124 AGRICULTURAL ENGINEERING 



irrigation water is needed most. Under other conditions, 

 reservoirs are little needed. 



Forests are natural reservoirs to the extent that they hold 

 the snow in mountainous countries and prevent a rapid sur- 

 face run-off of the water. In some localities the irrigation 

 water comes from glaciers, which have been found to regulate 

 the supply in a satisfactory and natural way. Thus the 

 maximum amount of water is furnished when the weather is 



Fig. 69. View of the Roosevelt Dam on the Salt River, Arizona. 

 (Bui. 235, Office of Experiment Stations.) 



hottest and the requirements are the greatest. Sometimes 

 reservoirs are placed at the end of the canal in order that the 

 supply of water may be on hand near the land to be irrigated, 

 so that, if a sudden demand for water is made which will 

 exceed the capacity of the canal, the water from the reservoir 

 may be released. 



Reservoirs have been used in connection with irrigation 

 since very early times. In India nearly ten million acres of 



