DISPOSAL OF RAINFALL 5.1 



A determination of the exact quantity of underground waters 

 is a very difficult problem. Numerous papers have been pre- 

 pared on the subject by different authors. Water Supply and 

 Irrigation Paper No. 163 of the United States Geological Survey 

 contains a bibliographic review and index of underground-water 

 literature published in the United States up to and including the 

 year 1905. 



Run-off. The run-off is that part of the rainfall which drains 

 off the surface of the watershed in visible streams. It is that part 

 of the rainfall which remains after nature's need of moisture has 

 been supplied in the form of evaporation and absorption. 



The close relation between these three subdivisions of rain- 

 fall has been referred to in the above, and it follows that the run- 

 off is affected, both directly and indirectly, by the same factors 

 that govern the rate of evaporation and absorption. 



It is often important to know the relation between rainfall 

 and run-off, as this may in many instances be the only way to 

 ascertain the flow of a stream. Rainfall observations have been 

 made for many years and it may be possible by knowing the ratio 

 between run-off and rainfall for a certain drainage area, to apply 

 this value to a watershed in another place. It is, of course, of 

 the greatest importance in such comparisons that the areas from 

 which the deductions are made must be of similar character. 

 Also that they are of approximately the same size, because smaller 

 drainage areas usually have a wider variation between maximum 

 and minimum run-off than large ones. 



It is apparent that there can be no constant relation between 

 the rainfall and the run-off for the whole country, although in this 

 respect the ratio for the Eastern States is much more constant than 

 for the Western States. There are also great variations in the 

 yearly as well as the monthly and daily run-off, and it is very 

 difficult to make accurate estimates as to what the two latter may 

 be expected to be; the daily being, of course, almost impossible to 

 foretell. The yearly run-off, however, bears a more nearly uni- 

 form ratio to the rainfall, so that with a good knowledge of the 

 presence of forests, character of soil, climate, etc., a fairly accurate 

 estimate of the yearly run-off may be made, based on known 

 values under similar conditions. 



As for rainfall, run-off is also usually expressed in inches, and 

 the map in Fig. 15 shows approximately the mean annual run-off 



