FLOW OF WATER THROUGH PIPES. I3S 



in twenty-four hours. At the end of nine days only 23 

 per cent of the rainfall had passed off in drainage. 



It must be admitted that there is a wide range of 

 differences in the recorded results of experiments made 

 for the purpose of determining the percentage of drain- 

 age to rainfall. One series of carefully constructed 

 observations gives the average filtration for a series of 

 years at 5 per cent of the rainfall. Another series ap- 

 parently as reliable gives 42 per cent. All of them, 

 however, give a small percentage for the months of 

 May, June, July, and August. The above observa- 

 tions were not made in this country, and furthermore 

 the means which were used in arriving at these conclu- 

 sions were crude. 



Soil water should occupy from 10 to 30 per cent of the 

 empty space in productive soils. Not until the quan- 

 tity exceeds this will the drains be called into action. 

 Again the subsoil for a foot or more above the plane 

 of the lateral drains may be saturated for a time and 

 no injury result to the upper soil or the growth upon it. 

 One benefit resulting from a system of underdrains 

 which is often overlooked is that soil water even when 

 excessive does not stagnate. It moves constantly in 

 the direction of the drains, and air fills the space for- 

 merly occupied by surplus water. 



There is another fact bearing on this question which 

 should be considered in this connection. Owing to 

 the rapidity of rainfall on some occasions, and the 

 inability of some soils to absorb the water quickly 

 enough, a certain portion of it flows over the surface 

 and collects in depressions near by and must be re- 

 moved from those points either through the underdrains 



