Historic floods and droughts - Climatic Change in Deserts - page 450

Libyan Desert had taken refuge in nearby low-lying caves to avoid the fighting, and
perished from drowning during a sudden flash flood that submerged the caves. In
similar vein, French legionaries bivouacked on the sandy bed of a normally dry wadi
in the Algerian Sahara were drowned by a sudden flash flood that originated in the
adjacent Atlas Mountains and of which they were entirely unaware (Sparks, 1972 ,
p. 331). After that, the French Army changed their standing orders to forbid bivouacs
in desert wadis.
In their detailed analysis of the interactions between desertification and climate,
Williams and Balling ( 1996 , p. 17) stated that drought 'denotes a regional deficiency
in soil moisture which may be caused by a combination of lower than normal pre-
cipitation and higher than average evapotranspiration'. However, they also pointed
out that drought has many wider connotations not covered by this definition. For
example, in the UNESCO-WMO report on Hydrological Aspects of Drought ,the
rapporteurs noted that 'drought is generally viewed as a sustained and regionally
extensive occurrence of below average natural water availability, either in the form of
precipitation, river run-off or groundwater' (Beran and Rodier, 1985 , p. 1). Depending
on when it occurred, a rainfall deficit might have little impact on local pastoralists
but a major impact on local cultivators. Likewise, a reduction in shallow groundwater
recharge might have no immediate impact but an adverse one in future years. These
differing aspects of drought are apparent in such concepts as 'agricultural drought',
'hydrological drought' and 'meteorological drought', and they are analysed in detail
elsewhere (WMO, 1975 ; WMO, 1990 ; Beran and Rodier, 1985 ). To these we might
add 'edaphic drought', which is when the soil has become so eroded and degraded
that it cannot absorb much moisture, and the resulting extreme run-off leads to flash
floods, as indicated in the quotation from Aldo Leopold ( 1949 )atthestartofthis
chapter. One consequence of widespread and prolonged drought is the reduction of
global primary production. Zhao and Running ( 2010 ) estimated that the large-scale
droughts during the warm decade 2000-2009 reduced global net primary production
by 0.55 10 15 g of carbon.

The aim of this chapter is to examine some of the more important factors that
have controlled the magnitude and frequency of historic floods and droughts and to
investigate the degree to which recent human activities may be contributing to floods
and droughts in the desert world and its margins. Along the semi-arid margins of
deserts, such extreme floods are more frequent and generally more widespread than in
the hyper-arid areas; they also appear to be occurring more often in certain areas than
in the time since flood records were first recorded. For example, in the Mojave Desert
of the south-west United States, in the lower Narmada Valley of north-central India,
and in the Fitzroy and Katherine rivers of northern Australia, recent flood levels have
reached heights above present river level last attained several thousand years earlier
(Baker et al., 1985 ; Ely et al., 1993 ; Baker et al., 1995 ; Ely et al., 1996 ).

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