March 08, 2021

Tile monitoring tracks nutrient loss

CHAMPAIGN, Ill. — Edge-of-field tile drainage monitoring provides an assessment of nutrient loss and documents the impact of conservation practices.

Laura Christianson, University of Illinois Department of Crop Sciences Extension specialist, detailed the procedures for collecting tile drainage water samples and measuring water flow as part of U of I Extension’s Crop Management Conference.

To provide a baseline “rule of thumb” snapshot of nutrient loss through tile drainage across the Corn Belt, Christianson referred to a large-scale review she did several years ago of every published drainage water quality study in North America.

“This gives us confidence that this is fairly representative across the Midwest, across the Corn Belt and across many cropping systems, primarily for corn and soybeans,” Christianson said.

The data included over 900 observations from 40 to 50 years of drainage studies that documented individual nitrate concentrations reported as flow-weighted and annual concentrations.

The analysis found the mean nitrate concentration was about 13 milligrams nitrate nitrogen per liter in all of those 900 drainage studies. The median value was about 12 milligrams of nitrate nitrogen per liter.

“So, if I wanted to give you a feel for what’s a normal or typical tile drainage concentration for corn and soybean cropping systems in the Midwest, I would say generally we’re talking about 10 to 15 milligrams nitrate nitrogen per liter based on the mean and median values,” Christianson said.

“I know what you’re saying. ‘Hold on, Dr. Laura, I’m doing cover crops, I’m following the ‘Four Rs’ of nitrogen management, my nitrate concentration should be a lot less.’ That may be the case, but I just want to give you a general ‘rule of thumb’ numbers for your back pocket for what normal values for nitrate concentrations are in our tile drainage across the Midwest.”

To put the nitrate nitrogen loss found in the data in perspective, the Illinois Nutrient Loss Reduction Strategy calls for a 25% reduction in both nitrogen and phosphorus losses into Illinois waterways by 2025 and a final goal of a 45% loss reduction. The “rule of thumb” across 900 observations is much higher in terms of drainage concentrations than the strategy’s goals.

Christianson gave step-by-step procedures to collect accurate readings of nutrient concentrations and flow in tile drains.

Sample Collecting

Tile water sampling can be done by hand or using automated sampling equipment.

“The selection of the method of collecting the samples obviously depends on the goals of your monitoring. It also depends on your resources, not just money, but also available labor, time and personnel,” Christianson explained.

“For a farmer or landowner you would probably use the manual sampling by hand rather than using an auto-sampler. You can collect from a control structure and you could also sample a title outlet if you have access to it.”

For hand collection, she recommended taping a bottle to a PVC pipe to collect samples from the tile outlet or control structure.

Rinse the sample collection container three times with water at the sampling location before filling the sample bottle to make everything in the bottle represents what’s being collected. Fill the sample bottle, cap it, label it and store on ice or frozen — usually on ice for transport and then store it frozen until it can be analyzed.

If funding is available, auto-samplers provide greater flexibility. There are auto-samplers with programmable electronic operation and memory; a sample collection pump; sample bottles — typical arrangements allow 1 to 24 sample bottles; and a stage recorder — usually, but not always.

The programmable computer tells the sampler to turn on and start pulling the water sample up and into the bottles.

The stage recorder is essentially a flow sensor that is down in the water that’s being sampled and “talks” to the auto-sampler. This allows for collecting flow-based samples.

There are two options for determining a sample interval when using an automated sampler. With the time-based option the auto-sampler can be programmed to collected, for example, every four hours or every four days. You don’t have to actually be there collecting samples every four hours or maybe every four days.

A second option is flow-based programming for the auto-sampler to sample every 2,000 gallons, every 20,000 gallons, or whatever is chosen for the water collection.

Christianson said of the sample frequency recommendations, “if you’re interested in nitrate the general rule of thumb is to collect tile drainage water samples at least once a week. If you’re interested in dissolved phosphorous in addition to nitrate levels, sampling should be done generally every day and one-half to every day.”

She noted another option for measuring nitrogen, phosphorous or other concentrations is using YSI handheld sondes that can give an immediate readout of nitrate concentration.

“There is no sample collection involved when using water quality sondes. Sondes have to be calibrated every couple of hours if they’re being used over the course of a day. They definitely need to be calibrated every time you go out,” she said.

“What I’m personally a little more excited about are real-time deployable nitrate sensors. These real-time sensors are still very expensive, anywhere from $15,000 up to about $40,000 apiece. However, they give us the capability to measure continuous nitrate concentrations in a stream, river or tile drain in real time.

“So, for example, rather than taking a sample today and going back and taking a sample two days later, I can deploy a real-time sensor and have a nitrate concentration every 30 minutes or every 15 minutes between those times.”

Water Flow

Monitoring water flow is also part of tracking nutrient losses into waterways. Christianson gave options for monitoring the flow through underground tile.

“One of the most common flow measurement methods for water which uses a stage discharge relationship or a stage discharge equation. Stage is a different way to say water depth or water surface level. We often talk about ‘flood stage’ for rivers. It means water depth that’s resulting in flooding,” Christianson noted.

Flow rates in tile drainage or in streams or rivers are very difficult to measure, especially if continuous flow rate measurements are needed. However, stage or water depth is relatively much easier to measure even continuously.

A stage-discharge relationship was developed so that whatever the water depth is in a stream or tile drain that water depth can be related to a flow rate.

Relating stage or water depth to flow rate can be determined by using a weir — an obstruction purposely placed in a channel or flow path over which water flows and can be measured.

The depth of water flowing over the weir is correlated with flow rate. A depth monitoring device can be placed behind or upstream of the weir and then you can relate the depth of water flowing over that weir to flow rate through a water sensor that’s continuously logging water depth.

Control Structure

A similar system can be used in a tile drainage system’s control structure.

Control structures are similar to small manholes that allow access to the underground tile. The control structures have stop logs or sometimes called plates, gates or drop shoots that can be deployed inside the controls structure.

“The real purpose of the control structures is for conservation drainage practices such as control drainage, bioreactors or saturated buffers. But control structures also offer an important opportunity to use stop logs to double as a weir for water monitoring,” Christianson said.

The depth of water that’s flowing over the top of the stop logs can be related to a flow rate and is determined by being calibrated in the lab.

Christianson said a “tried and true” method of measuring tile drainage flow rates is by using a calibrated bucket or calibrated container for volume accuracy and a stopwatch. This only provides an accurate snapshot of the flow rate at that specific by collecting the water as it flows out of a tile.

She recommended repeated measurements using this collection method at a given time to increase the precision of the flow rate.

Tom Doran

Field Editor