Towards a reconstruction of Blue Nile baseflow from Ethiopian tree rings

Tommy H.G. Wils, Iain Robertson, Zewdu Eshetu, Marcin Koprowski, Ute G.W. Sass-Klaassen, Ramzi Touchan, Neil J. Loader

Research output: Contribution to journalArticle

33 Scopus citations

Abstract

Most of the water in the River Nile originates from monsoonal rainfall over the Ethiopian Highlands. Despite warnings of future climate change, little is known about the historical variability in this supply, particularly at annual resolution. Development of tree-ring records in this region has been limited by the occurrence of bi- or multimodal rainfall regimes, causing the development of multiple growth rings that cannot be dated with confidence. In this study, we identified annual rings in 30 Juniperus procera trees from northwest Ethiopia by dendrochronology and AMS radiocarbon dating. Carbon isotope ratios (4 series) and ring widths (73 series) were measured.The carbon isotope series did not contain strong trends possibly attributable to increased anthropogenic atmospheric CO2 concentrations or the juvenile effect. Both carbon isotope values and ring widths were strongly correlated with Blue Nile baseflow, and from composite chronology indices (r=0.75, p < 0.01), a preliminary reconstruction of Blue Nile baseflow back to AD 1836 was developed. Subsample signal strength remained above 0.85 for most of the reconstruction. Uncertainty bands were relatively narrow and the reliability of the preliminary reconstruction was confirmed by correspondence with reported years of drought and famine. The preliminary reconstruction is characterized by an exceptional decline in baseflow during the late AD 1960s. Flows recovered during the late 1990s. Additional sampling is advised to increase replication, spatial coverage and length of the preliminary reconstruction.

Original languageEnglish (US)
Pages (from-to)837-848
Number of pages12
JournalHolocene
Volume20
Issue number6
DOIs
StatePublished - Sep 2 2010

Keywords

  • AMS radiocarbon dating
  • Juniperus procera
  • Sub-Saharan Africa
  • carbon isotopes
  • hydrological reconstruction
  • tropical dendrochronology

ASJC Scopus subject areas

  • Global and Planetary Change
  • Archaeology
  • Ecology
  • Earth-Surface Processes
  • Palaeontology

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