dkaijstage of ieeigated land. 



23 



irrigated region the seepage moves down the ridges more readily 

 than down the depressions, so it is generally necessary to modify 

 the drainage system by means of intercepting laterals cut through 

 the ridges near the upper 

 edge of the tract. These 

 are indicated at a, h, c, d, e, 

 and/, in figure 18. 



APPUCATION OF DOUBLE 

 LINES. 



If thfe depressions are 

 wide and the pervious ma- 

 terial does not extend un 

 der the soil in the depres- 

 sions, it will be necessary 

 to employ two lines, one 

 on either side of the de- 

 pression, cutting into the 

 pervious material. How- 

 ever, if it is possible to 

 completeh^ intercept the 

 seepage by means of later- 

 als at the upper edge of 

 the tract, it will be cheaper 

 to construct a smgle line 

 down the center of the de- 

 pression. Figure 19 shows 

 how a double-line system would be located. 



OTHER SPECIAL CASES. 



In some localities the soil is underlain at shallow depths by lava or 

 other rock formations, and the problem of drainage is exceedingly 



^Branch I ^///////?^mai N'^////y^''/'y///,/y/. 



Fig. is.— Plan and section illustrating application of natural 

 system modified by intercepting laterals. 



^pl^lpL:; 









F'lo. 19.— Cross section showlni; modification of niitnral system in wliidi two lines of tilo are oini)loyed, 

 one on either side of the wide depression and cultliiK through tlio jxTvions layer at \\w liiid of piiicliitiK 

 out. A .slni;l" drain In llio fr^ntcr of this dcpn',ssion would aw:om|)lisli very litlio. 



diflicult. ()ft(;n the formation is ho stratified and shatti^red that the 

 natural <hiiiriug(! seems to l)c very good. Water ontors tlie spaces 

 very readily, however, and the rock mjiss<!S themselves absorb but 



