DRAINAGE AND GROUND WATER SUPPLY 201 



the rather more complete data obtained on nearly 21,000 

 of these wells, the following conclusions are drawn : 



46.2 per cent show change in water level ; 



53.8 per cent show no change in water level. 



A part of these were dug wells and a part were drilled 

 wells. 



Of the dug wells 53.6 per cent showed change ; 



Of the drilled wells only 23.4 per cent showed change. 



Of the dug wells, for the period covered by the data : 



45.5 per cent showed a mean lowering of 4.31 feet ; 



17.5 per cent showed a mean rise of 3.68 feet. 



Of the drilled wells, for the period covered by the data : 



21.25 per cent lowered to the mean amount of 12.83 

 feet; 



3.3 per cent gave a mean rise of 11.08 feet. 



" The minimum lowering per decade for the entire 

 country is but 0.677 for the dug wells, and over three times 

 as much, or 2.167 feet for the drilled wells." 



263. Interesting facts concerning ground water-tables. 

 Of the nearly 29,000 wells, over 61 per cent have their 



water-table within 30 feet of the surface, and only 5.7 

 per cent (1635 wells) have their water-table below 100 

 feet from the surface, and nearly one-fifth of these (307) 

 are in one state. 



264. Chief causes resulting in lowering of ground 

 water. Several causes are suggested as probably having 

 a part in producing the lowering of ground water. The 

 ones most commonly offered may be grouped as : (1) 

 those resulting in increased losses by surface drainage 

 in increasing the run-off ; (2) those resulting in increased 

 losses by evaporation increasing the fly-off ; (3) the 

 removal of natural surface reservoirs; and (4) a direct 

 draft upon the waters themselves. 



