United States Department of Agriculture 388-CPS-1

NRCS, NHCP

October 2020

NRCS reviews and periodically updates conservation practice standards. To obtain the current 

version of this standard, contact your Natural Resources Conservation Service State office or 

visit the Field Office Technical Guide online by going to the NRCS website at 

https://www.nrcs.usda.gov/ and type FOTG in the search field. 

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Natural Resources Conservation Service 

CONSERVATION PRACTICE STANDARD 

IRRIGATION FIELD DITCH 

CODE 388 

(ft)

DEFINITION 

A permanent irrigation ditch, constructed in or with earth materials, to convey water from the source of

supply to a field or fields in an irrigation system. 

PURPOSE 

This practice is used to accomplish one or more of the following: 

• Improve distribution uniformity of water applied on irrigated land 

• Improve irrigation efficiency of water applied on irrigated land 

CONDITIONS WHERE PRACTICE APPLIES 

This standard is limited to open channels and elevated ditches of 25 cubic feet per second or less in

capacity and constructed of earth materials. 

This standard applies where field ditches are needed as an integral part of an irrigation water distribution

system designed to facilitate the conservation use of soil and water resources. 

CRITERIA 

General Criteria Applicable to All Purposes 

Ensure water supplies and irrigation deliveries for the area served are sufficient to make irrigation practical

for the planned crops and irrigation water application methods. 

Construct field ditches in earth material that contains sufficient fines to prevent excessive seepage losses

and where shrinkage cracks will not endanger the ditch or cause downstream water quality problems. 

Capacity requirements 

Design field ditches with adequate capacity to deliver the— 

• Design peak consumptive use of the crops to be grown in the field, with proper provisions for the

expected field irrigation efficiency. 

• Largest irrigation stream required for the irrigation methods planned for the field. 

Design the capacity of ditches to include the additional flow necessary to compensate for losses due to

ditch seepage and to safely carry surface runoff from adjacent lands that must be transported to

waterways or overflow points. 

For capacity design, select the value of Manning’s roughness coefficient “n” based on the materials of the

ditch, alignment, hydraulic radius, and additional retardance due to vegetation. For all ditches, the

maximum allowable Manning’s roughness coefficient “n” is 0.025. Ditches must meet the applicable
design criteria for field ditches in NRCS Conservation Practice Standard (CPS) Open Channel (Code

582). 

Velocities 

Design field ditches for flow velocities that are nonerosive for the soil materials of the ditch. Use local

information on the velocity limit for specific soils if available. If such information is not available, do not

exceed the maximum design velocity shown in NRCS National Engineering Handbook (NEH) (Title 210),

Part 654, Chapter 8, “Threshold Channel Design,” Figure 8–4, or other equivalent method, unless

protective measures are implemented. 

Cross section 

The required freeboard in field ditches is one-third of the maximum design depth of water, or 0.5 feet,

whichever is less. Ensure side slopes are stable against slope failure. Design the top width of banks, as

measured at the freeboard elevation, at least 1.0 foot and equal to or exceeding one-half the flow depth. 

If a field ditch is constructed on a fill section, ensure side slopes of the fill are not steeper than the values

shown in table 1. 

Table 1. Allowable Steepness of Slide Slopes for Field Ditches Constructed of Fill Material 

Water surface elevations 

Design all field ditches so that water surface elevations at field takeout points are high enough to provide

the required flow onto the field surface. If ditch checks or other control structures are used to provide the

necessary head, include the backwater effect in computing freeboard requirements. 

The required water surface elevations above the field surfaces will vary with the type of takeout structure

or device used and the amount of water delivered through each. Provide a minimum head of 4 inches.

Use energy dissipation devices where erosion is anticipated at outlets. 

Spoil 

Dispose of spoil material resulting from the construction of irrigation field ditches using NRCS CPS Spoil

Spreading (Code 572). 

Related structures 

Design and construct erosion control or water control structures, culverts, diversions, or other related

structures needed to supplement the field ditch to meet the applicable NRCS conservation practice

standards. 

CONSIDERATIONS 

There are potential water quality impacts resulting from soluble and sediment-attached pollutants. These

potential impacts can affect downstream flows or aquifers, which will affect other water users and aquatic

life. 

Construction of field ditches has the potential for uncovering or redistributing toxic material. Construction

and operation of irrigation field ditches may affect wetlands or water-related wildlife habitats. 

Sediment carried in the irrigation water for ditches constructed in soils that have the potential to crack may

have a positive sealing effect. 

Consider the effects of water level control on the salinity of soils, ground water, or downstream waters. 

Height of Fill to Water Surface on Centerline of Fill

(feet)

Steepest Allowable Side-slope of Fill

(horizontal to vertical)

< 3 1.5 :1

3–6 2:1

> 6 2.5 :1

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NRCS, NHCP

388

October 2020
PLANS AND SPECIFICATIONS 

Describe the requirements for applying the practice to achieve its intended purposes. As a minimum, the

plans and specifications shall include— 

• Cross section details (dimensions). 

• Embankment requirements. 

• Channel grades. 

• Appurtenant structural details. 

• Location of the irrigation field ditches on the plan map. 

• If applicable, provide information on recommended species of vegetative cover, establishment, and

maintenance. 

OPERATION AND MAINTENANCE 

Prepare an operation and maintenance plan for use by the landowner or operator. Provide specific

instructions for operating and maintaining the irrigation field ditch to ensure proper function as designed. 

Include— 

• Performing prompt repair or replacement of damaged components. 

• Removing any debris or foreign material from field ditches and other components that may obstruct

or hinder system operation. 

• Maintaining recommended vegetative cover on side slopes and in watercourses. 

• Scheduling regular maintenance (mowing or other disturbance of vegetative cover on side slopes or

in ditches) outside of the primary nesting season for local grass-nesting species. 

REFERENCES 

USDA NRCS. 2007. National Engineering Handbook (Title 210), Part 654, Chapter 8, Threshold Channel

Design. Washington, D.C. https://directives.sc.egov.usda.gov/.

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NRCS, NHCP

388

October 2020