Determining the Pace and Magnitude of Lake Level Changes in Southern Ethiopia Over the Last 20,000 Years Using Lake Balance Modeling and SEBAL

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 modeling 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 20-30% 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.

Modern Sedimentation and Authigenic Mineral Formation in the Chew Bahir Basin, Southern Ethiopia: Implications for Interpretation of Late Quaternary Paleoclimate Records

“…The Teltele plateau, which consists of Miocene basalts and rhyolite, trachyte and felsic tuff, is located in the eastern part of the Chew Bahir catchment (Figure 1d; see also Davidson, 1983). The northern part of the Teltele plateau extends to the Konso upland, which is drained by the Segen River, one of the major rivers draining to the Weyto River, and then to the Chew Bahir basin (Fischer et al, 2020).…”

“…The tropical African rain belt is commonly referred to as the intertropical convergence zone (ITCZ) but the twice-yearly passing of the tropical African rain belt may not necessarily be synchronized with the annual cycle of insolation (e.g., Nicholson, 2018). Catchment wide mean annual rainfall is in the order of 900-1,000 mm, and the boreal spring rainy season (i.e., March-May) accounts for >50% of the annual total (Fischer et al, 2020).…”

“…Water balance model estimates for paleo-lake Chew Bahir, based on the Surface Energy Balance Algorithm for Land, estimate an actual evaporation rate for the catchment of 847 mm/a (Fischer et al, 2020). Although mesoscale convective processes and topography also play an important role in the development of rain-bearing systems in eastern Africa (e.g., Hession and Moore, 2011), aridity in the catchment area is linked to large-scale divergence in the lower troposphere (Nicholson, 2016).…”

“…During the last African Humid Period (AHP), 20-30% more precipitation led to lake level rises of Lake Abaya-Chamo (Fischer et al, 2020). At that time, overflow spilled into the Chew Bahir basin forming paleo-lake Chew Bahir, which in turn overspilled into Lake Turkana after reaching the overflow sill at 543 masl (Fischer et al, 2020).…”

“…The potassium (K) record of the Chew Bahir basin provides an excellent example demonstrating the complexity of interpreting highs and lows in K content in the sediment as past water balance changes in the catchment (Foerster et al, 2012;Fischer et al, 2020). Foerster et al (2018) considered multiple hypotheses to explain the K record, concluding that K enrichment in the sediment could not entirely indicate episodes of enhanced detrital input, but rather incipient low temperature K-fixation during illitization associated with Mg uptake and clay mineral layer charge increase (Deocampo and Tactikos, 2010;Deocampo, 2015).…”