500-year reconstruction of Dez River discharge in southwestern Iran from tree rings

Long-term river discharge records are essential for water resource management, especially in semi-arid regions. However, the short instrumental records in Iran and the limited availability of local paleoclimate proxies to extend these records hinder our understanding of the full range of hydrologic variability. To help overcome these constraints, we have used annually-resolved tree-ring width chronologies from a network of sites spanning the eastern Mediterranean to the Tibetan Plateau to develop the first multi-centennial (∼500 years) river discharge reconstruction for the Dez River basin, a catchment that supports substantial water extraction in southwestern Iran. A Hierarchical Bayesian Regression (HBR) model accompanied by parsimonious predictor selection was used to derive the most robust model output. The selection method retained 26 precipitation-sensitive tree-ring width chronologies, mainly from regions with similar hydroclimate features to the Dez River basin. The resulting reconstruction of the most downstream gauging station accounts for 62% of the variance in river discharge observations, closely matching known historical events and other regional reconstructions. Notably, the underlying recurrence pattern of extreme flow conditions suggests floods were more frequent than low flow extremes (i.e., droughts) during much of the 18th and early 19th centuries. In contrast, during the early 17th century, droughts were more frequent. Worryingly, the frequency of both floods and droughts has increased simultaneously since the beginning of the twentieth century. Our reconstruction could be used to assess current water allocation strategies under conditions similar to past extended dry periods. When combined with future projections, they can help provide more robust assessment scenarios for water management.