TheImprovement ofTropical and Subtropical Rangelands

THE MARRl BALUOH OF PAKISTAN 289
valley, the main trend of the ridges is east-west. Valley bottoms and
plateaus in the main Marri tribal area vary from 800 to 1,000 m in
elevation, while hill elevations gradually increase from 1,000 m in the
south to over 3,000 m in the Pathan districts to the north. Passes
are few and widely scattered and paaage from one valley to the next
is often a problem. The result is a confused drainage pattern, the
east slopes of the Sulaiman mountains draining directly to the Indus,
whereas the remainder of the surface runoff is discharged toward the
southwest into the plains around Sibi.
Most of these streams flow during no more than two months of
the year (July and August), and the seasonal nature of the stream
flow is directly related to the paucity of the region's precipitation.
Rainfall totals are low, averaging 75-125 mm in the lowlands, while
totals as high as 300 mm have been recorded under orographic conditions
in the northern mountains. 6 Although rain occasionally falls
in winter-the result of very infrequent cyclonic storms of low mois.­
ture content that drift in from the eastern Mediterranean-the main
rainfall occurs in July, August, and early September. Coming at the
end of the summer monsoon, this rain, sporadic in distribution and
violent in character, falls in the form of torrential thunder showers
that give rise to heavyflooding. Although streams flow only during
part of the year, subsurface water in streambeds is usually available
with only a small amount of digging.
Not only is rainfall sporadic in distribution and sparse in quantity,
but it also occurs during the hottest part of the year, thus
lessening its efFectivenese. Temperatures in excese of SOOC are not
uncommon in the Sibi plains during the summer. Highland temperatures
tend to be cool in summer and cold in winter, 80 that a marked
daily and seasonal temperature range is characteristic. The combination
of high temperatures and low rainfall totals produces a scanty
vegetation cover, and vegetation, like temperature conditions, tends
to be distributed altitudinally.e On the higher elevations are found
IlCattered stands of juniper, pistachio, ash, and wild almond, while
acacias (AcGciG niloticG, A. modutG), olives, and occasional poplar
(Populu euphrGticG),T blue gum (Et&cGI"tu), and willow trees are
found in the lower hills. In the valleys, dwarf palm (NGnno"hop.
ritchieGnG), tamarisk (TGmGriz indicG), Zi.,phu nummulGriG, and
occasional myrtle groves are common. Annual grasses and herbs are
IlCattered everywhere, and it is their fluorescence in late spring and
continued presence in summer, together with that of the trees and
shrubs, that give the area its best pastures.'