48 GENERAL CONSIDERATIONS 



on special occasions to supplement the rainfall or 

 with special crops like rice that need an abundant 

 water supply at a certain period of growth. There 

 is no doubt that in the future irrio^ation will be 

 much more extensively practiced in what are known 

 as humid regions. 



In those parts of the tropics where there is a well- 

 marked wet and dry season the rainfall is ample or 

 even excessive for a part of the year, bnt is insufficient 

 for the best growth of crops at other seasons. Since 

 here the temperature conditions are suited to growth 

 at all seasons the annual production could be greatly 

 increased by supplying irrigation during the dry 

 season. This has already been done in certain 

 localities with very favorable results. 



Water for irrigation purposes is obtained in various 

 ways. The simplest and cheapest method, where this 

 is possible, is to utilize the water of running streams. 

 The water is diverted from the stream by means of a 

 small dam at some point far enough up so that it is 

 high enough to secure a free but not rapid flow in 

 the canal which leads it to the land to be irrigated. 

 The intake of the canal is so arranged that in time of 

 flood only the usual amount is admitted, the surplus 

 passing down the old channel. A gate is also pro- 

 vided by which all the water may be cut off from the 

 canal when it is necessary to clean it or make re- 

 pairs. From the main canal the water is distributed 

 to the fields in ditches that are laid out according to 

 the topography in such a way that the ditch always 

 passes along the highest side of the field which is to 

 be watered from it. The ditches are provided with 



