Adaptations to life in deserts - Climatic Change in Deserts - page 38

Figure 4.1. Afar women filling goatskin waterbags, Afar Desert, Ethiopia.
the higher plants, water is the limiting factor, because in contrast to animals that
can move to more favourable environments should the need arise, their mobility is
restricted by their root systems. Successful adaptation to desert living thus requires
an ability to make optimum use of the sporadic distribution of water in time and
space (Evenari et al., 1971 ; Stafford Smith and Morton, 1990 ; Morton et al., 2011 ).
As a general rule, the lower the total annual rainfall, the more variable it is from
year to year, so that areas that normally receive little or no rain may receive several
hundred millimetres of rain in a single, highly localised downpour, sometimes leading
to severe flood damage and loss of life. Dorothea Mackellar (1885-1968) caught this
distinction nicely in her poem My Country in which arid inland Australia is ' a land
of sweeping plains, of ragged mountain ranges, of droughts and flooding rains '. The
contrast here between plain and mountain is important, because the distribution of
surface and subsurface water is quite different in both zones. Indeed, rock type and
relief exert a dominant control on water availability in deserts, at all scales from
regional to local.
Nearly a century ago, the French geographers Emmanuel de Martonne and L.
Aufrere ( 1928 ) classified drainage systems into three broad categories: exoreic ,
endoreic and areic . Exoreic river systems flow from their upland headwaters to the
sea, with the Nile River being a well-known example. Endoreic river systems occupy
internal drainage basins and fail to reach the sea, sometimes terminating in inland