DRAINAGE OF IRRIGATED SHALE LAND. 19 
any flow at all, is a very uncertain matter, the amount of water 
developed in the bottom of the trench as it progresses is a very good 
indicator. Those points that develop the most water show that they 
are probably underlain by water-bearing zones and that the pres- 
sures are higher there than elsewhere. Evidently a well located at 
such a point will develop a larger flow and will be more beneficial 
than one placed elsewhere. Where the greater amounts of water are 
found in the trench and the higher pressures seem to prevail, wells 
~ should be located quite close together. They should be spaced further 
apart in the drier portions of the trench. Where the conditions in 
the trench are uniform, the wells should be spaced regularly if the 
resulting flows are uniform; but if, after putting down a hole or two, 
very little or no flow results, it is advisable to space them farther — 
apart until an area is encountered where more water is developed. 
DEPTH OF WELLS. 
The proper depth for relief wells is a matter for experiment in 
different localities. The maximum depth, however, usually is about 
20 feet below the bottom of the drains. It becomes difficult to drill 
wells deeper than this by hand, and as a rule their effectiveness is 
not increased by the additional depth. In any event, water encoun- 
tered at this depth probably is not contributing to the seepage area 
in the immediate vicinity. In beginning work on a new project. it 
always is advisable to drill the first two or three wells deep to de- 
termine where the flow is most apt to be encountered. While water 
will not be encountered at uniform depths, yet certain limits within 
which it is likely to occur will be determined. In most of the work — 
formirg the basis for the conclusions in this bulletin the approxi- 
mate depth at which the flows have occurred has been about 15 feet 
below the surface of the ground. 
AMOUNT OF WATER DEVELOPED, 
Tt will be found that some of the relief wells do not flow at all and 
that the discharge is small from many of those that do. In nearly all 
cases observed 2-inch wells have been of sufficient size to care for the 
water developed. Among other things, the amount of water devel- 
oped depends upon whether it is the season of low or high water 
table. Many wells that do not flow when they are installed in the 
season of low water table discharge when the water rises again. 
Figure 11 (p. 37) illustrates the variation in discharge from a 1,600- 
foot system of drainage, where 1,000 feet of the tile were in shale 
and a total of 35 wells were installed, ranging from 12 to 20 feet in 
depth. 
While there is no doubt that the relief wells furnish the larger 
part of the discharge from these systems, very few accurate data on 
