AVAILABLE WATER SUPPLIES. 



103 



Depth of annual runoff from drainage basins with- 

 in the arid region: 



FIG. 2. DRAINAGE BASINS. 



From which the run-off has been measured by the Geological 

 Survey. 



A few figures obtained for tributaries of the upper 

 Mississippi will serve to show the intermediate posi- 

 tion which these hold between the heavy runoff of the 

 East and the almost insignificant quantity of the West. 



Depth of runoff in upper Mississippi basin: 

 Mississippi at St. Paul, 9.0 inches. 

 Crow Wing, 12.0 " 



St. Croix, 16.0 



DATA FOR RUNOFF MAP, WESTERN HALF. 



The accompanying map, Fig. 2, of the western por- 

 tion of the United States, shows diagramatically the 

 areas for which data have been obtained concerning 

 the mean annual runoff for from one to five years. 

 It enables the observer to see at a glance the relative 

 proportion of the localities for which the runoff is 

 known, and to make comparison with those places 

 where it has been necessary to assume similar condi- 

 tions. 



The following table gives the years and depth of 

 runoff in inches, and from this a more definite knowl- 

 edge can be gained as to the amount of water coming 

 from the particular areas indicated upon this map by 

 the cross-hatching. 



RELATION BETWEEN RAINFALL AND RUNOFF. 



A comparison of the two maps of mean annual 

 rainfall and runoff waters shows, as previously 



noted, that the runoff is not absolutely dependent 

 upon the quantity of rain. It is the result not only of 

 the depth and distribution of precipitation, but also 

 of the character of the topography. That is to 

 say, the amount of water which flows off the 

 ground is influenced as much or more by the 

 slope of the surface than by the quantity of water or 

 by the time during which it falls. There are other 

 modifying circumstances, but these two are so much 

 more important that they should be dwelt upon. In 

 consideration of the water available, it is often 

 assumed that much study should be given to the rain- 

 fall, especially when it is desired to secure a supply 

 of water by means of reservoirs. It must be remem- 

 bered, however, that reservoirs do not catch the rain- 

 fall except to an insignificant extent. They receive, 

 not the water which falls as rain, but that which flows 

 off of the ground and for a time at least forms streams 

 or rivers; and therefore far more attention should be 

 paid to the water while flowing than to the quantity 

 which may occur as rain or snow. 



It is well known that more water comes from high 

 mountains than from plateaus, although the depth of 

 rainfall upon these may be nearly equal. But it is 

 not always as clearly recognized that as the rainfall 

 upon these high plateaus and mountains decreases, 

 the quantity of water from the nearly level places be- 

 comes far less proportionally than from the steeper 

 slopes. In the following diagram an attempt is made 

 to show graphically the relation of the depth of rain- 

 fall to depth of runoff upon the steeper slopes and 

 upon those more undulating or nearly level. In this 

 diagram the vertical lines indicate the mean annual 

 depth of runoff in inches and the horizontal lines the 



