6 BULLETIN 495, U. S. DEPARTMENT OF AGRICULTURE, 



large evaporation loss. In the arid region the truck farmer is in- 

 clined to make frequent applications — every 3 or 4 days — rather 

 than to apply the extra amount of water required in large applica- 

 tions which "will wet below the reach of the vegetable roots, while 

 the citrus grower applies from 4 to 8 inches each time. 



For spray irrigation sufficient water to cover the land to a depth 

 of 1 inch per week for humid regions and 1-1- inches per week for arid 

 regions is believed to be a safe estimate for designing purposes. A 

 spray plant should be large enough to supply these amounts of water 

 in a reasonable length of time. This is accomplished generally by 

 installing the system to spray from one-fifth to one-half of the total 

 acreage at one time, depending somewhat upon the type of distribu- 

 ti< n used and the available water supply. 



The capacity of a pump usually is stated in gallons of water per 

 minute of time. The following table of equivalents will assist in 

 converting local irrigation units into gallons and acre-inches for 

 estimating purposes: 



1 acre-inch equals 27.152 L T . S. gallons. 



1 acre-inch equals 3,630 cubic feet. 



1 acre-inch per hour equals 452.5 gallons per minute. 



1 acre-inch per hour equals 1.009 cubic feet per second. 



DEVELOPMENT OF WATER SUPPLIES. 



The writer will consider here only the water supplies which one 

 or more farmers can develop, for practically all spray-irrigation 

 installations are made by individuals and do not involve the de- 

 velopment and transportation of distant supplies, as is common for 

 community irrigation in the arid regions. 



There are two sources of water available for irrigation — surface 

 waters and underground waters. Surface supplies include streams, 

 lakes, springs, and stored drainage and rain waters. Underground 

 supplies are obtained from sumps, shallow wells, and deep wells. 



SURFACE SUPPLIES. 



Streams furnish the greater portion of the water used for irriga- 

 tion. 



In most of the "Western States methods of procedure for obtaining 

 the right to take water for irrigation purposes from public streams 

 have been outlined by law. Eastern States, however, have not en- 

 acted such laws, and this is apt to restrict the use of water from 

 many streams. This is true especially where the water is being used 

 for developing power, or where the entire flow is taken from the 

 stream, or when a riparian owner exerts his lawful rights. These 

 restrictions are not likely to apply to lakes unless sufficient water is 

 taken to lower considerably the water level in the lake, or the lake 



