Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 200

sedimentation. By land grading, the land surface is formed to the predetermined
grades so that each row or surface slopes to a field drain.
Land grading permits uniform and efficient application of irrigation water with-
out excessive erosion and at the same time provides for adequate surface drainage.
One of the objectives for grading or leveling agricultural fields is to improve the effi-
ciency of surface irrigation. Surface irrigation or gravity flow methods range from
furrow irrigation, whereby the field is irrigated by allowing irrigation water to move
across the field by flowing down the row furrows, to flood irrigation as used in rice
production, whereby several centimeters of irrigation water is maintained on the
field throughout the growing season. A plane surface (longitudinal and transverse
slopes) is easiest to manage and maintain. Land grading can reduce the number of
field drains; thus more land is available for use.
6.4.1.3 Essential Conditions and Applicability
All lands to be graded for irrigation should be suitable for use as irrigated land
and for the proposed methods of water application. Water supplies and the delivery
system should be sufficient to make irrigation practical for the crops to be grown
and the irrigation water application methods to be used.
Land grading practices are applicable at sites with uneven or steep topography
or easily erodible soils, because it stabilizes slopes and decreases runoff velocity. It
is necessary where grading to a planned elevation is necessary and practical for the
proposed development of a site, and for proper operation of sedimentation-control
practices. Grading activities should maintain existing drainage patterns as much as
possible.
6.4.2 Precision Grading
Precision grading is a land improvement that can increase the productivity and
value of agricultural crop land by improving field drainage or increasing the effi-
ciency of surface irrigation. In precision grading, instruments (Scapers) having
GPS/Laser-controlled cut-and-fill work facility are used. The cost of precision
grading represents a long-term investment in farm.
The efficiency of surface irrigation methods can be improved substantially by
precision grading. A field leveled to a precise grade will ensure furrow irrigation
water will travel the entire length of row and not be blocked or diverted by depres-
sions in the field. For flood irrigation, precision grading can allow for a reduction
in the minimum depth of water that must be maintained on the field to cover the
entire field. By increasing surface irrigation efficiency, irrigation costs can also be
reduced.
Precision grading land for the purpose of improving surface irrigation efficiency
is applicable to a wide variety of crop production situations. Approximately one
half of the total irrigated agricultural acreage in the United States is irrigated by
surface or gravity flow methods. Major crops utilizing gravity flow irrigation sys-
tems include corn, soybeans, rice, hay, cotton and orchards. Gravity flow systems