80 IRRIGATION PRACTICE 



total is considerable. The evidence of this is found in the 

 lean and washed-out soils of humid districts, where the 

 rainfall is large enough to permit steady drainage with- 

 out the counterbalancing effects of a tropical warmth. 

 In arid districts, likewise, where over-irrigation has pre- 

 vailed, soils have been permanently injured by the loss 

 of plant-food carried off in the drainage. 



The loss of plant-food is only one of the many injurious 

 effects of the excessive use of water. In arid districts the 

 drainage water, resulting from over-irrigation, frequently 

 accumulates in some lower-lying closed basin, such as in 

 the lowest part of a valley. At this point the ground 

 water rises higher and higher as excessive irrigation is 

 practised on the higher land, until the water-table is so 

 near the surface that water may be lifted from it to the 

 surface by capillary attraction. When this condition has 

 been reached, continuous evaporation from the soil sur- 

 face occurs. The soluble matters contained by the water 

 which is left behind increase, first, the concentration of 

 the ground water, and secondly, as evaporation goes on, 

 fill the upper layers of soil with soluble salts, often with 

 a formation of an alkali crust. Over-irrigation thus 

 becomes one of the chief sources of the dreaded alkali. 



The disastrous results of the excessive use of water 

 prevail over large areas in almost every irrigated section 

 of the world. Leaky canals have permitted large quanti- 

 ties of water to soak through great areas of fertile soils, 

 until, heavily charged with precious plant-food, they have 

 accumulated in lower basins. Farmers, anxious to pro- 

 tect themselves against the drought, and believing that 

 the more water used the more certain would be the crop, 

 have so over-irrigated their lands as to permit a more or 

 less constant drainage into subsoil and lower-lying places. 



