SIZE OF TILE. 173 



Drainage to be entirely successful must not depend to any extent whatever 

 ou evaporation ; that process removes heat from the soil and is consequently 

 hurtful to a great degree for most crops. 



Again drainage is of no value unless it frees the ground from surface 

 water during a wet time — the dry weather does not tax the drains, nor are 

 they especially needed at that time. It is seen from these considerations 

 that our drains must have a large reserve capacity; they must be of size to 

 remove the greatest excess of water that can reasonably be expected day by 

 day, yet we cannot afford to build drains for rain storms that come only once 

 or twice a year, because the absorption of the water by the soil will possibly 

 render such excess harmless. 



The important point to be borne in mind, is, that the factor which influ- 

 •ences the size of the tile is not the annual rainfall but rather the excess 

 above what is retained by the earth, which is brought down in heavy rain- 

 falls within short periods of time. In our locality we usually have, between 

 March and November, about twenty showers, in each of which more than one 

 inch of water falls within a period less than 34 hours. We have only about 

 three with rainfall of 1^ inch and one with a rainfall exceeding 3 inches, 

 while we may expect 285 showers with rainfall less than one inch. 



A long study of these considerations lead me to believe that the carrying 

 ■capacity of our drains should be sufficient to remove one-half inch of water 

 from the whole surface drained each day of 24 hours. I also believe that 

 there are but few places in the United States, or in any other ^lace, where 

 agricultural drainage is wanted but that drains of equal capacity will be 

 needed. 



CAPACITY OF TILE DRAINS AS COMPARED WITH OPEN" DITCHES. 



It is generally believed that for equal areas of cross section of water, the 

 tile drain has much greater capacity than the open ditch. This notion is, I 

 am sorry to say, founded on neither reason nor experiment, and the simple 

 act of measuring the flow in two such cases would soon convince a person of 

 5uch error. The retardation to the flow is caused by friction on the sides, 

 -and this is much worse in ordinary cases for tile drains than for open ditches. 



OPEN DRAINS. 



An approximate formula for flow of water in open ditches, which has been 

 found by experiment to give accurate results in ditch practice is: 



Q=a s/^^ 



Q=Cubic feet discharged per second. 

 a=area of section of water in square feet. 

 /=fall divided by distance. 

 P= Wetted perimeter. 



Example — Find discharge of ditch one foot wide on bottom, one foot deep ; 

 slope of sides, 45°; fall, one foot in 100. 



f a = 2 



In this case 



P = 3.82 



Compare this with 12 inch tile drain. 



