg, Koszalin, Łeba), then a bit less in the central and south-eastern parts of the country. Less days with intense precipitation have been noted on south-western part of the country with two statistically significant decreases for Śnieżka and Racibórz.
Changes in the number of days with very intense precipitation (equal to or greater than 20 mm; Fig. 4b) have a similar spatial distribution, but are greater. Increases are especially visible for Świnoujście (+ 116%, statistically significant), Szczecin (+ 70%, statistically significant), Chojnice (+ 34%), Białystok (+ 24%) and Włodawa (+ 24%, also statistically significant) (Fig. 4b). Decreases have been observed especially on the south-western part of Poland with a statistically significant change at Śnieżka, and then in the north-central part of the country.
One drought index—maximum dry period with daily precipitation below 1 mm during the summer half-year, from April to September—is getting longer (Fig. 5). Increases (statistically significant in three cases) are visible for 36 stations out of 46, the most for Lublin (+ 19%, statistically significant), Chojnice (+ 18%) and Łeba (+ 17%). For Kłodzko, consecutive dry days, CDD, decreased strongest − 11%.
Table 3 presents synthesis of results for particular indices of extreme precipitation in Poland. For nearly all indices, maximum increase is higher than maximum decrease, except for maximum 5-day precipitation total. Similarly, for nearly all indices, the mean change is positive with except of maximum 24-h precipitation for winter half-year. The highest mean increase was observed for maximum monthly precipitation total, and maximum increase was the largest for number of days with very intense precipitation (≥ 20 mm/day). Statistically significant increases are much more common (21; the highest number for monthly maximum precipitation total, 8) than decreases (6).
Changes in extreme precipitation for aggregated data from all 46 stations
Figure 6 presents long-term variability of mean percentage changes of extreme indices for all analysed 46 stations for 1961–2015, relative to the reference interval 1961–1990 (left column) and box plots for two studied intervals: colder 1961–1990 and warmer 1991–2015 statistical values: min–max, 25–75% and median (right column). Trends for all these indices are statistically insignificant. For five of them, trends are increasing; for one (maximum 24-h precipitation total for winter half-year), the trend is decreasing, and the number of days with intense precipitation (equal to or greater than 10 mm/day) does not show any changes.
The highest increase of maximum 24-h precipitation total for the summer half-year (Fig. 6b1) has been noted in 2010 (increase by nearly 40% relative to the mean 1961–1990) and then in 1997 (increase by nearly 30%). Box plots presented in Fig. 6b2 show minimum, median, 75 percentile and maximum of this index higher for the last 25 years in comparison with period 1961–1990.
Mean change of maximum 5-day precipitation total relative to the mean for the reference interval, 1961–1990, (Fig. 6c1) was the highest in 1997 (increase by nearly 50%), the second one occurred in 2010 (increase by above 40%). Values of 25 percentile, median, 75 percentile and also maximum (Fig. 6c2) were higher for the second, warmer period 1991–2015.
The highest percentage changes for these extreme indices occurred in 2 years with large floods, i.e. in 1997 and 2010, except for the maximum 24-h precipitation total for winter half-year. For this index, the highest value was recorded in 1975 (Fig. 6a1), where increase was by 70% relative to the mean for 1961–1990, as a result of a very wet October 1974 with a high sum of daily precipitation at many stations. For Ustka, monthly precipitation total for October 1974 was the highest (283.1 mm) for all available data from 1951 to 2015 (see Table 2). Comparison of two periods: 1961–1990 and 1991–2015 (Fig. 6a2), revealed that during warmer period minimum, 25 percentile and median were higher than for colder period, while 75 percentile and maximum were lower.
Figure 6d1 shows mean change of maximum monthly precipitation total. In 1997, the change was the highest (increase by 60%), also, another high value was recorded in 2011 (increase by nearly 50%) and a bit smaller in 2010. During the second warmer interval 1991–2015, all statistics presented in Fig. 6d2 were higher than for the first, colder period 1961–1990.
Number of days with precipitation equal to or greater than 10 mm was the highest in 2010 (increase by nearly 50%), and the second greatest value, smaller by 10%, was noted in 1970 (Fig. 6e1). Values of minimum, 25th percentile, median and maximum were higher for 1991–2015 (Fig. 6e2).
For the number of days with very intense precipitation (equal to or greater than 20 mm), mean percentage change was the highest out of all indices (Fig. 6f1). Increase in 2010 reached nearly 90%, relative to the mean for 1961–1990. Second highest record was noted in 1966 and was much smaller—increase by above 40%. Box plots in Fig. 6f2 show that the median and maximum values of this index are higher for the last 25 years than for the earlier period, 1961–1990.
Figure 6g1 presents mean percentage change of the drought index, CDD. The longest period with precipitation less than 1 mm occurred in 2009 (increase by above 40%), as a result of dry September. The second longest period has been noted in 2000, when dry days occurred at the turn of April and May (increase by nearly 40%). Also, very high temperatures were observed in April 2000. All statistics, (Fig. 6g2), except minimum, were higher in the recent period, 1991–2015, than in the earlier period.