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 

 forming 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 OE WATER DEVELOPED. 



It 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 sj^stems, very few accurate data on 



