Tree-ring width as an indicator for hydroclimate variability in the Lower Danube region

Paleoclimate reconstructions are increasingly used to characterize climate variability and change prior to the instrumental record, in order to improve our estimates of climate extremes and to provide a baseline for climate change projections. Most of these reconstructions are focused on temperature, precipitation, and/or drought indices, and to a lesser extent to reconstruct streamflow variability. In this study, a regional tree-ring width chronology (i.e. Quercus petraea), from Caraorman forest (Danube Delta, Romania), was used to reconstruct the last ~250 years of annual (from November previous year to July current year) streamflow of the Lower Danube River. The obtained results indicate a stable and significant correlation between the measured tree-ring width from Caraorman forest and Danube streamflow at Ceatal Izmail hydrologic station situated in the south-eastern part of Europe. The statistical parameters of the reconstruction model confirm that our model is stable and robust, explaining 44.9 % of the variance of the Lower Danube streamflow over the period 1920 – 2013. Interannual streamflow variation for the analyzed period indicates 11 extremely high flow years, with streamflow greater than 9200 m³/s (1170, 1771, 1799, 1836, 1838, 1839, 1871, 1876, 1877, 1940, and 2010) and 11 extremely low flow years, with streamflow lower than 5200 m³/s (1750, 1753, 1753, 1773, 1794, 1832, 1843, 1882, 1921, 1964, and 1994). The influence of the large-scale atmospheric circulation on the TRW variability was analyzed using the composite maps of the geopotential height at 500 mb (Z500) and sea surface temperature (SST). High TRW values (e.g. high streamflow periods) from the Caraorman forest are associated with a low-pressure system centered over Europe and positive SST anomalies over the Atlantic Ocean and negative SST anomalies over the Baltic, North, and Mediterranean Seas. These large-scale conditions favor the advection of moist air from the Mediterranean and the Black Sea towards the south-eastern part of Romania, which in turn leads to high precipitation rates over this region. Opposite to this, low TRW values (e.g. low streamflow periods) are associated with a high-pressure system centered over Europe, a northward shift of the storm tracks and negative SST anomalies over the Atlantic Ocean, and positive SST anomalies over the Baltic, North, and Mediterranean Seas. Based on our results we argue that the reconstruction of river streamflow data based on the tree-ring width has important scientific and practical implications for a better understanding of the streamflow variation of the past, necessary for water resource management, and environmental-hydrological protection.