92 THE STUDY OF PLANT COMMUNITIES • Chapter V 



especially if the soil is frozen, and be of no more significance 

 than that of an extremely heavy rain of short duration. That 

 same amount of snow, if it melts over a period of weeks, can re- 

 lease water so slowly that practically all of it will soak into the 

 soil, to become a part of a reservoir to be drawn upon during dry 

 periods weeks or months later. Again, under semidesert condi- 

 tions where the vapor pressure deficit is high, this may not be 

 true because, if the snow remains for long periods, much of it may 

 be lost by evaporation or sublimation. 



2.00 



1.60 



K 



O 1.20 



I 



i 



H 0.80 



0.40 



20 



40 



60 80 100 



MINUTES FROM START 



120 



140 



FlG. 39. A comparison of surface runoff and infiltration on forested pine- 

 land (55 yr.) and on bare, abandoned land in Mississippi when precipitation 

 was at essentially the same rate for both areas.— Adapted from Sherman and 

 Musgrave. 232 



This reserve of ground water derived partly from snow be- 

 comes of greatest importance where the total precipitation is 

 relatively low. The grasslands of our Middle West are much 

 more dependent upon the reserve of ground water than are 

 forested regions where the total precipitation is greater and is 

 more evenly distributed throughout the year. The success of 

 agriculture, especially wheat production in the mixed prairie 

 region of the Dakotas, Nebraska, Colorado, and Wyoming, is, to 

 a great extent, dependent upon the reserve of soil water derived 



