Advancements in technology can mean greater access. Farmers now have access to equipment to install their own drain tile systems, such as tile plows to install plastic corrugated tile with high-horsepower tractors (bypassing the need for commercial self-propelled trenching machines), and GPS technologies enabling the centimeter-level, real-time vertical slope accuracy for gravity-fed systems.
However, farmers should proceed with caution. Precision drainage is much more than using the latest tech to ensure the correct depth and pitch of drainage lines. Precision drainage is about using the soil characteristics and topography of the field effectively, to capture soil water in the most efficient and cost-effective manner, maintain a uniform water table across the field, and include the option to control the amount of water that leaves the field depending on seasonal trends and crop needs.
Farmers (and their contractors) should ask themselves if the drainage systems they’re implementing are true “precision” drainage systems, or might they be using precision technology to implement non-precision systems. For example, designing a drain tile system in parallel lines of uniform distances apart may be an indicator that precision drainage techniques were not used, as most fields are not that uniform in soils and topography.
Understanding the Soil’s Characteristics
Drainage Intensity (DI)
Drainage Intensity represents the actual rate at which water can move through soil and into the tile lines when the water table is at the surface. Think of DI as the “speed limit” of your soil’s hydraulics. DI is not about the size of pipe installed, but about how water moves through the soil. Sandy soils have a high DI. Heavier clay soils have a low DI. The soil DI can vary horizontally across the field and vertically within the soil profile, meaning the top one foot of soil may have a different DI than the lower layers of soil.The term for how easily water moves through your specific soil type is “soil hydraulic conductivity”.
Restrictive Layers
A soil restrictive layer is defined as a soil layer whose soil flow rate through it is one-tenth or less than the above layers. Identifying the restrictive layer is important to know what depth your tile should be put in.Thinglink Interactive Image Drainage Basics
Understanding Water Flow through Soils
Since the DI is how fast or slow water flows through the layers of the soil, DI can and does change depending on the soil type and layers. The soil layer the tile is laid in is the ultimate determiner of the flow of water into the tile line and out of the field.We know water flows downhill on the surface, but it can also flow downhill below the ground surface. If tile lines are installed going up a hill, some water will flow in between the tile lines. Instead, you can install tile lines along the contour to intercept that water rather than counting on it moving horizontally flowing crossways to the tile lines.
In addition, water will be extremely limited to enter the tile line if you install it below the restrictive layer. If you put the tile at the restrictive layer, water will also be limited to enter the tile line as most drainage water enters from the bottom of the tile line. A properly placed tile line sits above the restrictive layer to allow water to flow down at a sufficient rate and also to flow “up” towards the tile line bottom.
The most effective tile line placement intercepts the natural flow of water that existed prior to drainage being installed.
Understanding Water Flow Through Tile Lines
While DI measures water flowing through the soil, the Drainage Coefficient (DC) measures the quantity of water flowing through the tile lines – both the laterals (tributaries) and the mainline (collector). The DI and DC determine how much water can leave the field and at what rate, and they both have to be aligned. If the DI is less than the DC, the DI is the limiting factor. And also, if the DC is less than DI, then the DC is the limited factor.In other words, if you place your laterals close together to remove a ½ inch of water in 24 hours, but your tile collector can only remove ⅜ inch of water in 24 hours, you will not receive the benefits of the lateral spacing.
Understanding the Water Flow in the Ditch or Receiving Body
A farmer can have an optimally designed system for their field, but if they overlook or ignore the Drainage Coefficient of the receiving body, they may not reap the full benefit of their system. Like their field main or collector line, the receiving body also has a DC for all the fields it drains. Depending on a variety of factors, a farmer’s field drain system may have to first wait for the ditch to drain significantly for their field to drain if the field DC is larger than the ditch DC.These issues raise a question: does the farmer want to pay for a field system that has a ¾ inch DC that flows into a ditch with a ½ inch DC? They may if they think someday the ditch system will be made larger, but that should be thoroughly researched if they want to optimize their investment.
Drainage Basics
Understanding the basic physics of soil capacities, field topography and slopes, and water flows to, through, and out of the drainage system is critical whether one applies precision drainage systems or not.The next article in our series on drainage will discuss how to use drainage basics to optimize water table management and water flows in precision drainage systems.
Additional Resources
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