IBHIGATION. 899 



mense plantations of date-palms and other tropical fniits. 

 But where water comes within twenty feet of the sur- 

 face and the supply is plentiful, the modern gasoline 

 pumps raise and carry the water to elevated reservoirs 

 at a cost so moderate that it is trifling when compared 

 with the usual cost of water in the arid districts. 

 Indeed, the reservoir system has been the main one used 

 in ancient and modern times. In the arid regions the 

 water of the wet season is stored in immense ponds or 

 lakes for use in the dry period, usually in mountains, or 

 by damming mountain streams. In the dry season this 

 stored water is used on large tracts during a long period. 

 In the humid States the dry period is short. Hence our 

 friends from the arid States need not make sport of' the 

 smaller ponds filled by pumping from stream-beds or wells. 



403. Sub-irrigation. — The best practical illustration of 

 what is known as sub-irrigation is found in the raisin- 

 producing district near Fresno, California. ISTo water is 

 applied to the surface. Six feet below the surface is found 

 a nearly water-tight clay deposit with a porous soil above. 

 The water runs in ditches down to the clay and seeps 

 under the vineyard, rising to the roots by capillary attrac- 

 tion. 



Another kind of sub-irrigation is found in California, on 

 the bottom lands, on which sugar-beets are grown, in 

 China, and at other points. These lands are in the stream 

 valleys and are sub-irrigated by the seepage water from 

 orange and other irrigation on the higher elevations. 

 The Chinese gardeners also seek such land for vegetable- 

 growing in California, near Phoenix, Arizona, in Colorado, 

 and at other points where the seepage water from higher 

 land sub-irrigates lower-lying land without making swamps 

 or ponds, which often happens. 



In Wisconsin, near Sparta, and at other points, sandy 



