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 14 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 accomplshed generally by 
installing the system to spray from one-fifth to ene-half of the total 
acreage at one time, depending somewhat upon the type of distribu- 
tion 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 U. 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. 
i 
The writer will consider here only the water supples 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 


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