Hydrological Change and Human Occupation in Deserts

Viewing the diagrams incorporated within the mentioned sources enhances this post.

Hydrological shifts in desert regions have had significant impacts on the viability of the landscape for inhabitation. As a consequence, populations of deserts have fluctuated dramatically. One major controlling factor of the availability of water in deserts is climate. I would like to provide two examples of the influence of climatic change on desert hydrological regimes.

The western coast arid zone of South America includes the Peruvian and Atacama deserts. Initiated by uplift of the Andes and the development of the Antarctic bottom waters and the Peruvian-Humboldt current, the Atacama is generally accepted as being very old (Goudie, 2002). Hyper-aridity has predominated since the middle to late Miocene. Betancourt et al (2000) present evidence of hydrological changes over the last 22,000 years from the Atacama. Vegetational and groundwater change were reconstructed from radiocarbon dated fossil rodent middens and wetland deposits. Rodent middens provide high taxonomic resolution of past vegetation and are used in this study in terms of rainfall seasonality. Diatomaceous wetland deposits provide a more continuous record. Data reveals increasing summer precipitation, grass cover and groundwater from 16.2-10.5ka. Summer-flowering grasses spread to regions now lacking water and vegetation. In addition, another pluvial period occurred from 8-3ka. Betancourt et al suggest teleconnections or insolation forcing to be attributable.

Moving from South America to Africa, Damnati (2000) investigated lake fluctuations as a measure of palaeohydrological and palaeoclimatic change in the Sahara and Sahel. Lake status can be derived from a variety of data including stratigraphical, geochemical and palaeoecology, but also archaeological data since lakes are an important source of food and water in arid and semi-arid regions. It is clear from studying Damnati’s maps of lake status that there was a major pluvial phase 10 to 7ka. At 7 to 6ka many of the lakes shifted to an intermediate or low water balance. From 5ka to the present, lake levels in the Sahara and Sahel have continuously deteriorated to a point where over 95% of the sample have low or intermediate lake levels.

Climatic shifts determining water resources inherently influence occupation of arid and semi-arid environments. Kuper and Kropelin (2006) describe human occupation of the Sahara during the Holocene. Notable occupation events seem to correlate with Damnati’s findings. A major reoccupation of the Sahara occurred at 8.5 to 7ka as monsoonal rains transformed the landscape into a savannah-like environment. The Formation phase 7 to 5.3ka ended abruptly in congruence with diminishing lake levels. During this phase, however, there was the introduction of domestic animals including sheep and goats The Regionalisation phase saw retreat to highland refugia with greater precipitation or temporary lakes. Refugia included the Gilf Kebir and the Sudanese plains. Finally, 3.5 to 1.5 ka, human activities were restricted to northern Sudan. This is known as the Marginalisation phase where rains ceased even in the ecological niches. 

References

Goudie, A.S. (2002) Great Warm Deserts of the World, Oxford: Oxford University Press.