24 BULLETIN 906, IT. S. DEPARTMENT OF AGRICULTURE. 
Five- foot contour intervals may be close enough for estimating the 
location of the main pipe in hilly country, but it may be necessary 
to plot 1-foot or even 6-inch contour intervals on flat valley land. 
Furthermore the entire system should be designed before any con- 
struction work is begun. 
Tracts for new colonies under the control of the Land Settlement 
Board of the State of California have been carefully surveyed 
throughout. The location of pipe lines has been determined and 
the sizes and location of each subdivision carefully considered. 
Contour intervals have been plotted accurately enough to allow 
all surface grading to be laid out before the farm is ready for 
water. The size of each subdivision is determined by the character 
of the soil and the methods adopted for irrigation and drainage. 
The location of the laterals is also determined by the prevailing 
grades of each unit to be watered and the kind of crops to be grown. 
Land for alfalfa usually has comparatively flat slopes, while orchard 
land may be very uneven providing pressure pipe is used. 
Hilly land that has recently been subdivided- for citrus orchards is 
frequently irrigated with little or no grading, even though the 
separate tracts may be very uneven. In this case pipe lines are con- 
structed along ridges, feeding all the high knolls. Tree rows may 
be laid out in squares, or may be set out with respect to contours 
only. If contour or terrace planting is necessary it is usually possible 
to irrigate in two directions from one field lateral. 
The sizes of pipe needed for a given tract will depend upon the 
acreage to be served, the grades of the pipe lines, the smoothness of 
the interior surface of the pipe, the water requirements of crops, the 
character of the soil, and the slope of the land. If the source of 
water is a reservoir at a considerable elevation above the lands to be 
irrigated, it may be possible to install pressure pipe that will carry 
water at a high velocity. If pressures exceed 50 feet in head it is 
common to install steel, reinforced concrete, or wood pipe. It is often 
practicable, however, to control the pressure of the main by suitable 
relief stands so that concrete pipe of larger diameter can be used. 
The latter practice is usual in recent installations. The smoothness 
of the interior of the pipe will also affect its carrying capacity. 
The following table is compiled from data collected in the field 
by Fred C. Scobey, senior irrigation engineer of the Bureau of Pub- 
lic Boads. 3 The table is not complete but has been arranged to give 
approximate carrying capacities of concrete pipe under ordinary 
working conditions and for average pipe. Especially smooth pipe 
will have larger capacities and very rough pipe much lower. For 
average conditions small sizes should not be figured too closely. 
Probably pipe up to 12 inches diameter should be computed to 
3 « xhe Flow of Water in Concrete Pipe." Bulletin 852, U. S. Dept. of Agr. 
