432 



Irrigation and Drainage 



at the places designated by the small numbered cir- 

 cles. 



Referring to the two figures, it will be observed 

 that there is a marked tendency for the ground- 

 water surface to stand highest where the level of the 

 field is also highest, and that there are valleys in the 

 ground -water surface beneath the valleys in the field. 

 It will be seen that the water rises as the distance 

 from the lake increases, and that in places it stands 

 10 and even 20 feet higher. 



This distorted surface of the ground water cannot 

 be a condition of rest, for gravity tends continually to 

 force a flow from the higher toward the lower levels 

 along the lines indicated by the arrows shown in Fig. 

 133. Since the further this water must travel through 

 the soil to reach the lake the more resistance it must 

 meet, it is plain that a greater pressure will be re- 



Fig. 134. Diagram of lines of flow of water in the drainage of a river valley. 



quired to overcome this resistance, and hence the 

 water must stand higher in the ground the farther the 

 distance to the drainage outlet. The space enclosed 

 by the rectangle in Fig. 133 is an area which required 

 underdraining to fit it for farm crops, and the reason 

 it did is clearly shown by the contours of the two 



