Description: The main objective of this study is to identify different types of drought (moderate, severe and extreme) in the Dobrogea region based on three indicators: the Standardized Precipitation Index (SPI), the Standardized Precipitation and Evapotranspiration Index (SPEI) and the Standardized Flow Index (SFI). The dry periods, from a meteorological point of view, were identified based on a drought index that takes into account only precipitation (SPI) and another one that takes into account both precipitation as well as mean air temperature (SPEI). To highlight the dry periods from a hydrological point of view we applied the procedure for calculating the SPI to monthly discharge time series, through the Standardized Flow Index (SFI).

Description: In this study we have examined the spatial and temporal variability of winter (DJF) streamflow over Romania as recorded at 46 hydrological stations over the period 1935 -2010. An empirical orthogonal function analysis (EOFs) was employed to characterize the spatial variability of winter streamflow. The dominant mode captures in-phase variability of river flow anomalies over the entire country. The second mode is characterized by a north-south dipole, emphasizing the influence of topography over the streamflow variability. Both modes are related with large-scale atmospheric circulation and sea surface temperature patterns. We show that the Arctic/North Atlantic Oscillation, East Atlantic, East Atlantic/Western Russia and Scandinavian patterns control a significant part of the interannual winter streamflow variability as captured by these two modes. Moreover, we show that the winter streamflow is very sensitive to the influence of winter temperatures. Positive streamflow anomalies are recorded during warm winters, which are favorable to precipitation fallen as rain, while cold winters can favor snowy winters and frozen ground and hence reduced winter discharges.

Description: In 2015 large parts of Europe were affected by drought. In this paper, we analyze the hydrological footprint (dynamic development over space and time) of the drought of 2015 in terms of both severity (magnitude) and spatial extent and compare it to the extreme drought of 2003. Analyses are based on a range of low flow and hydrological drought indices derived for about 800 streamflow records across Europe, collected in a community effort based on a common protocol. We compare the hydrological footprints of both events with the meteorological footprints, in order to learn from similarities and differences of both perspectives and to draw conclusions for drought management. The region affected by hydrological drought in 2015 differed somewhat from the drought of 2003, with its center located more towards eastern Europe. In terms of low flow magnitude, a region surrounding the Czech Republic was the most affected, with summer low flows that exhibited return intervals of 100 years and more. In terms of deficit volumes, the geographical center of the event was in southern Germany, where the drought lasted a particularly long time. A detailed spatial and temporal assessment of the 2015 event showed that the particular behavior in these regions was partly a result of diverging wetness preconditions in the studied catchments. Extreme droughts emerged where preconditions were particularly dry. In regions with wet preconditions, low flow events developed later and tended to be less severe. For both the 2003 and 2015 events, the onset of the hydrological drought was well correlated with the lowest flow recorded during the event (low flow magnitude), pointing towards a potential for early warning of the severity of streamflow drought. Time series of monthly drought indices (both streamflow- and climate-based indices) showed that meteorological and hydrological events developed differently in space and time, both in terms of extent and severity (magnitude). These results emphasize that drought is a hazard which leaves different footprints on the various components of the water cycle at different spatial and temporal scales. The difference in the dynamic development of meteorological and hydrological drought also implies that impacts on various water-use sectors and river ecology cannot be informed by climate indices alone. Thus, an assessment of drought impacts on water resources requires hydrological data in addition to drought indices based solely on climate data. The transboundary scale of the event also suggests that additional efforts need to be undertaken to make timely pan-European hydrological assessments more operational in the future.