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This paper characterizes temporal and spatial changes of humidity in Poland during April–September in 1981–2020 and its decades. For this purpose, absolute values of climatic water balance (PEI) and its standardized values (SPEI) were used, based on data from the Institute of Meteorology and Water Management – National Research Institute (IMWM‑NRI). The PEI component, potential evaporation, was calculated using the Penman-Monteith method. The average PEI totals were found to be negative across most of Poland, with the lowest values below −250 mm in the central part of the country. A particularly pronounced decrease in the PEI and the SPEI was noted during the decade 2011–2020. Temporal variability of the SPEI indicates that in the last two decades there has been a noticeable increase in moisture deficit. This is particularly noticeable from 2011–2020 when changes in the mean SPEI values were statistically significant (α=0. 05= 0. 05). During the period 1981–2020, the 6‑month area-averaged PEI totals showed a negative trend (15 mm/decade). The minimum and maximum PEI values also showed negative trends. The rate of change of the minimum PEI values was almost 3 times higher than that of the maximum PEI values. The trends for the minimum PEI values are statistically significant (α=0. 05= 0. 05). After the year 2000, a higher prevalence of negative SPEI was found, as well as its intensity in the decade 2011–2020. All negative SPEI categories show positive trends. In contrast, positive SPEI categories are characterized by little or no negative trends, as in the case of extremely wet conditions. Frequency trends for all SPEI categories are not statistically significant at α=0. 05= 0. 05. SPEI changes in Poland confirm the observed, increasing trend of moisture deficit in central and southern Europe as a response to increasing air temperature and evapotranspiration.
Urban et al. (Tue,) studied this question.