up, leaching out the soluble salts from the upper containing rock until the water there- 

 in has little dissolved matter to take up. 



Several examples of this type were developed in the Western Desert during the war, 

 the most notable being at Fuka(^), Its structure^ with a bed of clay separating two 

 limestones and folded into an elongate basin, has been fully described. From measure- 

 ments of exhaustion and replenishment after rain, it was calculated that 25,000 gallons 

 a day could be taken out without risk of failure and that the structure carried a reserve 

 equal to five years' supply to tide over any winters of low rain replenishment. Hence 

 there would appear here to be the possibility of a small irrigation scheme (but only of 

 about 20 acres, if we are to accept David's figures and allow also for use of the winter 

 rains) provided that the water was good enough. Actually the average of several ana- 

 lyses shows : 



Total solids 1400 parts per million 



Sodium chloride 750 " 



Sodium carbonate 50 " " " 



Sodium sulphate 160 " 



Calcium (plus magnesium) carbonate 330 " " " 



It is thus somewhat above the limit mentioned earlier, but in view of the fact that 

 there is here a winter rainfall of perhaps 6 inches which would tend to dissolve out 

 such salts as had been deposited in the soil during the preceding summer, an irrigation 

 scheme appears possible. 



What must be emphasized, however, is the pitiful inadequacy of this, our only 

 spectacular example of perched water - a structure of 170 acres, capable of irrigating 

 20 acres, in a desert that was considerably if not exhaustively probed over perhaps 

 8000 square miles. 



Wartime experience in the eastern Egyptian desert (Red Sea Hills), where rainfall 

 is extremely small and sporadic, showed that by careful attention to geology, aided by 

 geophysical measurements, underground reservoirs of drinkable water could be found • 

 Of 10 wells with drinkable water, 5 were of irrigation quality; but the yields were only 

 of a few hundred gallons an hour with a limited life, and so useless for irrigation 

 schemes. 



I feel therefore, that it should be emphasised that in hot deserts, with their very 

 low rainfall, the derivation from this of underground supplies large enough and fresh 

 enough for irrigation must be an exceptional occurrence, the result of a combination of 

 geological accidents that can only occur very rarely. 



There remains one other source of hope. Some deserts may have a geological 

 structure where a sedimentary formation, occurring at depth, eventually outcrops in an 

 area of normal rainfall beyond the confines of the desert. Provided this formation is 

 water- conducting and insulated from contamination with whatever higher saline water 

 exists, it may be entered by borings and good water may be obtained. Notable examples 



(1) Shotton, F.W., 1944, The Fuka Basin. Roy. Engrs. J.. 107-9, 1946. Wat. & VJat. Engng 

 49. 257-263. 



(2) Paver, A.L., 1946, V/at. & V/at. Engng. 49, 653-662. 



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