drain tile

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NDSU hosted a Tile Design workshop via Zoom on Thursday morning, March 25. The workshop was hosted by NDSU Extension Agricultural Engineer Dr. Thomas Scherer and NDSU Extension Agronomist Dr. Hans Kandel.

Scherer presented information on drainage and lift station design. The purpose of installing drain tile in a field is to try to control the water table so that plants can grow at maximum ability. A high water table is due to a restrictive sub-layer below. The upper layer of the water table is called the capillary fringe. Tile is installed so that only water that rises above the capillary fringe goes into the tile and drains from the field.

The steps for planning out tile drainage for a field includes the preliminary site evaluation (soils, drainage coefficient, and topographic features), a field visit to develop a topographic map, and tile layout selection (grade, spacing and size).

Soil texture is a combination of sand, clay and silt. Scherer’s tool of choice for determining soil texture was Google Earth with the soilweb.kmz overlay available from California Soil Resources. He demonstrated how you can pull up the soil composition and in-depth analysis within areas of your own fields, including the Ksat value - water able to be drained off a field per hour.

Drainage coefficient is the depth of water to be removed from the soil in 24 hours, expressed in fractions of an inch per acre. Field crops with good surface drainage need ¼ to ½ inch; high value crops, such as vegetables or potatoes, with good surface drainage need ½ to ¾ inch.

Light Detection and Ranging (LIDAR) is a method of gathering and recording topographic information to make maps. North Dakota LIDAR information can be accessed at http://lidar.swc.nd.gov (ND Soil and Water Conservation) to get a topographic map of your field.

The site visit checklist includes: buried lines (electric, gas, oil, rural water, etc), identifying wetlands, noting any boundary conditions (roads, ditches, depths, culverts, flow from other fields), the current surface drainage system, soil types (any salinity or sodium issues), high and low spots with water problem areas (draw on your map), and possible outlet locations.

Tile grade is the rise of the land divided by the distance of that rise (“rise over run”) and is expressed in a percent grade. For example, a one foot rise over 1000 feet would equal a 0.1 percent grade. Grade can change along a tile as long as it is always sloping in the same direction. Tile grade should not be allowed to rise and drop as that will cause pooling and sediment.

For tile spacing, you want to set the distance between tiles so the water table doesn’t get too close to the surface of the field. There is a drain spacing calculator developed by Dr. Chris Hay at http://www.igrowdrainage.org/#/. You need to know the drainage coefficient, depth of the restrictive layer, the minimum water table depth you would like, and the hydraulic conductivity (Ksat value) in order to calculate drain spacing. The nice feature of the calculator is that you can change the tile diameter and depth and recalculate to see your options.

Scherer shared a tile sizing slide rule to calculate tile sizes for different walled tile. There are also a number of calculators on the same iGrow.org website for pipe size and area drained. With those tools and information at your disposal you can design the tile layout. There are likely several options for the layout of the system, and you will need to assess the pros and cons of each. The cost of the project will include feet of pipe in all sizes needed, fittings, end caps, splices, tees, pipe connections, labor, and machine cost.

In choosing an outlet, salts need to stay dissolved and water needs to keep moving. Everything has to drain to the outlet, and all mains and submains have to go uphill from that point. Tile should be at least 2 feet below ground surface at its high point, and this determines the final depth of the downhill outlet.

If you don’t have a deep enough outlet, you need a lift station. It is usually most economical to locate the pump station near the power source, so evaluate the cost of extending electric lines versus the cost of extending piping. A few lift stations have been powered with a portable generator, propane, or solar. Determine the drainage area and include future development; some use one pump station for two fields. One of the most important factors for lift station location is the impact on your neighbors.

There are two types of pumps, fixed speed (on/off controlled by floats) and Variable Frequency Drive (VFD) with submersible and above ground options. Maximum flow rate needed, the height the water needs to be lifted (called head), horsepower, and sump storage volume are factors in pump selection. Fixed speed pumps need more water storage in the sump to keep from having to cycle on and off too frequently which is hard on the motor. Instead of increasing the size of the sump, a 2 foot pipe extending horizontally can be installed.

A lift station will add costs for the pump, sump construction, installation, extended electric connections, and dewatering (a system of small wells to keep the water table low around the sump) if needed. Pumps and floats need to be checked after large rains and need annual maintenance.

Kandel presented the agronomic considerations to tiling. Delayed planting due to wet fields reduces yield. When the water table is high, the availability of oxygen to the roots is decreased creating shallow roots and de-nitrification. Surface drainage and ditching are options, but subsurface tiling was proven to increase yields more and have less variability.

Northeastern ND had the highest percentage of soil samples in the state with salinity above 1.0 in 2020. Kandel presented several research studies as to how much saline soils stunt plant growth; when soil salinity reached 1.0, soybean plant height was cut in half; tile drainage has shown to reduce salt levels in topsoil but may take several years to reach low levels; crop yields increase in tiled vs. untiled fields, with corn, wheat, and sunflowers showing the highest increases; and wheat, soybean, and sugar yields increased around 10 percent with a drainage coefficient of ¼ inch per acre.

Wet soil conditions also result in delays, ruts, and sunken equipment at harvest time. It is slightly more efficient to unload on the go vs. unloading at the end of the field.

In summary, the advantages of tiling are more consistent yields, earlier and more timely planting, more nitrogen retained per rainfall event, increased organic matter and residue resulting in less blowing soil, reduced cleaning of ditches, and more efficient use of equipment for seed bed preparation, seeding and harvesting. Other external benefits are timely application of herbicides and fungicides, better crop quality, and fewer hassles. In summary, you should see increased crop production, less production risk, increased land value, and tile can be depreciated.

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