UNDERGROUND WATER 127 



necessarily accumulate on its surface. The inclination of the 

 clay bed determines also the direction in which natural drainage 

 will occur. 



When the complications of local clay beds are absent, the 

 level of the underground water is found to follow generally 

 the contour of the land surface, but in a less exaggerated 

 manner, and with a decline in level in the direction of the 

 drainage outfall. King points out that, as a result of this 

 greater height of the water level under high ground, the land 

 lying at the foot of hilly ground receives a continuous supply 

 of underground water even in time of drought, amounting 

 under favourable conditions to a veritable sub-irrigation. 



When the distance of the stratum of saturated soil from the 

 surface is considerable, this store of underground water is of 

 no direct benefit to plants. We have already mentioned, that 

 at Wisconsin a water level 6 ft. below the surface does not 

 prevent the effects of drought being manifested by the crops, 

 though doubtless they derive some benefit from the water. 

 On the other hand, King tells us that the water level should 

 not come nearer than 4 ft. to the surface if ordinary cereal 

 crops are to be grown, as a higher water level prevents a 

 proper development of the roots. With crops having short 

 roots, as grass, the water level may with advantage be consi- 

 derably higher. A water level may generally be lowered to 

 any desired point by the insertion of drain pipes. The water 

 level in a soil is raised by systems of irrigation. 



The height of the water level in a soil varies considerably at 

 different seasons of the year : this is best seen by measurement 

 of the heights of water in wells. In deep wells in the chalk 

 at Harpenden, Herts, the water is found at its lowest level in 

 September, October or November. The rise commences at 



