Assessing seasonal drought variations and trends over Central Europe

The relevance of drought is still often underestimated for temperate climate regions like Central Europe that are characterized by on average ample precipitation. Nonetheless, several drought events in recent years (e.g. 1992, 2003, 2015 and 2018) demonstrated that droughts are a relevant factor for several economic activities (e.g., agriculture, water dependent industries, energy supply, etc.) in Central Europe. This is particularly true for the vegetation period, where increasing evapotranspiration rates due to rising atmospheric temperatures are intensifying existing drought conditions that originally developed from rainfalls deficits. The contribution of this study is an assessment of the long-term variability of drought conditions and seasonal climate trends within 1951–2015 based on a collective of 91 climate stations from the national meteorological services of Germany, Poland and the Czech Republic. Using a set of eight drought and three heavy precipitation indices an aggregated evaluation of seasonal precipitation characteristics is done and the driest seasons are identified for the entire study area as well as for four sub-regions. It is shown that the choice of the study period matters (1951–2015 vs. 1961–2015) as the pronounced (multi-)decadal variability of drought conditions restricts the temporal stability of computed trends. The drought trends computed for 1951–2015 are similar in direction, but generally smaller in magnitude than those of the ten year shorter period 1961–2015, as the 1950s have been a very dry decade in Central Europe. Seasonally, drying trends were observed for spring and less pronounced for summer, while autumn and winter show wetting trends. The seasonal trends are sensitive to shifts in the season definition by one month. Vegetation period I (VP-I) shows stronger drying trends, but less increases in heavy precipitation than spring, while the drought trends are less pronounced in vegetation period II (VP-II) as compared to the summer season, but more trends towards heavy precipitation increases occur in VP-II. These differences are explained by the daily trends in the seasonal cycle that show the strongest drying in April, June and the beginning of August and the strongest wetting in March and September. Generally, heavy precipitation increases prevail over decreasing trends in all seasons, whereby stations with strong drought trends generally have smaller positive or even negative heavy precipitation trends. A simultaneous occurrence of drought and heavy precipitation increases is observed in spring at several stations, particularly in sub-region West.