732 
IOWA DEPARTMENT OF AGRICULTURE 
terranean currents, and does not reach the drain, and besides that the 
rain does not fall at a uniform rate and it must be cared for as soon as 
it comes. So we must know the greatest rainfall likely to come in any 
one day, and the portion that is likely to reach the drain. It is safe to 
count that practically all of the water falling on the untillable land 
should be carried by drains. The portion coming from the tillable land 
depends largely on the surface and subsoil and is largely of necessity a 
matter of judgment. Steep hills with clay subsoil furnish more than 
gentle slopes with gravel subsoil. Hills with sand or gravel subsoil ab- 
sorb the water and give it out in seepage at the foot of the hills; but in 
this case it comes to the drain much more slowly. In computing size of 
drain I usually compute tillable land sloping to the drain at about one- 
fourth the area of untillable land, but this can not be considered a safe 
rule for all cases; sometimes it should be more and sometimes less. We 
occasionally have a rainfall of two inches, and on rare occasions, like 
June, 1864, and May, 1902, we got four or even five inches. But good 
authorities use a maximum of one inch for a basis from which to compute 
the size of tile. In ascertaining the slope of the drain you must measure 
and find how much lower the bottom of the drain is at the outlet than the 
bottom of the drain will be at the upper end, and divide the amount by 
the length of the drain. For the purpose of computing the size of the 
drain, this measurement can be made by any intelligent man with a 
carpenter's level. As to measurements in general I will speak in another 
part of this paper. 
After you have learned the slope and the volume of water to be moved 
you will be in position to judge of the kind of drain to make. You may 
find that a tile large enough to drain the land would cost more than you 
can afford. In that case you will find an open ditch less expensive but not 
so good. Small sized tile should never be used on flat slopes nor for 
large areas. You must have the drains large enough and near enough 
together to carry away a large rainfall before it drowns your crop, or your 
money is wasted. There were thousands of rods of perfectly working 
tile in this county last year that drained the land splendidly, but left it 
bare of crop because it was too slow. This was a waste of money. If 
you compute the gallons of water to be moved in a day, the following 
table will help you to compute the size of tile needed: 
TABLE SHOWING NUMBER OF GALLONS PER MINUTE C^VRRIED BY 
DIFFERENT SIZES AT DIFFERENT SLOPES. 
Size 
Slope .13 
Slope .25 
Slope .50 
Slope .75 
Slope 1.00 
4 
36 
54 
84 
10 i 
144 
232 
267 
470 
830 
1,200 
52 
78 
120 
160 
208 
2.30 
377 
680 
1,000 
1,600 
76 
111 
169 
220 
304 
470 
463 
960 
1,400 
1,900 
92 
134 
206 
280 
368 
570 
655 
1,160 
1,800 
2,800 
108 
159 
240 
330 
432 
660 
803 
1,300 
2.KX) 
3,300 
5 
G - 
7 
8 
y 
10 . 
12 
14 _.- 
16 
