Upper Nile River flow reconstructed to A.D. 1784 from tree-rings for a long-term perspective on hydrologic-extremes and effective water resource management

Abstract

The Nile River is one of the principal rivers in Africa, with Blue Nile (BNRiF) and Tekeze-Atbara (TARiF) rivers being its largest tributaries. However, long-term hydrological information is lacking in the Nile basin, which is a shortcoming to design and implement sustainable water management. We reconstructed river discharge since A.D. 1784 using tree-ring proxy data to (1) extend the short existing discharge records (2) examine long-term flow variability, and (3) identify characteristics of high- and low-flow periods and their connection with large-scale climate forcing factors like the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole Mode (IOD). The chronology (RPC#1) correlates significantly with annual BNRiF (r = 0.62, p < 0.001) and TARiF (r = . 66, p <. 001) discharge. Reconstructed river discharge showed significant high-frequency variations at 2- to 4-year cycles, and sub-decadal and decadal periodicities at 7–10 and 10–14 years, respectively. The inter-annual discharge deviations from the mean during pluvial (dry) periods reached up to 38% (−32%) in BNRiF and 76% (−65%) in TARiF. El Niño and La Niña events matched with 40% and 59% of extreme-dry and extreme-pluvial episodes, indicating teleconnections influencing the regional rainfall and hydrological system. Reconstructed river discharge showed significantly positive spatial relationships with rainfall and negative spatial correlations with temperature across northern Ethiopia and large parts of the Sahel belt and the White Nile swamps in South Sudan. The short instrumental period did not adequately represent the full range of annual to multidecadal discharge variability present in the reconstruction. Hence, the data presented are crucial to extend hydrological records and to revise existing worst-case scenarios and water management strategies developed based on short instrumental records for water supply and energy production across the Nile basin.