HYDROLOGIC PRINCIPLES 21 



isolated mountainous areas throughout the west. The remain- 

 der of the western United States constitutes the arid and semi- 

 arid region where precipitation is perennially insufficient to 

 sustain most crops. Figure 1 shows the areas where average 

 annual precipitation is less than 20 inches. 



the surplus: stream flow 



Stream flow is measured at more than 6,000 gaging stations. 

 These gaging stations provide a quantitative measure of the 

 visible outflow from the drainage-basin tributary to a stream 

 at the point of measurement, as distinguished from the sam- 

 pling methods that must be used for determinations of pre- 

 cipitation upon an area. Records show that the runoff from 

 an area is nearly always less than the precipitation upon that 

 area: for the United States as a whole about 8% inches, or 28 

 per cent, of the average annual precipitation is carried by 

 streams to the oceans. In Fig. 2, the areas where the average 

 annual runoff is less than 1 inch are all in the western half of 

 the country; for the most part they are similar to those areas 

 shown on Fig. 1 as receiving less than 20 inches of precipita- 

 tion a year. The higher mountains and plateaus form humid 

 "islands" in this broad arid region, from which streams flow 

 out upon the arid lands. 



THE RETURN TO THE ATMOSPHERE: EVAPOTRANSPIRATION 



Instruments have not yet been perfected to measure the re- 

 turn of water from the earth to the atmosphere, and there is 

 no adequate means of knowing the distribution of evapotran- 

 spiration in space or time. The difference between precipita- 

 tion and runoff provides an estimate of the evapotranspiration, 

 which may, however, be considerably in error if there are 

 significant changes in storage of soil moisture, ground water, 

 or surface water during the period. For the nation as a whole 

 it has been estimated that about 21% inches of water, or 72 

 per cent of the average precipitation, is returned to the atmos- 

 phere annually by evapotranspiration. 



Thornthwaite has pointed out the significance of "poten- 



