STREAM-FLOW 53 



for the country. By comparing this map with that of rainfall 

 in Fig. 1 1 , a fairly good idea of the relation between rainfall and 

 run-off may be had. Table XXII furthermore gives the run-off 

 for various watersheds in the United States. 



4. STREAM-FLOW 



Definition of Terms. The volume of water flowing in a river 

 is generally defined as " stream-flow " and is expressed in various 

 terms depending upon the particular class of work for which it is 

 to be used. The term used in the reports of the U. S. Geological 

 Survey are: Second-feet, second-feet per square mile, acre-feet 

 and depth in inches. Of these the first two represent the rate of 

 flow only, while the two latter represent the actual quantity of 

 water. They are defined in the Survey Reports as follows: 



" Second-foot " is an abbreviation for cubic foot per second 

 and is the unit for the rate of discharge of water flowing in a stream 

 1 foot wide, 1 foot deep, at a rate of 1 foot a second. It is gen- 

 erally used as a fundamental unit from which others are com- 

 puted by the use of the factors given in the following table of 

 equivalents. 



" Second-feet per square mile " is the average number of cubic 

 feet of water flowing per second from each square mile of area 

 drained, on the assumption that the run-off is distributed uni- 

 formly both as regards time and area. 



" Depth in inches " is the depth to which the drainage area 

 would be covered if all the water flowing from it in a given period 

 were conserved and uniformly distributed on the surface. It is 

 used for comparing run-off with rainfall, which is usually expressed 

 in depth in inches. 



An " acre-foot " is equivalent to 43,560 cubic feet, and is the 

 quantity required to cover an acre to the depth of 1 foot. The 

 term is commonly used in connection with storage for irrigation. 



The direct course of stream-flow is the visible run-off from the 

 watershed and that part of the rain-fall which was absorbed by 

 the soil and which slowly finds its way to the stream bed in the 

 form of an underground flow. 



Variation in Stream Flow. There is a very considerable vari- 

 ation in the flow of rivers not only during the various months of 

 the year, but from year to year as well, and the variation is greater 



