DRAINAGE OP lEEIGATED LAND. 11 



in gravel to a number of feet in fine, silty sand, or clay, soils; the rise 

 may even extend to many feet in special soils containing a high per- 

 centage of gypsum or of calcium chlorid. Average soils show a 

 range of from 1^ to 4 feet. 



Plants in the arid region are unusually deep rooted, and they can 

 not thrive unless the air and moisture conditions are properly bal- 

 anced. Therefore, the plane of supersaturation must be kept below 

 the root zone, and since this plane is several feet above the free water 

 level it is necessary to give drains a considerable depth. 



The presence of alkah salts complicates the problem of depth, for 

 not only is the capillary rise of the water thus increased and expe- 

 dited, but it is essential that the injurious salts themselves be kept 

 down. It is highly important that a downward movement of the 

 water in the root zone be maintained to offset the natural upward 

 movement due to capillary attraction and evaporation. Owing to 

 the presence of animal and worm burrows, cracks, root spaces, and 

 other noncapillary openings, a great deal of water moves downw;.ard 

 without coming into contact with salts; but the upward capillary 

 movement is entirely through the capillary pores where the salts 

 are confined. The natural tendency, therefore, is for the salts to 

 move upward rather than downward. Drains should never be less 

 than 5 feet deep, and experience has shown that depths of from 6 

 feet to 8 feet are much more efficient. The optimum depth for 

 drains is that which will prevent fluctuations of the ground-water 

 level within the root zone, and yet will keep capillary water within 

 reach of the plant roots. 



In determining proper depths the location of any stratum which 

 is either more or less pervious than the adjacent soil is of great im- 

 portance. This involves a careful study of the structure of the soil 

 for considerable depths. It is quite possible to construct a well- 

 arranged system of considerable depth which will be absolutely inef- 

 fective but which would have been highly effective if the depth had 

 been increased less than 1 foot. Figure 4, a, illustrates an actual case 

 of this kind. This system would have been successful if the tile had 

 been laid as is indicated in b. The porous stratum carries water 

 from higher lands, and where the stratum pinches out the water is 

 forced to the surface, forming a bog. In the system as constructed, 

 the tile was laid above the wat(^r-bearing stratum, in dry material, 

 and the water contirmcd to pass under it. Had the system been laid 

 a foot deeper the stratum would have been cut and the flow inter- 

 cepted. 



Figure 5 shows a case where a tile line has been laid 5 feet deep in 

 a soil underlain l)y a stiff, impervious clay at a depth of 6 feet. Here 

 again the wutfr passes und(!r tlu- drain and tlu^ syst(Mn is a failure, 

 though it could he made successful merely by d(!epcning the drain. 



