380 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



to measure and calculate the rate and amount of evaporation from 

 open water and land surfaces and the transpiration of plants. Evapo- 

 ration may be estimated by measuring the loss of water from open 

 pans on the land surface, but application of the evaporation rates to 

 lakes and swamps can be misleading. Eecent studies of energy 

 budgets and heat transfer provide more reliable means of calculating 

 total evapotranspiration than we have had in the past. In the fu- 

 ture we may look to measurement of atmospheric moisture at eleva- 

 tions high above the ground and to the use of instruments carried in 

 aircraft to give us gross figures on the total evaporation from a par- 

 ticular area. 



The water on and below the surface of the ground is not pure; it 

 contains varying amounts of different chemical substances in solution. 

 The amount of material that is carried in solution by the water de- 

 pends upon the solubility of the rocks with which the water comes in 

 contact and the length of time of contact. Eesearch into the physical 

 and chemical properties of water and rocks and the interrelations of 

 the water and the dissolved mineral matter are of extreme importance 

 because of the uses to which we put water. Industries, municipalities, 

 and irrigation all require water that is within certain but different 

 limits of chemical quality. Ground water at a particular place gen- 

 erally has a fairly constant chemical quality, but water in streams in 

 the same area may vary greatly in chemical quality during the year. 

 The constancy of ground-water quality is an attractive feature for 

 consumers whose water-quality requirements are not flexible. 



WATER PROBLEMS OF THE UNITED STATES 



It is easy to see that we cannot manage water as a whole throughout 

 the Nation, because water problems are not the same everywhere. We 

 have therefore oversimplified the major water problems in the follow- 

 ing discussion. The general problem of water in the United States 

 can be broken down into six problems of major importance which 

 plague the Nation's water resources. Plate 1 shows the major areas 

 affected by the problems listed. 



The first problem, and one that comes most quickly to mind, is that 

 of deficient supply — not enough water. Deficient supply is primarily 

 a problem of the southwestern United States, although, as with other 

 problems, it occurs in some areas elsewhere. In such areas, the total 

 supply of water is not sufficient for the demands made upon it. Defi- 

 cient supply may be in part an economic problem. Water may be 

 available, but the cost of obtaining it can be prohibitive. 



The Navajo country of Arizona, Utah, and New Mexico covers 

 about 25,000 square miles and is a good example of an area of deficient 

 supply. It is sparsely settled and probably never will have the prob- 

 lems of an increasing population similar to those of the nearby cities 



