TheImprovement ofTropical and Subtropical Rangelands

ORJTERIA FOR PLANT SBLEOTION 191
70
eo
~ Growlng HUOn
P-257mm
~;.;~mm (lure)
35
32.5
30
50
25
40
20
35
30
15
25
20
10
15
10
5
I
o 0
J FMAMJJ AS ON 0
EL DJEM (Tunlal.)
FIGURE 8-1
Typical ombro'hermic diacram.
and (2) a mean monthly temperature above lOoe (500F) or a mean
monthly minimum temperature above 5°e (42°F).
It may be ueeful or necessary at times to 888e88 the probability
of drought. In a first approximation, this may be evaluated from
the coefficient of variation of monthly and annual rainfall. A good
indicator of drought probability is the coefficient ofvariation (c.v.) of
annual rainfall (standard deviation over mean). In the Mediterranean
Basin, for instance, a C.v. of annual rainfall equal to 25-30 percent
would indicate eemiarid conditions with a drought probability of
~25 percent; a c.v. of 50 percent, arid conditions with a drought
probability of 40-60 percentj and a c.v. of 100 percent or above,