Recent changes in reference evapotranspiration in Romania

In the last few decades, climate changes have become the most important topic in the field of climatology. Reference evapotranspiration (ET0) is often used to identify regions prone to drought or aridity. In this paper, we used monthly data recorded in 57 weather stations in Romania over the period 1961-2007. The FAO Penman-Monteith method, based on air temperature, sunshine duration, relative humidity and wind speed, was employed in order to calculate ET0. Seasonal, annual, winter wheat and maize growing seasons data sets of ET0 were generated. The trends were detected using the Mann-Kendall test and Sen's slope, while an ArcGIS software was employed for mapping the results. The main findings of the study are: positive slopes were found in 71% of the data series considered and almost 30% of the total number of series were found significant at α = 0.05; the highest frequency of the increasing trends as well as their absolute maximum magnitude were detected during summer and maize growing season; in winter, significant increasing changes are specific mainly to the extra-Carpathians regions; in autumn decreasing ET0 is specific to more than 80% of the locations, but the significant decrease characterizes mainly the southern half of the country; during the growing seasons of maize and winter wheat, the increase of the ET0 is dominant for the entire country. The relative change decreases with the increase of the length of the period considered: the most intense changes were detected for climatic seasons, followed by crop growing seasons and annual values. Among the climatic seasons, the highest relative increase is specific to winter followed by summer, spring and autumn, while for the crop growing seasons the values detected are similar.