TheImprovement ofTropical and Subtropical Rangelands

PASTORAL REGIMES 0' MAURITANIA 217
a barren one,2 replete with coutal dunes and deprellSions, such as
the Aftout aa-Sahali, while sand dunes interspersed with barren regs,
or stony surfaces, stretch deep into the interior. Once away from
the littoral zone, it is pOllllible to find some grazing areu despite the
generally barren nature of both dunes and regs, but vegetation is
severely limited by scanty and often saline water resources. In the
interior of the country, occasional plateaus rise above the level of
the surrounding plains, and some of these, most notably the Adrar,
contain important oases. North and eut of the Adrar are several
northeast to southwest oriented bands of sand dunes (extensions of
Erg Chech and Erg Iguidi) that give way still farther north to the reg
deserts of GhaUamane and Yetti. s Occuionallow mll88ifs (al-Hank,
Zemmour Labyad), often with steep cliffs, as well 118 the Hamada
of Tindouf and Wadi Draa, add variety to an otherwise sterile and
monotonous landscape.
The southern and central regions ofMauritania are influenced by
a tropical climatic regime, the impact of which extends as far north
as the Tropic of Cancer to the northwest of the Adrar. But eut
of the Adrar, the 5O-mm isohyet that marks the northern boundary
of the tropical influence dips below the parallel of 18 0 N.· Rainfall
totals increase rapidly south of this line, and the 500-mm isohyet,
marking the southern boundary of the Sahel, extends westward from
just north of Dakar across northern Senegal and along the southern
boundary of the Hawd. Although the exact onset of the rains is
variable, most of it falls during the summer months. The rainy
season usually begins in May around S6libabi and N6ma, and in
July around Nouakchott, Boutilimit, and Tidjikja. As the rains
proceed northward, the variability in their occurrence from year to
year increases steadily (118, for example, the recording of 247 mm at
Atar in 1927 followed by only 31 mm in 1928)6 and their inception is
sometimes delayed until 118 late as August in Tagant and the Hawd.
These summer rains are part ofthe Inter Tropical Convergence Zone,
and their arrival is signaled by the outburst of violent thunderstorms
that also characterize the Kel Tamasheq country farther to the eut.
Along the entire coast rainfall is light, but the effects of the summer
heat are somewhat modified by the influence of the cool offshore
current 118 well as by the frequent occurrence of fog and dew. In the
south, three m~or seasons can be recognized: a hot, humid summer
from June until the end of August, during which most of the year's
precipitation occurs, and a winter season that is subdivided into a
cool, dry period and a hot, dry period!'