c 



Ground Water Gradient 435 



It will be clear, therefore, from the cases cited, 

 that wherever the moving sheet of ground water ap- 

 proaches within capillary range of the surface of the 

 ground, there the soil is liable to be too wet for crops 

 unless underdrained. 



RATE AT WHICH THE GROUND -WATER SURFACE 

 RISES AWAY FROM THE DRAINAGE OUTLET 



In well 29 of Fig. 133, situated 150 feet from the 

 lake, the water stood 7.214 feet above the level of the 

 lake June 27, 1892, thus showing a rise of 1 foot in 

 every 24.4 feet. At another place in the same locality, 

 but not shown in the map, a well 1,250 feet from the 

 lake shows the ground -water surface to stand 52 feet 

 above, thus giving a gradient of 1 foot in 24 feet. 

 Later in the season, w r hen the ground had become 

 dryer, the gradient at well 29 became 1 foot in 

 35.86 feet. 



Between tile drains 33 feet apart and 4 feet deep, 

 laid within the rectangle of Fig. 133, measurement 

 showed the surface of the water to rise at the mean 

 rate of 1 foot in 25 feet 48 hours after a rainfall of 

 .87 inches, and the shape of the ground -water surface 

 at the time in question is represented in Fig. 137. 

 Of course, after the lapse of a longer interval of 

 time the gradient here would have become less steep, 

 just as was the case in the other instance cited. 



The subsoil in which these gradients were observed 

 was a fine sand, in some places with grains so small 

 as to approach the character of quicksand, and they 



