ductivity. Unless man finds such water in quantity, he can neither grow regular crops 

 for their own sake nor as fodder for his herds. The truth of this is exemplified in the 

 Australian artesian basins, where very few of the thousands of boreholes are used for 

 irrigation schemes, because of the amount of dissolved salts. Consequently most of 

 the boreholes lie in the semi-desert, where rainfall, exceeding 10 inches a year, pro- 

 vides natural grazing. If long continued drought causes this to fail, water from bore- 

 holes may prevent the cattle dying of thirst but not of starvation. 



If, therefore, the desert is to blossom, water of irrigation quality and quantity has 

 to be found. Underground water is dependent on rainfall, for we can discount juvenile 

 water in this connection. Probably no desert is completely rainless, but a low perco- 

 lation is accompanied by a paucity or absence of springs and that, it its turn, means a 

 slow underground movement of water with ample opportunity to dissolve salts from the 

 containing rocks. Along the Palestine coastal plain, which is not a true desert, we 

 can see this relationship between declining rainfall and increase in the mineralisation 

 of the water until at Rafah, on the Sinai frontier, it is not easy to obtain even satis- 

 factory drinking water. Before reaching this point we can see the increasing difficulty 

 of running satisfactory irrigation schemes. My experience of North Egypt and Libya 

 during the war convinced me that a water table could be found almost everywhere in 

 this desert but usually of such high salinity that a random well has small chance of 

 finding drinkable water and next to no chance of water of irrigation quality. 



Although this taking up of salts in solution is controlled also by the characters of 

 the rock holding the water, I think it is a fair assumption that irrigation quality water 

 is not to be expected in a desert from its own local and limited rainfall unless excep- 

 tional conditions exist. Of a number of such conditions, two may be mentioned. The 

 first occurs when newly -percolated rain, making its way to the water table, finds dif- 

 ficulty 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 2000 parts per million in a vast area where normally the salinity stood at 5000 

 or 6000 (i.e. unpotable) and exceptionally 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 inevi- 

 table 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 to be discussed is that of perched water, where geological 

 structure causes the holding up of water above and quite separate from the main table. 

 The controlling factor is often a bed of shale or clay occurring as a lens or a fold be- 

 tween two aquifers. Such a structure has its limits and as rain joins it, there must be 

 an overflow from the perched position either as a spring (which is unusual in deserts) 

 or underground to the main water table. In this way, a one-way movement may be set 



(1) Shotton, F.W., 1946, Vi/at. & U'a<. Engng. 49, 218-226. 



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