PROPORTION REACHING THE SEWERS 63 



suburban territory in Berlin to J of a rainfall of 2.9 inches in 

 Koningsberg 



After a careful study of his records Kuichling formulated 

 the following conclusions: 



" i. The percentage of rainfall discharged from any given 

 drainage-area is nearly constant for rains of all considerable 

 intensities and lasting equal periods of time. This can be 

 attributed only to the fact that the amount of impervious 

 surface on a definite drainage-area is also practically constant 

 during the time occupied by the experiments. 



"2. The said percentage varies directly with the degree 

 of urban development of a district, or, in other words, with the 

 amount of impervious surface thereon. This fact is clearly 

 shown by the large percentage derived from the relatively 

 most developed district, X, in contrast with the smaller per- 

 centages from the relatively less developed districts, IX, IV, 

 and XVII, and the least improved district, I. 



"3. The said percentage increases directly or uniformly 

 with the duration of the maximum intensity of the rainfall 

 until a point is reached which is equal to the time recorded for 

 the concentration of the drainage-waters from the entire tributary 

 area at the point of observation; but if the rainfall continues at 

 the same intensity for a longer period, the said percentage will 

 continue to increase for the additional period of time, but at a 

 much smaller rate than previously. In other words, the pro- 

 portion of impervious surface slowly increases with the duration 

 of the rainfall. 



" 4. The said percentage becomes larger if a moderate 

 rain has immediately preceded a heavy shower, thereby partially 

 saturating the permeable territory and correspondingly increas- 

 ing the impervious surface. 



"5. The sewer-discharge varies immediately in all appre- 

 ciable fluctuations in the intensity of the rainfall, and thus 

 constitutes an exceedingly sensitive index of the rate and its 

 variations of intensity. 



"6. The diagrams also show that the time when the rate 



