Page 333 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

LANDSCAPING AND AGRICULTURAL GRADING 7.27

Large farms or ranches may control an entire irrigation system, but water is usually distributed
to a number of users by a water district or other government agency, or by a water company. One
water gate or weir is usually provided for each land unit, and distribution within that unit is cared
for by the farmer.
This may be handled by running a pipe line along one edge of the field, with standpipes and
valves at 10- to 20-foot intervals. If the field is long, several distribution lines may be used, con-
nected by a main.
Ditches may be used instead of pipes, but they require more maintenance and may interfere
with tillage.
The direction of irrigation is the direction the water flows on the surface of the field from the
distribution pipe or ditch. It is generally at right angles to the pipe.

LAND LEVELING

Slope Patterns. Land leveling may be divided into six classes, according to the result obtained:
1. Spot grading
2. General downward slope away from water supply—for sprinklers
3. Uniform grade in direction of irrigation
4. Uniform grade in direction of irrigation and at right angles to it
5. Uniform grade in direction of irrigation and exactly level at right angles to it
6. Exact level
Spot grading consists in removing humps or filling hollows, without establishing a uniform
grade in any direction. It is sometimes done in advance of better leveling for irrigation, and is of
general use to make possible faster tillage and more even production.
If water distribution is to be by means of sprinklers, perfectly uniform slopes are not required.
For water distribution, it is only necessary that the land have a general slope down from the source
of water. In climates where deep freezing of soil occurs, the slope should be uniform enough to
make possible drainage of sprinkler pipe laid at a fairly regular depth.
When the water reaches the individual plants by flowing on the surface of the ground, it is necessary
to have an almost uniform slope in the direction of irrigation. The steepness of slope may be determined
by the character of the soil, the crop to be planted, the original grade, and the rate of water use.
Economies may be affected on many plots by leveling only in the direction of irrigation, and
following the original profile at right angles to it. This type of job is used chiefly in orchards
which can readily be cultivated into ridges that will regulate water drifting across the field.
Choice between the fourth and fifth methods will depend largely upon economies in working
over the natural grade. In very large fields, the two-way slope will facilitate movement of water
through the cross-distribution pipes. The cross grade should be so slight that even light ridging
will prevent sideward drift of the water.
Entirely level plots are usually limited to rice fields, and alfalfa and other crops which can tolerate
flooding.
Flow and Absorption. The rate of water flow and absorption should balance, so that water will
reach the lower end of the slope in sufficient quantity for a crop without flooding or running off.
In practice this balance may be difficult to achieve, and provision is made for draining off excess
water when necessary.
Increase of gradient will accelerate the flow and decrease the rate of penetration. Light sandy
or gravelly soils absorb water rapidly and require steeper grades than clay, which may be almost
waterproof unless freshly loosened.
The maximum gradient would be below that which will cause the soil to wash and gully dur-
ing irrigation or heavy rains. The minimum is flat.