34 THE RELATION OF DESERT PLANTS TO 



tion of 470 per cent is produced. No observations of wind velocity on 

 Tumamoc Hill were made, but the air, as has been remarked, is seldom 

 at rest, and strong gales of a velocity probably far surpassing 50 kilos 

 per hour are frequently experienced and often last for hours.* 



EVAPORATION FROM THE SOIL. 



As has been said already, the surface layers of the soil on Tumamoc 

 Hill are air-dry during most of the year. After a shower they dry out 

 rapidly and in so doing shrink in such a way as to be somewhat loosely 

 porous to a depth of several centimeters. The deep cracks so charac- 

 teristically produced in many similar soils upon drying from a puddled 

 condition are not prevalent here. Cracks indeed often form, but these 

 are small and close together and seldom penetrate more than a few 

 centimeters below the surface. 



The high evaporating power of the desert air removes water from 

 these surface layers much more rapidly than the movement in the soil 

 films can supply it from below, and this soon results in the air-dry con- 

 dition just noted. Thus the evaporating surface retreats farther and 

 farther into the soil, evaporation being hindered more and more by the 

 thickness of the nearly air-dry layer through which the water vapor 

 must diffuse upward, and finally an equilibrium must be reached where 

 the rate of upward movement of water in the soil films will equal the 

 rate of evaporation. This point is attained in the rock-bound pockets 

 of the Laboratory hill at a depth of less than a meter, as is shown 

 by the actual amounts of water noted in the dry season, and possibly 

 also by the position of the caliche layer, which may mark roughly the 

 position of the average evaporating surface throughout many centuries. 



Thus the surprisingly large amounts of water found comparatively 

 near the soil surface even at the end of the dry season are undoubtedly 

 due, as has been already remarked, to the presence of a thick layer of 

 air-dry soil, acting like the dust mulch of the agriculturists. If we 

 suppose a soil to be saturated and supplied with water from below, and 

 if it be supposed to be losing water by evaporation at its upper surface, 

 whether or not a dry mulch will be formed will depend upon the rate of 

 water loss as related to the rate of water movement through the soil. 

 With a sufficiently low rate of evaporation water will be supplied from 

 below as fast as it leaves the upper surface, and therefore during a 

 long period of drought much more water should be lost, and this from a 

 much greater depth, under these conditions, than would be the case if 

 the evaporation rate were high enough to far exceed the rate of water 



*0n the influence of wind velocity upon the rate of evaporation, see Hondaille 

 (1892, 1 and 2), Russell (1895), and Davis (1900). 



