272 ENGINEERING ON THE FARM 



the work should be planned and carried out in a thorough and 

 scientific manner. A complete analysis of all the conditions 

 affecting the working of the system should be made. The 

 source of the water which saturates the soil needs attention 

 This may be due to rainfall only, as in flat lands which 

 are protected by intercepting ditches or levees. There may 

 be an overflow coming in a very thin sheet from surrounding 

 lands with a slightly greater elevation. Such an overflow 

 is often so evenly distributed over the entire surface that 

 it is overlooked and drainage capacity for a given area is 

 provided when in reality there is moisture to be removed 

 from several times that area. Seeps and springs may occur, 

 caused by the water being carried by a porous stratum from 

 higher levels. Such water may be distributed on the sur- 

 face at a low velocity over a wide area and, instead of pro- 

 ducing a running stream, keep the surface saturated for a 

 long period. Drainage channels, unless conducted back 

 into the dry land a sufficient distance to cut off the water- 

 bearing stratum, will not give the desired results. The 

 distribution of rainfall and conditions favoring evaporation, 

 absorption, and percolation are all factors which have a 

 bearing upon the drainage problem. These last are discussed 

 in the chapter on climatology. 



Elevation. To determine the rate at which the lands can 

 be relieved of water it is necessary to know the fall per unit 

 of distance, and to estimate cost it is necessary to know the 

 depth of cut through the ridges or high lands. Therefore a 

 determination of the elevation of the lowest point to be 

 drained, with reference to the outlet, and the highest points 

 through which the proposed channel will pass, with reference 

 to the lowest points, is essential. The general surface con- 

 ditions will determine the number of these elevations neces- 

 sary. A flat surface sloping in one direction requires few 

 elevations for a drainage plan, while a rolling country 

 with deep depressions and intervening ridges will require a 

 great many elevations for determining the depth of channel 



