122 THE FUTURE OF ARID LANDS 
is sufficient for certain types of crops, and grass is an important 
element of the natural vegetation unless overgrazing has replaced 
it by brush. In the absence of other data these criteria are often 
of considerable value. Where rainfall data are available, even if 
only from scanty records, arid areas are sometimes taken to 
include those with less than 350 millimeters (13.8 in.) and the 
semi-arid those with less than 750 millimeters (29.5 in.). In 
North Africa Tixeront and Drouhin consider as arid lands those 
with an annual rainfall of less than 500 millimeters (see page 
85). An area is classified as “extreme arid” if in a given locality 
at least twelve consecutive months without rainfall have been 
recorded, and if there is not a regular seasonal rhythm of rainfall. 
Some of the territories, e.g., Northern Kenya, Aden, parts of 
Bechuanaland, are known to have an average annual rainfall of 
less than 12 inches, while in most of them the average annual 
rainfall is less than 30 inches. Moreover, in most of these territories 
the annual precipitation occurs within a period of four months or 
less, so that for the remainder of the year the conditions are often 
truly arid. Some account of the water supply conditions in these 
territories and of the water supply investigation and development 
programs carried out in them in recent years has already been 
given (5, 6, 7, 8, 10). But whether arid, semi-arid, or humid, the 
principles involved in the hydrogeological investigation of these 
territories are the same, and the lessons gained in any one of 
them relating to the location, occurrence, or movement of ground 
waters are very largely applicable to the others. Moreover, the 
question of drought, a relative term, affects all of them, although 
in the drier areas droughts are of fairly frequent, but irregular, 
occurrence. 
There are certain other features which are common to all the 
drier territories to be described, namely, their isolation, poor 
communications, the general lack of basic hydrological data relat- 
ing to precipitation, runoff, evapotranspiration and percolation, 
the high cost of labor, transport, and material required to amelio- 
rate the unsatisfactory water supply conditions, and the in- 
ability of the sparse population to pay for such amelioration 
except where there is an economic outlet for their stock or for 
