How much water does a drain tile remove?

By Vinayak Shedekar, The Ohio State University

Have you ever wondered how much water a drain tile removes? Does a larger outlet pipe drain a field faster? Are there differences in drain pipes that would affect the amount of drainage? Let’s explore some of these questions.

Does drainage water always flow downhill? Drainage theory seems simple: water flows downhill! In other words, gravity plays a major role in driving the movement of drainage water from one location to another. However, the actual flow lines of drainage water leading to lateral pipes are not straight lines. Because the actual opening on a drain pipe is much smaller than the 4-inch diameter, the flow lines converge radially around the drain pipe. So in reality, it may look like the water is moving up in the soil profile against gravity.

What role does the soil play? Certain soil properties control how quickly water can move within the soil profile towards the drain tiles. The permeability of a soil dictates how slow or fast the water is supposed to move towards the drain pipe. Each soil has a certain amount of “drainable pore space”. In other words, there is only so much water that can drain out of the soil by gravity. Typically, the drainable porosity of the majority of soils requiring drainage in Ohio ranges from 3% to 11%. For a drainage system installed at a 3-ft. depth, this would translate into about 1 to 4 inches of drainage discharge! Now, most drainage systems are designed to lower the water table by about 1 ft in the first 24 hours. Based on the 3% to 11% drainable porosity, this translates into about 0.4 to 1.3 inches of drainage discharge in 24 hours. Thus, with a properly installed drainage system, you should be able to remove 0.4 to 1.25 inches of excess water in the first 24 hours! This maximum rate of water entry into the drainage system is often referred to as “Drainage Intensity”.

What role does the pipe play? Pipe type, drain spacing, drain depth, and installation grade can affect how much excess water can be extracted from the soil and carried through the pipe network to the drainage system outlet. Most laterals are 4-inch corrugated plastic pipes with perforations. Do you know that the type and number of perforations can affect how fast or slow the water enters the lateral pipes? Regardless of how water enters and moves through the drainage system, the system’s capacity to drain is ultimately limited to the maximum discharge that the main outlet pipe can carry. Even if the soil can supply more water, the outlet pipe limits how much can leave the system. Thus, even if soil allows 1 inch of drainage discharge per day, an outlet pipe allows only 0.5 inch of discharge/day, then that’s the actual drainage amount in 24 hours! This maximum carrying capacity is what we call a “Drainage Coefficient”. You will find two fascinating videos that explain these concepts at go.osu.edu/drainagebasics.

How much water does a drain tile remove? To figure out how much water is discharged from your drainage system in a day, multiply the drainage coefficient by the contributing drainage area, then convert the units. Refer to the graph that summarizes estimated discharge in ft3/sec or gallons/minute based on the acreage you drain and a drainage coefficient your system is designed for. Most soils in Ohio would use a drainage coefficient ranging from 3/8th to ¾ inches. Another way to estimate the tile discharge is to use an online calculator that allows you to enter the outlet pipe size (assume a grade of 0.1%). Visit go.osu.edu/draincalc for links to access these calculators.

For example, with a drainage coefficient of 0.5 in. and an 8-inch. A single-wall outlet pipe would drain about 15 acres (0.31 ft3/sec), and a dual-wall pipe of the same size would drain about 20 acres (0.41 ft3/sec). When you start multiplying these numbers by the duration for which the tile drains run, you will realize that a typical farm field discharges millions of gallons of water in a day! As the growing season progresses towards warmer, drier weather, it may be worthwhile to consider storing some of this water in the profile using approaches such as controlled drainage and drainage water recycling.

This column is provided by the OSU Department of Food, Agricultural, and Biological Engineering, OSU Extension, International Program for Water Management in Agriculture, Ohio Agricultural Research and Development Center, and the College of Food, Agricultural, and Environmental Sciences.

Dr. Vinayak Shedekar is the Assistant Professor of Agricultural Water Management and director of the Overholt Drainage Research and Education Program in the OSU Department of Food, Agricultural, and Biological Engineering, and can be reached at shedekar.1@osu.edu