Changes in potential evapotranspiration and atmospheric evaporative demand in Slovakia since 1961

Our contribution is aimed on the investigation of changes in humidity characteristics at the most homogeneously considered stations in Slovakia. We used data on the Penman and the Budyko – Zubenokova´s potential evapotranspiration, Tomlain's irrigation indicator and the SPEI to investigate changes in the moisture characteristics. Tomlain's indicator is calculated as the annual sums of the difference between precipitation and potential evapotranspiration (PET) data. The PET for this indicator was estimated using to the Budyko-Zubenokova´s method. The SPEI is used in operational way for meteorological drought monitoring in Slovakia. The PET considered in the SPEI is estimated according to the Penman-Montheit method, which is also recommended by WMO. To obtain the regions with negative atmospheric evaporative demand, we used the relative SPEI using the coefficients of the theoretical distribution estimated for the spatial averaged data of the PET and precipitation. These coefficients were applied to calculate the SPEI using the station data. The stations were chosen to represent the northern, south-western and eastern parts of Slovakia, as they cover a very diverse orography and they have the longest series of observations. Three reference periods were compared, namely the 1961-1990, 1981-2010 and the most recent reference period, 1991-2020. The main goal of the work was to investigate how the potential evapotranspiration characteristics were changed and to what extent the country has a drying tendency. The comparison of the different reference periods has shown that the annual precipitation sums increased slightly, but their increase does not cover the increase of the PET resulting in the drying tendency. The highest negative change was observed in the south-western and south-eastern part of Slovakia hitting the lowland regions with intensive agricultural land use.