64 



H. DEANE. 



It is of importance that at times there should be an excess of 

 rainfall over that required to effect saturation of the soil as water 

 is required for drinking and other purposes. A season may be a 

 good one from an agricultural point of view if the soil is kept in 

 a fair condition of moisture throughout the period, although no 

 surplus may run off. On the other hand a season of occasional 

 heavy rainfalls with droughty conditions in between may as 

 regards water supply be superior to the other. 



It is worth while inquiring what are the conditions producing 

 a drought. We know the effect of a drought in losses of crops 

 and stock, but it is as well to notice how these are brought about. 

 A drought is marked chiefly by its effect on vegetation. It is a 

 period during which the drying process of the soil is of such 

 duration as to cause vegetation to wither and eventually fail 

 altogether. In wheat districts a drought may be short and yet 

 destructive if it comes at a time when the crops should be growing. 

 In the pastoral areas longer duration is necessary to produce 

 harmful results. A drought is not measured wholly by the 

 amount of the deficiency of rainfall, but by the accumulated 

 result of the evaporation of the moisture in the soil without the 

 compensation of rain to restore what is being thus lost, this con- 

 dition of things being much intensified by strong winds and high 

 temperatures. It is the progressive drying which I have endea- 

 voured to represent diagrammatically. 



A few more observations are desirable before I proceed to 

 explain the diagram. 



The drying process which goes on in the soil consists partly of 

 evaporation at the surface of the ground of the moisture rising by 

 capillary attraction, and partly of the exhaustion of its moisture 

 due to the evaporation from leaves and external surfaces of plants 

 of the water which has been drawn up out of the soil by their 

 roots : partly also, no doubt, by the gradual effect of gravitation 

 on the suspended moisture in the soil, but I think this is very 

 small. 



