378 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



The general disposal of the 30 inches of precipitation received is 

 shown diagrammatically on plate 1, in wliich the quantities are 

 rounded off to whole numbers. 



The 81/^ inches of runoff represents, virtually, our manageable water 

 supply. Management of water consists of solving the general prob- 

 lem — national, regional, or local in scope — of how to obtain perennial 

 supplies of water of usable quality at a reasonable price at desired 

 locations for specific uses. Each area of the Nation has several water 

 problems that require solutions, and these may differ greatly from 

 those in other areas of the country. 



WHAT WE KNOW OF THE HYDROLOGIC CYCLE 



Much is known about the fundamentals of hydrology and the laws 

 that govern the occurrence and movement of water on and below the 

 surface of the earth. Research directed to finding these facts has been 

 a continuing and accelerated program throughout the United States 

 and elsewhere and has been carried on by governmental agencies, uni- 

 versities, and foundations. A great deal has been learned, but a great 

 deal more remains to be understood. 



The mechanics of precipitation of water from atmospheric vapor 

 as rain, hail, and snow is fairly well known. However, we still do 

 not understand the distribution of precipitation in time and space 

 well enough to predict how much rain or snow will fall where and 

 when — to say nothing of being able to influence them. Stations for 

 the recording of precipitation and temperature are scattered through- 

 out the United States, but most of them are in heavily populated 

 areas. However, much of the precipitation falls where the population 

 is small and scattered, such as the mountainous areas of the West, 

 which furnish a large part of our water, and records from these areas 

 are spotty and inadequate. More has to be learned about the distri- 

 bution of precipitation in these areas before we can understand and 

 predict our primary source of water (Langbein and Hoyt, 1959, 

 p. 41). 



When water reaches the land surface, a portion soaks into the 

 ground and is stored as soil water which is available for the growth 

 and nourishment of plants. This zone of soil moisture may at times 

 be completely saturated with water — that is, all the pore spaces be- 

 tween the grains of soil and rock may be filled with water, or they may 

 be only partially filled. We need to know more about the mechanism 

 of the filling and draining of the soil-moisture zone. We also need 

 to know a great deal more about how much water is extracted by 

 crop plants and native plants from this soil-moisture zone and how 

 much water is evaporated from the land. 



Between the zone of soil moisture and the water table is the zone of 

 aeration. Water in excess of the amount ("field capacity") that the 



