220 ENGINEERING FOR LAND DRAINAGE. 



soil produced which makes it capable of retaining a 

 greater amount of capillary water, the greater depth of 

 soil from which plants may draw nutriment, the circu- 

 lation through the soil of air from which the moisture 

 is condensed in its cooler recesses, all contribute ta 

 the power of a drained soil to resist the ejfifects of a 

 protracted dry spell during the growing season. 



The experience of farmers who have drained clay or 

 alluvial soils sustains these conclusions as to value of 

 underdrainage in time of drought. 



Benefits from Open Channels* — Their value consists 

 in making more complete drainage possible. When- 

 ever the value of complete underdrainage is estab- 

 lished, the value of open ditches is established if they 

 become a necessity for properly carrying out the work. 

 In the absence of natural receiving channels, artificial 

 ones must be constructed, or old natural channels im- 

 proved so that drainage may be completed by the own- 

 ers of individual tracts of land. Open ditches often 

 serve as the outlet of the soil drainage of towns and vil- 

 lages, or for the effluent of sewage after it has been 

 treated or filtered in such a manner and so effectively 

 that it will be harmless. In such cases assessment 

 should be made on the corporation for its share of the 

 cost of the ditch. Such an assessment cannot be made 

 upon the same basis as upon agricultural lands, but 

 upon the valuation of the property of the corporation 

 compared with the value of a similar area of farm 

 property. 



These ditches are of marked benefit to the highways 

 of the tract which they drain in giving outlets for road 

 drains and in the work incidentally effected by the gen- 



