Drain Spacing Calculator Guide

The drain spacing calculator estimates the required drain spacing between parallel drains necessary to promote trafficability and prevent crop damage from excess water in the root zone by using the Hooghoudt[1] equation. The drain spacing calculator gives the recommended drain spacing given the choice of drainage coefficient and the soil and system parameters. However, actual choices for spacing and drainage coefficient should be based on balancing the costs with the expected benefits.

The required parameters for the drain spacing calculator, with typical values for the Upper Midwest, and a reference diagram are as follows:

• Drainage Coefficient: Typical range (0.25 to 0.5 inches/day). The drainage coefficient is the design capacity of the drainage system and is typically expressed as a depth of water removed in 24 hours (inches/day). A drainage coefficient should be chosen that will economically remove excess water from the top part of the root zone within 24 to 48 hours. Climate, soils, and crops grown will all influence the choice of drainage coefficient, and drainage coefficient choices are typically based on local conditions, experience, and judgment. Typical drainage coefficients in the Corn Belt range from 0.25 to 0.75 inches/day. Drainage coefficients are generally lower (0.25 to 0.5 inches/day) in the northern and western Corn Belt (Dakotas and western Minnesota) and higher (0.375 to 0.75 inches/day) to the south and east.

• Tile Diameter: Different pipe sizes have different opening areas, which affects water flow into the pipe and has an impact on spacing requirements.

• Tile Depth: Typical range (3–4 feet). Typical tile depths (W) for agricultural drainage are 3–4 feet. The depth of the drains affects the hydraulic head (h) of water driving flow to the drains and the distance between the drains and the restrictive layer that is available for water flow. Shallower drains will require a narrower spacing for the same drainage coefficient.

• Depth to Restrictive Layer: Typical value (known value from sampling or soil survey or an arbitrary value, 10 feet). A restrictive layer is a layer of soil that limits the vertical movement of water, which can lead to a perched water table. A restrictive layer is often considered to be a layer where the saturated hydraulic conductivity (Ksat) is less than 10% of that of the soils above it. The depth to the restrictive layer is the combination of the drain depth (W) plus the depth from the drain to restrictive layer (D). The depth to the restrictive layer (W + D) affects the flow patterns of water to the drains and the required drain spacing. A shallower depth to the restrictive layer will require narrower drain spacing. The drain spacing calculator assumes that the drains are above the restrictive layer. If the restrictive layer is very shallow and the drains cannot be placed above it, a more detailed design procedure is required. If the depth to the restrictive layer is not known (deeper than the depth reported in the NRCS Soil Survey or deeper than any in-field sampling), an arbitrarily deep depth (for example 10 feet, as used in the NRCS Minnesota Drainage Guide) can be used to estimate a drain spacing. Depths deeper than this will have increasingly less influence on the drain spacing results.

• Minimum Water Table Depth: Typical value (1 foot). Most agricultural crops cannot tolerate a water table within 1 foot of the soil surface for more than 24 hours, so the minimum water table depth (H) is typically set to 1 foot. Values less than 1 foot will result in wider drain spacings, and values greater than 1 foot will result in narrower drain spacings.

• Saturated Hydraulic Conductivity (Ksat): Typical range (0.1–50 feet/day, depending on soil type). The saturated hydraulic conductivity is a measure of the ease with which the pore spaces in a saturated soil will allow water to move through that soil. The saturated hydraulic conductivity is the most important soil property affecting drain spacing, but it is highly variable and thus difficult to find accurate, representative values. Saturated hydraulic conductivity estimates can be obtained from field measurements using the auger hole method, looking up reported values from a soil survey (the NRCS Web Soil Survey and the UC Davis California Soil Resource Lab’s SoilWeb are online sources of soils data for looking up estimates of Ksat values), or from handbook data of typical values by soil texture. Saturated hydraulic conductivity is reported in a variety of units, so several units options are provided in the dropdown box. The required drain spacing increases as the saturated hydraulic conductivity increases.

Average Saturated Hydraulic Conductivity Values

	Saturated Hydraulic
Soil Texture	Conductivity (ft./day)[2]
Sand	49.9
Loamy sand	44.2
Sandy loam	9.84
Silt loam	2.04
Loam	1.97
Sandy clay loam	1.79
Clay loam	0.694
Silty clay loam	0.482
Sandy clay	0.615
Silty clay	0.292
Clay	0.363

Source: Clapp and Hornberger (1978)[3]