Abstract
The Ethiopian rift is known for its diverse landscape, ranging from arid and semi-arid savannahs to high and humid mountainous regions. Lacustrine sediments and paleo-shorelines indicate water availability fluctuated dramatically from deep fresh water lakes, to shallow highly alkaline lakes, to completely desiccated lakes. To investigate the role lakes have played through time as readily available water sources to humans, an enhanced knowledge of the pace, character and magnitude of these changes is essential. Hydro-balance models are used to calculate paleo-precipitation rates and the potential pace of lake level changes. However, previous models did not consider changes in hydrological connectivity during humid periods in the rift system, which may have led to an overestimation of paleo-precipitation rates. Here we present a comprehensive hydro-balance modelling approach that simulates multiple rift lakes from the southern Ethiopian Rift (lakes Abaya, Chamo and paleo-lake Chew Bahir) simultaneously, considering their temporal hydrological connectivity during high stands of the African Humid Period (AHP, ~15–5 ka). We further used the Surface Energy Balance Algorithm for Land (SEBAL) to calculate the evaporation of paleo-lake Chew Bahir’s catchment. We also considered the possibility of an additional rainy season during the AHP as previously suggested by numerous studies. The results suggest that an increase in precipitation of 20 to 30% throughout the southern Ethiopian Rift is necessary to fill paleo-lake Chew Bahir to its overflow level. Furthermore, it was demonstrated that paleo-lake Chew Bahir was highly dependent on the water supply from the upper lakes Abaya and Chamo and dries out within ∼40 years if the hydrological connection is cut off and the precipitation amount decreases to present day conditions. Several of such rapid lake level fluctuations, from a freshwater to a saline lake, might have occurred during the termination of the AHP, when humid conditions were less stable. Fast changes in fresh water availability requires high adaptability for humans living in the area and might have exerted severe environmental stress on humans in a sub-generational timescale.
Highlights
Eastern African precipitation is characterized by large interannual to centennial variability (Lamb et al, 1998; Nicholson, 2000; Junginger et al, 2014; Liu et al, 2017), controlling water resource availability in the region
When considering a third rainy season to have been active during the African Humid Period” (AHP), the threshold precipitation to fill paleolake Chew Bahir up to its overflow level would have increased by 6.9% (TSCAB = TS+6.9% = 13.4%) compared to the modern day precipitation amount (Figures 5A,C)
We developed a comprehensive Lake Balance Model (LBM) for the southern Ethiopian rift with focus on paleo-lake Chew Bahir and its catchment as a first contribution to better understand the potential response of paleo-lake Chew Bahir to precipitation changes
Summary
Eastern African precipitation is characterized by large interannual to centennial variability (Lamb et al, 1998; Nicholson, 2000; Junginger et al, 2014; Liu et al, 2017), controlling water resource availability in the region. An increase in arid conditions has been documented during the termination of the AHP ∼5 ka BP in eastern Africa (e.g., deMenocal et al, 2000; Barker et al, 2004; Foerster et al, 2012; Junginger et al, 2014; Bloszies et al, 2015) This aridification trend coincides with societal adaptations, such as the introduction of pastoralism to eastern Africa, and the emergence of highly organized and complex state-level societies (demenocal, 1995; Brooks, 2006; Kuper and Kröpelin, 2006; Garcin et al, 2012; Foerster et al, 2015). To better understand potential links between cultural changes and climate variability, paleoclimate records can be used in conjunction with archaeological records to reconstruct major influences on modern civilizations
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