46 



HYDROLOGY 



TABLE XXI 

 TYPICAL AVERAGES OF RAINFALL 



Inches Annual 

 Rainfall. 



Approximate 



Mean Annual 



Run-off, Inches. 



North Atlantic States. . 



Gulf States 



Lake Region 



Mississippi Valley 



Mountain Region 



Plains 



Pacific Coast, north. . . . 

 Pacific Coast, south. . . . 



40-50 

 50-60 

 30-40 

 30-60 

 10-20 

 0-10 

 40-60 

 10-30 



Over 20 



Over 20 



10-20 



10-20 



2- 5 



0- 2 



10 Over 20 

 2-10 



State. 



Spring. 



Summer. 



Autumn. 



Winter. 



Total. 



Massachusetts 



Georgia 



Michigan 



Missouri 



Colorado 



Nevada 



Oregon 



California .... 



11.6 



12.4 



7.9 



10.0 



4.2 



2.3 



9.8 



6.2 



11.4 



15.6 



9.7 



12.0 



5.5 



0.8 



2.7 



0.3 



11.9 



10.7 



9.2 



9.1 



2.8 



1.3 



10.5 



3.5 



11.7 



12.7 



7.0 



6.5 



2.3 



3.2 



21.0 



11.9 



46.6 

 51.4 

 33.8 

 38.0 

 14.8 

 7.6 

 44.0 

 21.9 



The annual as well as the monthly rainfall varies irregularly 

 from year to year, and the amount of these variations is greater 

 in some localities than other. While they may remain within 

 certain limits, the totals are made up of still greater variations in 

 individual storms. 



The rainfalls to be considered for practical purposes are the 

 average monthly and the monthly of the driest year, both of 

 which affect the supply, while a knowledge of the maximum rainfall 

 is essential for determining the discharge. 



Rainfall Record. The United States Weather Bureau main- 

 tains several thousand stations for recording the rainfall of the 

 country and the number of points at which such observations 

 is increasing from year to year. There are some places where 

 observations have extended for over fifty years and sufficient 

 information can therefore usually be obtained from the bulletins 

 of the Weather Bureau. Where small watersheds are under in- 



