Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 204

The degree of slope acceptable for irrigation development and therefore the
cost of land leveling depends on the anticipated method of irrigation, intensity and
amount of rainfall, susceptibility of the soil to erosion, and planned cropping system.
In the United States, gravity irrigation on slopes greater than about 12% is seldom
practiced. With sprinkler or drip systems, limitations on slope due to an erosion haz-
ard or the operation of farm machinery are important. Slopes of 20% are currently
considered the maximum acceptable in the United States for cultivated crops irri-
gated by sprinklers. In areas that experience severe thunderstorms, the maximum
usable slope may be less. Land devoted to dense cover crop or grass may permit
irrigation of steeper slopes than for row or field crops.
Although excessive slope is the most frequent problem, lack of slope may also
be a limitation. Excessive flatness may result in higher grading costs to increase
the slope and achieve the smooth uniform surface necessary for uniform distribu-
tion of irrigation water. Extremely gentle gradients may make irrigation of slowly
permeable soils difficult because standing water induces scaling and waterlogging.
Very permeable soils and extremely flat topography may prevent uniform irrigation
without excessive deep percolation and water use. On the other hand, very flat land
provides an opportunity to use really efficient surface irrigation methods such as
basin and border strip, where soils are suitable.
6.4.4.1 Furrow Grades
Land graded for irrigation with subsequent drainage will have a slope in the row
direction between 0.1 and 0.5% on deep alluvial soils. There should be no reverse
grade in the row direction. The most desirable surface is a plane. Fields graded to
minimum slopes will require more maintenance of grade than steeper slopes.
Design grades on claypan soils may have furrow grades up to 1.0% to avoid
exposing large areas of subsoil in cut areas. Special residue management may be
necessary to minimize erosion where furrow grades exceed 0.5%.
6.4.4.2 Cross Slope
Cross slope (slope perpendicular to row slope) is permitted in order to reduce cut
volume or to establish the “plane of best fit.” Cross slopes must be such that “break-
throughs” from both irrigation water and runoff from rainfall are held to a minimum.
Terraces should be used on cross slopes of 2% or more. Where terraces are neces-
sary on fields to be irrigated, rows should be parallel to terraces. Land forming may
be necessary between terraces to eliminate reverse grade in irrigated rows.
6.4.4.3 Maximum Length of Runs for Irrigation
Maximum length of runs for irrigation should be limited by furrow flow rates avail-
able, furrow cross-sectional area, erosion hazard to the furrow, and water intake
characteristics of the soil. Erosion hazard is a function of soil texture, crop residue