212 ELEMENTS OF GENERAL SCIENCE 



It must not be forgotten that underground drainage is ad- 

 vantageous for other purposes than the removal of surplus 

 water, though even at times irrigated farms need it for that 

 purpose also. The failure to introduce underground drainage 

 in irrigated districts is now beginning to prove a limiting fac- 

 tor in the production of plants, and doubtless such drainage 

 will soon become usual in irrigated lands. 



233. Source of water for irrigation. The larger part of our 

 irrigation projects consist in diverting mountain streams into 

 nearby valleys. Sometimes streams are dammed (fig. 103) 

 either to assist in diverting the water or to store the water 

 for later use. Tunnels have been made through mountains, 

 and elevated aqueducts have been built over valleys to carry 

 these mountain streams. In some regions irrigation by the 

 use of deep wells is practiced. Water is pumped into reser- 

 voirs, and from these it is led off through ditches to the fields. 

 In case of rivers which flow through flat valleys, the water 

 is sometimes pumped into irrigation ditches or into elevated 

 reservoirs. 



(Read a description of one of the larger irrigation projects 

 and report upon it in class.) 



234. Dry farming. In some parts of the United States 

 there is an amount of rainfall which would give the soil suf- 

 ficient water to produce a crop if evaporation could be pre- 

 vented. In some such regions the experiment has been tried 

 of keeping the soil under surface cultivation for a long time. 

 First deep plowing is done, furnishing a deep soil for the ab- 

 sorption of rainfall. Then constant surface cultivation retains 

 most of the moisture, and after a time enough moisture may 

 thus be caught and held to make it possible to grow a crop. 

 In some cases where this type of farming is done, the rains 

 are so scanty that a crop can be grown only once in two 

 years. Possibly some water may be drawn upward from 

 the deep soil, but most of it must come from the scanty and 

 infrequent rains. 



