CHAPTER XVIII 

 OVER-IRRIGATION AND ALKALI 



OVEK-IRRIGATION and alkali are the two chief evils 

 that endanger irrigation as a permanent system of agri- 

 culture. These result from the unwise use of water and 

 from peculiar soil conditions that may be aggravated by 

 excessive irrigation. 



222. Seepage from reservoirs and canals. Soil is a 

 porous mass. Water added to the soil forms a film around 

 the soil particles which thickens as more water is added, 

 until the film becomes so thick that the liquid water 

 slides through the capillary spaces to the greater depths 

 of the soil. When the annual rainfall is from 20 to 30 

 inches, approximately one-half of it seeps through the 

 soil beyond the reach of plants. Under a high rainfall 

 more is lost. This seepage continues until the water 

 reaches an impervious layer of rock, clay or other sub- 

 stance, where a permanent water table is formed; and the 

 more water is added from above, the thicker becomes the 

 water table. It does not necessarily stand still, but usually 

 flows gradually hi some direction depending upon the 

 inclination of the impervious bottom. According to the 

 most recent estimates there is such a layer of water 100 

 feet thick, on the average, under the whole surface of the 

 earth. 



Much of this subterranean water is derived from the 

 rainfall directly; much water seeps also from the river 

 beds, which are covered with water practically the whole 



(371) 



