20 CONSERVATION OF GROUND WATER 



periods. The total amount of water returned to the atmos- 

 phere by evapotranspiration is derived by subtraction, but we 

 do not know enough about ground water or soil water, or 

 about infiltration, evaporation, or transpiration to make a 

 quantitative evaluation of the movement of water in those 

 phases of the hydrologic cycle in any drainage basin, except 

 for a few experimental watersheds. 



The records collected to date show everywhere marked 

 changes from day to day, from season to season, from year to 

 year, in the quantities of water passing through each phase 

 of the hydrologic cycle. They also show that some communi- 

 ties, some states, are habitually more favored than others in 

 water resources. The long-term average conditions, rather 

 than the short-term fluctuations, are the limiting factors in the 

 sound development of the water resources. The following 

 paragraphs stress those average conditions within various geo- 

 graphic subdivisions of the country, in order to point out the 

 areas chronically deficient in water supplies, where water be- 

 comes the limiting factor in economic development. As is 

 shown later, the regional water deficiency — as against what 

 civilization might consider desirable — applies to ground water 

 as well as to other phases of the hydrologic cycle. 



THE GROSS WATER SUPPLY: PRECIPITATION 



Precipitation is measured and recorded at about 10,000 

 points distributed throughout the United States. It is recog- 

 nized that existing data are inadequate for many regions — 

 particularly the sparsely inhabited mountainous and desert 

 areas — and the network of stations is being extended every 

 year. The records from these stations form the basis for com- 

 putations of the gross water supply of the Nation. A summary 

 of records from 5,000 stations for the 40 years from 1899 to 

 1938 x shows that the average precipitation over the Nation 

 was about 30 inches; it exceeded 20 inches throughout the 

 eastern half of the country, along the northwest coast, and in 



i Kincer, J. B., Climate and Weather Data for the United States, in "Climate 

 and Man," US. Dept. Agri. Yearbook, p. 711, 1941. 



