Reference evapotranspiration during the growing season in Poland (central Europe) in response to ongoing climate changes (1966-2020)

Evapotranspiration is an important part of the water cycle and influences, for example, water management and agricultural production. The article presents temporal and spatial changes in reference evapotranspiration (ETo) in the growing season (April-September) against the background of climatic conditions in Poland in the years 1966-2020. ETo was calculated using the Penman-Monteith method using data from 37 meteorological stations in the network of the Institute of Meteorology and Water Management-National Research Institute. For the growing season of April-September, the average sum of ETo for Poland was 529 mm. In terms of spatial distribution, the values were lowest in northern Poland (Kolobrzeg, 480 mm; Lebork, 488 mm) and highest in the central lowlands (Warsaw, 570 mm; Poznan, 566 mm). For individual months, the average sum of ETo ranged from 53 to 63 mm in April and September to 108-112 mm in June and July. In Poland, in the years 1966-2020, ETo showed a statistically significant upward trend of 16.4 mm/10 year. This is the result of, inter alia, statistically significant increases in sunshine duration and air temperature and a decrease in relative humidity (respectively: 62 h/10 year, 0.40 & DEG;C/10 year and -1%/10 year) during the growing season in Poland. Within the multiyear study period, the maximum ETo values occurred in 2018, reaching values of >700 mm. At that time, Poland was under the influence of very intense anticyclonic circulation. This is confirmed by the relationship between ETo and the anticyclonic circulation index for central Poland, which is statistically significant for the 55 study years.