186. CONSERVATION OF GROUND WATER 



cover in checking soil erosion and sedimentation of reservoirs 

 outweighs by many times the value of the slight amount of water 

 it uses. 



Although there has been much research into the use of 

 water by various types of cultivated and native vegetation — 

 grasses, shrubs and trees, water-loving types and desert types — 

 we need to learn much more in order to resolve some of the 

 apparent conflicts of existing data. For instance, we do not yet 

 know in detail the variations in use by individual species of 

 plants or the effects of changes in temperature or availability 

 of moisture. Comparison of records of precipitation and stream 

 flow in Tennessee Valley has indicated that annual evapo- 

 transpiration from that watershed is a relatively constant quan- 

 tity. 23 Thorntlnvaite has suggested that in a region where 

 adequate moisture is available, the use of water by vegetation 

 of any type will be dependent only upon the temperatures and 

 hours of sunlight. 24 On the other hand, researches in the West 

 indicate a wide variation in rate of use by various species of 

 plants. 25 



There are increasing indications that the use by a single 

 species is not constant and that many types of vegetation will 

 use up all the water that is available to them in the growing 

 season. 26 In confirmation of this, studies of several ground- 

 water reservoirs recharged chiefly by precipitation have show r n 

 that there is little or no recharge during the growing season, 

 even though rainfall is abundant. A very desirable product 

 of research would be a plant that uses only small amounts of 

 water even when there is plenty available, and yet provides 

 an ample protective cover against erosion. 



Another important question for research is to what extent 

 the water used by vegetation depletes the net water supplies 



23 Fry, Albert, Oral communication. 



24 Thornthwaite, C. W., An Approach to a Rational Classification of Climate, 

 Geog. Rev., vol. 38, pp. 55-94, 1948. 



25 Young, A. A., and H. F. Blaney, Use of Water by Native Vegetation, Calif. 

 Div. of Water Res. Bull. 50, 1942, 160 pp. 



2« Musgrave, G. W., Oral communication. 



