CHAPTER X 



IRRIGATION 



Drainage means the removal of water from land where it 

 Objects of exists in too large a quantity. Irrigation means the reverse, 

 namely, the application of water to land where it does not 

 exist at all, or where its quantity in the soil may be increased 

 so as to benefit vegetation. A good deal has been said 

 about the necessity of water to fit the soluble part of the 

 soil for plant food. Without water, therefore, plants cannot 

 Arid regions, live, and in the large, rainless tracts of land in Peru, Bolivia 

 and Chili, and other parts of the world, there is no vege- 

 tation, except i?t those places to which water has been carried 

 by irrigation. Water may be conveyed to the land by 

 canals, or pipes or spouts, from rivers, or lakes, or ponds, 

 or it may be pumped up from wells and thence distributed 

 over the soil. 



The art of irrigation comprises two essential particulars, 

 namely, to give a supply of water to growing plants ; and, 

 at the same time, to prevent the supply being so great as to 

 cause the soil to remain sodden with moisture. If, there- 

 Provision fore, canals and trenches be made to carry water, to irrigate 

 made to get the soil, particular care must be taken to provide canals for 

 water^"'^^''^^ taking away the water after it has passed through the land. 

 In some cases, the water is simply conducted through the 

 land by the canals, the soil obtaining its supply by the water 

 percolating through the porous earth. Another system is to 

 have a series of transverse trenches on the sloping land. 



