of this effect are provided by the deep artesian wells of Tunis and Tripolitania, by the 

 Nubian Sandstone (Cretaceous) which outcrops in the Sudan but gives good water to the 

 depressions of Kharga and Dakhla, 650 miles to the north and, of course, by the vari- 

 ous artesian basins of Australia. 



It is undoubtedly in such large-scale geological structures that the main hopes 

 for irrigating deserts lie. It would be well not to exaggerate those hopes. Artesian 

 supplies in a desert can only materialise in large and useful amounts (a) if the aqui- 

 fer outcrops outside the desert region, i.e. in an area of adequate rainfall ; (b) if the 

 aquifer has high permeability beneath the desert, so that there can be underground 

 transfer of water and the obtaining of high yields; (c) if the quality of the water re- 

 mains suitable for use in its long underground journey from the intake; (d) if the water 

 budget is balanced — i.e. the extraction is balanced by intake. 



Mainly on the second ground, Du Toit held out little hope of artesian supplies from 

 the Karroo. In the Great Artesian Basin of Australia, much of the water is mineralised 

 as a result of its underground journey from intake to well, to the extent that although 

 it is acceptable to cattle and to man, it is not usable for irrigation. 



The great African hollows of Kharga and Dahkla, where artesian springs were once 

 abundant, are notable examples of large-scale irrigation (mainly of date groves) from 

 artesian water of good quality in an area which is virtually rainless ; but attempts to 

 intensify that cultivation have been accompanied by a continuous demand for more and 

 deeper boreholes, with a steady lowering of water pressure and the drying up of springs. 

 The artesian water,here, then, is a slowly wasting asset on the present scale of culti- 

 vation. Such a picture does not encourage any belief in a spectacular increase in the 

 use of the Nubian Sandstone water in the Western Desert, even if low- lying areas can 

 be found where the water may be met at depths sufficiently shallow to allow economic 

 pumping. Nor is there much hope that this good quality water extends as far north as 

 the Egyptian and Libyan coast, in view of the bad water of the northern oases of Siwa 

 and Jiarabub and the saline water in a deep bore at Tobruk. 



The deserts mentioned above are more fortunate than others which seem to have 

 no hope of a deep- seated supply. The great Arabian Desert, for instance, probably 

 has no suitable geological structure for artesian supply and even if it had, it could not 

 satisfy the necessary condition of an outcrop in a region of good rainfall beyond the 

 confines of the desert. 



This paper is a very general survey, biassed perhaps by the deserts which I know 

 personally. It will have fulfilled its purpose if it sounds a warning against the opti- 

 mism which sometimes pervades discussions on the conversion of deserts to useful 

 productive land. Again it must be emphasised, however, that only full desert has been 

 under discussion. In those fringe areas (South Palestine and parts of Jordan are good 

 examples) where nature has provided at some period of the year an adequate rainfall 

 and yet turns the country to arid desert in the summer, there is every incentive to 

 search for underground water and to use it to balance out the irregularities of the rain- 

 fall. The search may often be long and difficult and the results must always conform 

 to the law that more water cannot be taken from the ground than soaks into it; but sub- 

 ject to those limitations, there is a future for parts of the semi-desert earth which 

 most of the true desert cannot hope to share. 



17 



