258 Ground Water, Wells and Farm Drainage. 



granite, there may be as much as .4 of a pound of water 

 in each cubic foot, but here again the texture is too close to 

 permit such water to become available in wells. 



323, The Ground Water Surface. As the rains which fall 

 in a given locality percolate beneath the surface they fill 

 the pore spaces between the soil grains and raise the level 

 of the ground water. If none of this water drained away 

 and none of it were lost by evaporation the whole soil 

 would have its pore spaces filled with water and the surface 

 of the ground water would coincide with the surface of the 

 land. As it is, as soon as the surface of the ground water 

 oeases to be level drainage begins and the water under the 

 higher land is lowered until a condition is reached when 

 the rate of drainage laterally exactly equals the rate of ac- 

 cumulation of water from the rains. 



In Figs. 88 and 89 are shown the contours of the surface 

 of a section of land and of the ground water beneath, both 

 sets of contours being referred to the same datum plane, 

 Lake Mendota, into which the water is draining. Here, it 

 will be seen, the ground water stands highest where the 

 surface is highest and lowest where the land is lowest. The 

 arrows show the lines of flow and make it clear why the tile 

 drained area needed that treatment. 



FIG. 90. Showing lines of flow of ground water during seepage into a 



stream. 



324. Seepage. Almost everywhere under the land areas 

 there is a slow movement of the ground water from higher 

 to lower levels destined ultimately to reach some drainage 

 outlet. This movement is known as seepage and Fig. 90 is 



