A 200-year reconstruction of Kocasu River (Sakarya River Basin, Turkey) streamflow derived from a tree-ring network.

The Sakarya River Basin (SRB) contains one of the most important agricultural areas for Turkey. Here, we use a network of 18 tree-ring chronologies and present a reconstruction of the mean June-July Kocasu River discharge, one of the main channels in the SRB, during the period 1803-2002 CE, and place the short period of instrumental flows (since 1953 CE) into historical context. Over the past two centuries, we found 33 dry and 28 wet events and observed the longest wet period between the years 1880 and 1920. The driest years were 1845 and 1873, and the wettest years were 1859 and 1960. Our reconstruction showed that the extreme short-term drought events that occurred in recent years were minor compared to the severity and duration of droughts that occurred previous to instrumental data. We found four pre-instrumental severe and sustained low streamflow events during the periods 1819-1834, 1840-1852, 1861-1875, and 1925-1931, during which historical records show reduced agricultural production, death, famine, plague, economic crisis, and widespread human migrations. More concerning, however, are current hydroclimate conditions in the SRB, marked by decadal-scale mean flows that dip below the long-term mean (1803-1953) in the late 1970s and have since failed to recover. With the Mediterranean region currently likely experiencing the worst drought in the past ca 1000 years due to human-induced climate change, the future outlook of water resource availability in the SRB could prove catastrophic for human and natural systems.