Bid wet spots adieu with blind inlets

by Courtney Llewellyn

Spring rains are coming. Generally, that is seen as a good occurrence in farming, but what happens if there’s too much water?

Draining wet spots can make portions of fields – or entire plots – more arable, and that’s what Dr. Ehsan Ghane, assistant professor and Extension specialist with Michigan State University, discussed during the recent MI Ag Ideas to Grow With.

The water cycle is important to consider – precipitation, transpiration and evaporation. But farmers also need to look at infiltration and surface runoff. Surface runoff occurs when the rainfall rate exceeds a soil’s filtration rate or it rains on already saturated soil (the water table is at the soil surface) or it rains on frozen ground. Under the ground, there may be deep percolation into the water table, capillary rise (which, depending on the soil, will bring water up to the root zone), lateral seepage (which moves water side to side) and drainage discharge – a very important part of this.

Ghane said there are basically two ways to drain wet spots: targeted subsurface drainage or a blind inlet. When it comes to subsurface drainage layouts, there are several options to consider. Parallel systems, with straight lines beneath a whole field, create a more uniform crop yield. More sloping land may use a herringbone layout. Targeted drainage, which focuses on areas of ponding, is an older system, historically. (Ghane noted clay tiles from Europe were installed in Geneva, NY, hundreds of years ago – and that required a lot of effort to dig up the ground, lay the tiles and cover them again.)

If the wet spot and its surroundings have poorly drained soil (heavy clay soil, for example) and the source of excess water is the naturally shallow water table, you’ll need to remove water from the soil profile, often by installing perforated pipes in the ground.

A blind inlet can be used if the wet spot and its surroundings have well-drained soil and the source of excess water is surface runoff, Ghane said. Blind inlets replace surface (or open) inlets – the pipes that stick up out of the ground. Surface inlets are not part of a good conservation practice, Ghane added, because they move a lot of phosphorus. The good news is a surface inlet can be converted into a blind inlet.

Building a Blind Inlet

Before trying this method of draining wet spots, farmers need to consider the size of the watershed draining into the depression area. The overall area should be less than 20 acres, and soil erosion should be a small factor, as sediment clogging reduces the lifespan of the filter material. The actual inlet can be as large as 50-by-50 feet or as small as five-by-five feet.

To create the blind inlet, dig a trench a minimum of 1.5 feet deep. Lay a geotextile material (usually made of polypropylene or polyester, often woven, needle-punched or heat-bonded) on the bottom. Place a coarse material, such as gravel, on top of it, and run your perforated pipes through this layer. Above that, you want a minimum of one foot of sand, which filters the soil and traps phosphorus. Water will be moved away either through the existing drain system or to an outlet.

“Why use blind inlets?” Ghane asked. “There’s no interference with field operations or equipment and they reduce sediment and particulate phosphorus loss and erosion.”

He noted that experiments have shown that if steel slag is used as the coarse material, dissolved reactive phosphorus can be removed as well. If woodchips are used, nitrate can be removed (and steel slag and woodchips can be mixed). The materials will not be considered environmental hazards if you need to remove them.

Maintenance of blind inlets is required when sediment and residue build up over the sand layer. All that needs to be done is removal of the old sand, followed by replacement with new sand. Ghane said you’ll know when the system is clogged because it doesn’t drain as quickly.

The lifespan of a blind inlet depends on the sediment from each farm’s practices, but they can last up to 10 years. Tillage and soil disturbance will increase erosion and therefore decrease the lifespan. Conversely, reduced tillage and using cover crops, manure/compost and diverse rotations improves soil health, which reduces erosion and leads to a longer life expectancy for the inlet.

Ghane recommended visiting egr.msu.edu/bae/water/drainage for more information on blind inlet usage, among other drainage topics.

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