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.

During the past 25 ka, southern Ethiopia has undergone tremendous climatic changes, from dry and relatively cold during the Last Glacial Maximum (LGM, 25–18 ka) to the African Humid Period (AHP, 15–5 ka), and back to present-day dry conditions. As a contribution to better understand the effects of climate change on vegetation and lakes, we here present a new Predictive Vegetation Model that is linked with a Lake Balance Model and available vegetation-proxy records from southern Ethiopia including a new phytolith record from the Chew Bahir basin. We constructed a detailed paleo-landcover map of southern Ethiopia during the LGM, AHP (with and without influence of the Congo Air Boundary) and the modern-day potential natural landcover. Compared to today, we observe a 15–20% reduction in moisture availability during the LGM with widespread open landscapes and only few remaining forest refugia. We identify 25–40% increased moisture availability during the AHP with prevailing forests in the mid-altitudes and indications that modern anthropogenic landcover change has affected the water balance. In comparison with existing archaeological records, we find that human occupations tend to correspond with open landscapes during the late Pleistocene and Holocene in southern Ethiopia.

<p>Modern-day southern Ethiopia exhibits a complex mosaic of vegetation types. These types range from desert scrubland along the shores of Lake Turkana, to woodlands and wooded grasslands in the Omo-River-Lowlands and Chew Bahir catchment, and Afromontane forests of the Ethiopian Highlands. Over the past 20 ka, this region has experienced a variable climate, from the dry Last Glacial Maximum (25-18 ka BP) to the wet African Humid Period (15-5 ka BP), and back to present-day dry conditions. These oscillations likely had an impact on the biosphere and its human inhabitants. The biosphere, especially climate-induced changes in vegetation, in turn have a feedback effect on the local climate – and must therefore be considered in climate models and hydro-balance models. However, there are hardly any data on changes in vegetation during the dry-humid-dry transition of the AHP that could be used to parameterize such models.</p><p>As a contribution to an enhanced understanding of the role that paleo-vegetation could have played during those transitions, we present here a new comprehensive vegetation model. This study links a Predictive Vegetation Model (PVM) with the available vegetation-proxy records from southern Ethiopia, including a new phytolith record from Chew Bahir. The PVM uses an 18-year averaged time series of the Global Precipitation Measurement as well as SRTM elevation data to predict an 18-year averaged time series of MODIS landcover and vegetation parameters using boosted regression trees. We linked the PVM and resulting surface parameters (moisture availability, surface drag coefficient, albedo) with an existing hydro-balance model of the southern Ethiopian Rift to calculate precipitation during the AHP and hence also model the paleo-vegetation during this period. Available paleo-vegetation data including a new grass phytolith record from the sediments of an 11 m-meter long sediment core from the margin of paleo-Lake Chew Bahir were then used to compare model and proxy results. Being able to validate our new model data with actual vegetation proxy data for the first time enables us to gain valuable insights into the paleo-dimension of the vegetation mosaic of southern Ethiopia, a possible habitat of early<em> Homo sapiens</em>.</p>