THE PHYSICAL ASPECTS OF DRY DESERTS 



Brigadier R.A.Bagnold, F.R.S. 

 (London) 



Cause and General Character of Dry Deserts 



A desert can be defined as a region where the physical conditions are adverse to 

 human ecology, beyond some agreed stage. But there is no reason to confine ourselves 

 to human economy, and in any case we ought to think of the economy of desert folk 

 rather than of western civilisation. This kind of definition would be all right if the 

 physical conditions were uniform from one area of the region to another and from year 

 to year. But they are not. Hence for the proper study of the biology and productivity 

 of deserts we must have a clear idea not only of the general physical factors but also 

 of their variation from place to place and from year to year. 



In what follows I shall include the more arid and the less economically inviting 

 desert conditions because I feel one can often see a particular important but narrow 

 part of a wide range of conditions in far better perspective after having seen the ex- 

 tremities of the range. And unfortunately very few biologists have had personal access 

 to the arid extremity. If I shall sin at times as a layman by straying into the realm of 

 biology, it will be for the same reason. 



The primary cause of the great sub- tropical deserts is quite clearly meteorologi- 

 cal, though it is not heat but lack of moisture. Life can thrive in the very hottest 

 spots known. Desert regions lie beneath more or less permanent anticyclones where 

 the dry upper air descends to the ground. Atmospheric moisture is therefore low, rain- 

 fall is slight and precarious. The sun's radiation is for long periods unshielded by 

 cloud. Summer day temperatures are high. The downward seepage of water through the 

 sub- soil is so infrequent that salts tend to rise and accumulate at the surface in ex- 

 cess, through evaporation. The soil is dry for such long periods and to such a depth 

 that young replacement plants may not mature, and ultimately even deep-rooted plants 

 may not be able to exist. The resulting lack of vegetation cover allows a high rate of 

 erosion both by wind and rain. 



The sub- tropical anticyclones like the trade winds are due to geophysical causes, 

 and so are quite unalterable by human agency. The fitful cloud and rainfall on their 

 borders depends on the degree to which disturbances whether local or from outside can 

 upset the general anticyclonic regime. This degree may vary from time to time but the 

 cores of the great deserts have most probably been relatively arid from far back into 

 geological time and must remain so into the future. 



Factors Affecting the Availability of Desert Moisture 



The simple measurement, or classification of desert conditions in terms of lack of 

 moisture is not possible. Too many factors enter. First one must be careful to dis- 

 tinguish those special areas which do not rely at all on the present day rainfall of the 

 region because they get adequate and reliable water some other way. Second we have 



