VARIABILITY AND PREDICTABILITY OF WATER SUPPLY 129 
(11) giving figures expressed as total solids have set limits even 
beyond those of Jack. It is clear that both man and his stock can 
drink water which is of a quality not infrequently obtainable in 
deserts. 
To obtain water of a quality suitable for crop irrigation is a 
far more difficult matter, for there are certain salts—the alkali 
carbonates and bicarbonates (black alkali)—which are only 
acceptable to plants in very small concentration. A practical 
limit of only 700 parts per million has been given for the total 
solids permissible in irrigation water, and unless man finds such 
water in quantity, he can neither grow regular crops for their 
own sake nor as fodder for his horses. Shotton’s experience in 
northern Egypt and Libya during World War IT convinced him 
that a water table could be found almost everywhere in this 
desert, but usually of such high salinity that a random well had 
small chance of finding drinking water and next to no chance of 
water of irrgiation quality. He considers it a fair assumption 
that irrigation quality water is not to be expected in a desert 
from its own local and limited rainfall unless exceptional condi- 
tions exist. Of a number of such conditions two are mentioned. 
The first occurs when newly percolated rain, making its way to 
the water table, finds difficulty in mixing with the general body 
of saline water. In the Western Desert during the war many 
water points were established through this cause, with salinities 
from 200 to 2,000 parts per million in a vast area where normally 
the salinity stood at 5,000 or 6,000 (i1.e., unpotable) and ex- 
ceptionally went up to 60,000. Characteristic of such wells were 
the thin depth of good water (typically only a few feet), the very 
sporadic distribution of these patches (undrinkable water could 
exist only 100 yards away), the gradual tendency to become 
more saline with pumping, and the small yield which rarely 
exceeded a few hundred gallons an hour. Indeed, the smallness 
of yield is an inevitable corollary of the fresh water—a fissure or 
pore system open enough to give a large yield would not permit 
the fresh water to remain unmixed with the salt in the first place. 
Such wells, therefore, have no importance in irrigation prospects. 
The second possibility is that of perched water, where geological 
