U3 



CHAPTER 8 

 SUMMARY 



Precipitation in arid regions is largely consumed by the 

 native vegetation, and desert plants are adapted to an extreme 

 economy in their use of water. As precipitation increases, the 

 dominant type of growth changes in response to the augmented 

 water supply. In areas of high ground water are found those 

 plants which send their roots to the water table or into the 

 adjoining capillary fringe. Also, there are plants which live 

 with' their roots in water and are responsible for a considerable 

 draft upon the general water supply of the region. 



In closed basins having little or no outflow, the consump- 

 tive use of the native vegetation growing therein is a practical 

 measure of the amount of underground water recoverable from the 

 basin for other uses. This may be determined by study of native 

 vegetation, the position of the water table, and the quantity of 

 water each species uses annually. 



Investigations of use of water by various species of native 

 vegetation have been made under different conditions of climate 

 and depth to water table. The species studied have been limited 

 principally to grasses, small shrubs and water-loving plants 

 adapted to growth in metal tanks. The difficulty of growing 

 larger vegetation in tanks is obvious. 



Depth to water table is an important factor in the quanti- 

 ties of water consumed by vegetation. A high water table results 

 in increased growth and consumptive use. As depth to water be- 

 comes greater, vegetation uses decreasing amounts in practically 

 a straight-line ratio, provided the soil within the root zone is 

 reasonably homogeneous. As the limiting depth at which it is 

 possible for the roots to function is approached, vegetation typ- 

 ical of the area becomes progressively smaller and scarcer and 

 gradually changes from one dominant type to another. 



