When the Sahara Was Green by Martin Williams

Author:Martin Williams [Williams, Martin]
Language: eng
Format: epub
Published: 2021-08-02T00:00:00+00:00

FIGURE 7.3. Sketch of a wave-cut bench eroded in older delta deposits within Deriba caldera, Jebel Marra volcano, Darfur Province, western Sudan (28 January 1976). The delta is older than the 5-metres-deep lake, which is dated between 23,000 and 19,000 years ago, and was deposited when the lake was 25 metres deep.

The response of different parts of the desert to this general increase in precipitation would have varied from place to place, depending on geomorphic factors like relief, position in the landscape, nature of the soils or sediments, bedrock type and amount exposed, and proximity to the water table. For example, lakes fed primarily from groundwater tend to be less responsive to minor climatic fluctuations in contrast to lakes fed solely from local rainstorms and runoff. Lakes with a very large catchment area relative to their size will respond more rapidly to changes in runoff than lakes confined within small catchments. Lakes located within volcanic craters like the lakes inside Jebel Marra volcano69 (map 1) in northwest Sudan will depend mostly upon local rainfall and so act as rain gauges (fig. 7.3).

In a classic study of the distribution of trees in relation to rainfall and soil published in 1949, the forester J.D. Smith70 investigated the trees growing along a 1000-kilometre north-south transect across the Sudan (now two countries: Sudan and South Sudan). He found that in the drier regions of the Sudan, acacia species that were growing on sandy soils only needed about two-thirds of the precipitation required than when the same acacia species was growing on clay soils. The reason for this is that water within sand is freely available for plant roots to use, but in clay, water is held under tension and is harder for plants to access. This discovery has some interesting implications. For instance, sand laid down across preexisting clay soils during a time of windier and drier conditions would in due course be able to support a denser tree vegetation than previously even though the precipitation had diminished—a counterintuitive occurrence.

Another example of a counterintuitive response to climate change concerns sand dunes. The activity of any dune is governed by a variety of factors, including the obvious ones of strong winds and a depleted plant cover. Aridity is of course conducive to depleting the plant cover, but an important factor that is sometimes forgotten is sand supply. Dunes in many humid coastal areas are often active even when the vegetation is quite dense because the winds are very strong and there is an abundant supply of beach sand. Rivers flowing through semi-desert areas often carry a sizeable bedload of sand. If local winds blow strongly from one direction and the riverbanks are reasonably free of any dense sand-trapping vegetation, long, narrow dunes form and extend downwind (see Chapter 5, fig. 5.8b). These dunes, known as ‘source-bordering dunes’, are not diagnostic of extreme aridity because all that they require is a regularly replenished sand supply if they are to continue to grow.71 Seasonally flowing rivers in

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