Determining the proper spacing of drain ditches or subsurface drains can be a tedious task, but careful assessment of site conditions is fundamental to a successful drainage system. Producers, landowners, farm managers, and contractors should be aware of the soil types in the field where installation of a new drainage system is being considered.

Soil texture is a vital consideration in determining the proper drainage spacing. To select the proper drain depth, you need to determine the soil layering and textural classes. In general practice, subsurface drains and field drainage ditches are installed as deep as the outlet will accommodate, allowing for a wider drain spacing to meet drainage needs. Wider spacing minimizes installation costs because the amount of drainage pipe or ditch excavation required is reduced. However, regardless of outlet characteristics, avoid placing subsurface drainage pipes in a restrictive soil layer, such as a clay that reduces water movement, or the system will fail. We recommend first performing a soil profile assessment and comparing the results to the United States Department of Agriculture's Natural Resources Conservation Service (NRCS) Web Soil Survey database.

Hydraulic conductivity describes the ability of soils to move water through the profile. This property, which is related to soil texture, is key to determining the proper drain spacing. The Web Soil Survey database contains information on the estimated values of the hydraulic conductivity for different soil series. When feasible, you should determine the hydraulic conductivity based on site-specific measurements. It is also critical to consider the subsurface drainage intensity (DI). The DI is the rate at which water can move through the soil profile to the drain when the water table midway between drains is at the ground surface (Skaggs 2017). The DI required for a properly designed drainage system varies with climate and therefore location. For example, rainfall during the growing season is greater in North Carolina than in Illinois, so the DI required for North Carolina systems should be greater than in Illinois (Skaggs et al. 2006).

Research at North Carolina State University has shown that the average DI required for row crops in eastern North Carolina varies from 0.4 inch per day for lands with good surface drainage to 0.5 inch per day for those with poor surface drainage (Skaggs and Nassehzadeh-Tabrizi 1986; Skaggs 2017). You should design and install subsurface drains at a depth and spacing such that the rate of drainage through the soil to the drain is equal to the DI.

The design drainage coefficient (DC) is another key factor affecting drain spacing. The DC is the theoretical maximum amount of water that can be removed by the drainage system in a 24-hour period. The DC quantifies the hydraulic capacity of the installed drainage system; it should not be confused with the DI. The DC is typically expressed as the depth of water that can be removed per unit area in a day. You should choose the size (diameter) of the main or collector drains that satisfies the DC. The DC should not be less than the design DI and should be greater than the DI if provisions are made for surface runoff to enter the laterals, mains, or submains. For example, if the DC for a drainage system is less than the required DI, then the system will not remove enough water in time to maximize crop yield in wet years. Most subsurface drainage designs for eastern North Carolina are based on a DC of 0.5 to 1.0 inch per day. The selected value will depend on the purpose of the system (for example, drainage, subirrigation, or surface inlets) and the value of the crop being produced.

Online calculators are available to determine both the DI and DC based on specific user-supplied inputs of site and system design characteristics. It is important to understand that the calculators do not determine the required DI and DC for your field; rather, they allow you to verify that your drainage system will achieve the DI and DC required for optimal water removal. Useful online DI and DC calculators are available via the Iowa Soybean Association (ISA). To use the DI calculator, you will need to provide the inputs for drain spacing, depth, and diameter. These parameters depend on the drainage design and should be adjusted to meet the required DI for your location and purpose. The depth to the restrictive layer and hydraulic conductivity above and below the drain lines are also needed for the calculator; these inputs are unique to the specific soils and soil layering at the field. They can be estimated utilizing the Web Soil Survey; however, using the survey should not be a substitute for obtaining actual measurements from your field. Field-specific information can often be more accurate than values supplied from the Web Soil Survey.

The ISA's online DC calculator utilizes specific characteristics of the installed drainage system. The drainage area includes all upstream areas that drain into each receiving drain. The drain grade is the actual final installed grade of a particular drain line. The drain diameter and drain material describe the drain line. To ensure that downstream components do not limit the flow from upstream components, you should calculate the DC for each drainage component (lateral, submain, and main) in a drainage system independently. Use the calculator for each of the individual components of a designed drainage system, starting with the most upstream component and proceeding toward the outlet. Each component should be sized appropriately to transport all flow received from upstream drains.

Figure 1 gives typical spacing for 4-foot-deep subsurface drains based on soil textures and relative conductivities adapted from the American Society of Agricultural and Biological Engineers (ASABE 2015). The sample values will give you a good starting point for selecting proper drain spacing. However, proper spacing is also a function of the drain depth, which is typically controlled by the outlet conditions and soil layering. Shallower drains will require narrower drain spacing for the same DI.

In addition to online calculators, hydrologic models such as DRAINMOD are good tools to use in designing water management systems. DRAINMOD is a computer simulation model created at NC State that can predict crop yield benefits using historic weather conditions, site-specific soil information, and drainage system design criteria to evaluate your long-term potential return on investment. Though the resources mentioned here are valuable tools for planning your drainage system, we strongly recommend consulting with a professional on the design of the system before installation.

Figure 1. Typical drain spacing ranges by different soil types.

Adapted from ASABE 2015.

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Figure 1. Typical drain spacing ranges by different soil types.

Adapted from ASABE 2015.

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American Society of Agricultural and Biological Engineers (ASABE). 2015. Design and Construction of Subsurface Drainage Systems on Agricultural Lands in Humid Areas. EP260.5. ASABE. ↲

Skaggs, R. W. 2017. “Coefficients for Quantifying Subsurface Drainage Rates.” Applied Engineering in Agriculture 33 (6): 793–799. ↲

Skaggs, R. W., M. A. Youssef, and G. M. Chescheir. 2006. “Drainage Design Coefficients for Eastern United States.” Agricultural Water Management 86 (1–2): 40–49. ↲

Skaggs, R. W., and A. Nassehzadeh-Tabrizi. 1986. “Design Drainage Rates for Estimating Drain Spacing in North Carolina.” Transactions of the ASAE 29 (6): 1631–1640. ↲