Land grading is reshaping the surface of land to planned grades for irrigation and subsequent drainage. In general, the terms land grading, land levelling and land forming are synonymously used in literature. Land grading permits uniform and efficient application of irrigation water without excessive erosion and at the same time provides for adequate surface drainage.
A plane surface (uniform row and cross slopes) is easiest to manage and maintain. In dryland agriculture, land grading aids in soil and moisture conservation.
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 operations for an area depends on topography of the area, soil type, soil depth, crops to be grown, source of water supply and method of irrigation.
Design Criteria:
Soils should be deep enough so that, after the needed grading work is done, an adequate, usable root zone remains over most of the field that will permit satisfactory crop production with proper conservation measures. Limited areas with shallower soils may be graded to provide adequate drainage, irrigation grades or a better field arrangement.
All grading work for drainage or irrigation should be planned as an integral part of an overall farm system to conserve soil and water resources. Boundaries, elevations and direction of slope of individual field grading jobs should be such that the requirements of all adjacent areas in the farm unit can be met. Designs for the area being graded should include plans for removing excess irrigation and storm runoff water from the fields.
Excavation and fill material required for or obtained from such structures as ditches, pads and roadways should be planned for as a part of the overall grading job and the yardage included when calculating cut-to-fill ratios. The cut-to-fill ratio will normally be between 1.30 and 1.50 to allow for losses due to compaction, hauling and undercutting.
Land to be levelled shall be suitable for irrigation and for the proposed methods of water application. Water supplies and irrigation deliveries to the area to be levelled shall be sufficient to make irrigation practical for the crops to be grown and the irrigation water application method to be used.
Soils shall be deep enough so that after levelling an adequate, usable root zone remains that will permit satisfactory crop production with proper conservation measures.
Limited areas of shallower soils may be levelled to provide adequate irrigation grades or a better field arrangement. The finished levelling work must not result in exposed areas of highly permeable materials that can inhibit proper distribution of water over the field.
All levelling work shall be planned as an integral part of an overall farm irrigation system to enhance the conservation of soil and water resources. The boundaries, elevations, and direction of irrigation of individual field levelling jobs shall be such that the requirements of all adjacent areas in the farm unit can be met. This includes adequate land area to install a tail water system, rainfall runoff collection or water redirection system to prevent negative effects on other crops and land units.
Field Grades:
If more than one method of water application or more than one kind of crop is planned, the land must be levelled to meet the requirements of the most restrictive application method and crop. All levelling work must be designed within the slope limits required for the methods of water application to be used, to provide for the removal of excess surface water and to control erosion caused by rainfall. Reverse grades in the direction of irrigation shall not be permitted.
Slope:
Slope for level irrigation methods:
The maximum fall for level basin or level border irrigation in the direction of irrigation shall not exceed one-half the design depth of application for a normal irrigation. The difference in elevation across an individual border strip shall not exceed 0.1 feet.
Slope for graded irrigation methods:
The maximum slope in the direction of irrigation if rainfall erosion is not a significant problem shall be as follows:
1. Furrows: 3 per cent
2. Corrugations: 8 per cent
3. Borders for nonsod-forming crops, such as alfalfa or grain: 2.0 per cent
4. Borders for erosion-resistant grass or grass-legume crops or for nonsod-forming crops on sites where water application by the border method will not be required until after good crop stands have been established: 4.0 per cent.
In areas where potential for rainfall erosion is great, the maximum slope for furrows shall be 0.5 per cent and 2.0 per cent for borders for sod forming grasses and 0.5 per cent for other crops.
On slopes in the direction of irrigation of more than 0.5 per cent where levelling designs provide for increasing or decreasing slopes, the following limits shall apply:
1. The maximum slope in an irrigation run shall be no more than twice the minimum
2. The change in slope in any 100 foot reach shall not exceed one-half the maximum permissible change along the length of run, except, short, level sections are permissible at the upper or lower ends of irrigation runs to facilitate water control or to reduce runoff.
3. The maximum permissible slope change is the difference between the flattest and steepest design slope along the length of run.
Slope for Subsurface Irrigation Methods:
In areas where irrigation is practiced through groundwater level control the field surface shall be shaped to parallel the expected subsurface water elevations. The design shall consider the desired depth from the soil surface to the elevation of the groundwater.
Cross Slope:
Maximum cross slope for borders shall be 0.1 feet per border strip width, but less than the slope in the direction of irrigation. The allowable cross slope for furrows and corrugations depends on the stability of the soil, the size of furrows that are to be used and the rainfall pattern in the area. Cross slopes must be such that “breakthroughs” from both irrigation water and runoff from rainfall are held to a minimum.
Farm irrigation systems shall include plans for removing or otherwise controlling excess irrigation and storm water. Levelling designs must provide field elevations and field grades that will permit proper functioning of the planned drainage system facilities.
All levelling work shall be designed to permit delivery of needed irrigating streams onto the highest point on the field surface. The field elevation shall be at least 0.33 feet below the water surface elevation at the point of delivery.
Borrow shall be obtained from the required cut excavation or from designated sites specified in the design.
The effects of crop residue and other vegetative material that may materially affect the land levelling operations and earthwork volumes should be evaluated.
Cut fill ratios should be based on local experience, but the volume of cut should exceed the volume of fill by no less than 30 per cent. Rarely will cut volumes be twice that of the fill volumes (2:1 C/F ratio). Soil texture, soil moisture, large cut and fills depths and equipment traffic routing will affect the cut/fill ratio.
Adjustments may be required for soil conditions during construction. Levelling operations should not be performed if the ground is frozen or if soil moisture conditions will excessively damage soil structure.
Considerations:
In the design, the following needs consideration:
1. The excavation and fill material required for or obtained from such structures as ditches, ditch pads, and roadways. The appropriate yardage shall be included when balancing cuts and fills and determining borrow requirements
2. Related structures and measures needed to control irrigation water and/or storm water runoff
3. Irrigation water management elements such as: crops, method of irrigation, soil intake rates, field slope, irrigation stream size and resulting deep percolation and runoff when determining or evaluating length of irrigation runs
4. The depth of cuts and the resulting available plant rooting depths to saline soils and to shallow water tables. In areas with sediment laden irrigation water consider increasing the required height of the water surface at the point of delivery
5. The effects on irrigation efficiencies, especially on volumes and rates of runoff, infiltration, evapotranspiration and deep percolation
6. The effects on water flows and aquifers and the affect to other water uses and users. Consider the effects on adjacent wetlands.
Plans and Specifications:
Plans and specifications for irrigation land levelling shall be site specific and shall show the requirements for installing the practice to achieve its intended purpose. Site specific typically include field boundaries, planned cuts and fills, earthwork volumes, cut/fill ratio, direction of irrigation, design down slope and cross slope, required water surface and location of irrigation water delivery, tail water disposal and other structures.
The maintenance on levelled fields includes the periodic removal or grading of mounds and/or depressions. Land grading may periodically be needed to restore the design gradient.