TITLE: USDA LTAR Common Experiment measurement: Discharge from artificial subsurface  drains
AUTHORS: Gary W. Feyereisen, Kevin W. King, Kevin J. Cole, Mark R. Williams, Rob W. Malone, David D. Bosch, Claire Baffaut

[DESCRIPTION]
Subsurface drain discharge, sometimes simply referred to as drainage, is a process by which water is removed from a soil profile or area (ASABE, 2015). On agricultural lands with intermittent high-water tables due to precipitation or melting snow, water is removed using artificial (generally referred to as tile) drains or ditches to allow timely field operations and protect growing crops from being waterlogged. This discussion is limited to artificial subsurface (tile) drain discharge. In agricultural regions where crop production requires artificial subsurface drainage, drain discharge can represent a substantial portion of the annual water budget. Thus, measuring this water flow is important in understanding the water use of crops and potential impacts on downstream water movement. The measurement of subsurface drain discharge, together with a measurement of sediment and water pollutants, provides an assessment of the losses of these constituents from agriculture to the environment. Tracking them over time quantifies the beneficial effects of improved agricultural practices.

[GUIDELINES]
Site considerations

Agricultural plots or fields with artificial subsurface drains are typically flat or have little slope. In some cases, subsurface drain discharge is measurable using a weir and depth gauge at the tile outlet, where it empties into a drainage ditch. Because of slope limitations, an access pit must often be dug at the edge of the plot or field down to the drain line (Lawlor et al., 2008). Hydrologic isolation of study plots, or groups of plots, needs consideration. If subsurface water flow into a site from the surrounding area is concerning, install a perforated drain around the site perimeter. Keep drainage water from this perimeter drain separate from the plot collection system and route it downstream. Accomplish further isolation of plots within a site by installing a plastic-lined curtain between each group of plots or between plots to reduce lateral flow between plots (Kaspar et al., 2012). Such curtains need installation depths below the drain lines. For additional recommendations, refer to the section titled “Placement and site maintenance” in the USDA LTAR Common Experiment measurement: Best practices for collection, handling, and analyses of water quantity measurements (Baffaut et al., 2024).

[STEPS]
SECTION: Data collection
1. Measurement

Subsurface drain discharge can increase rapidly in response to precipitation events, so the stage is recorded at short time intervals (seconds to hours). 
Most often, discharge measurement devices are connected to an electronic data logger, which records stage in the device as well as additional parameters of interest, e.g., temperature. 
Construct a simple measurement system with a tipping bucket connected to a simple tip recorder with clock-recording capabilities.
See https://transformingdrainage.org/videos/monitoring-tile-drain-flow for a video review of various techniques for measuring subsurface drain discharge.
SECTION: Data collection
2. Site maintenance

Maintain equipment in a pit or field enclosure such that flooding is avoided during large events and rodents are excluded. 
Summer maintenance includes weed control. 
Depending on local conditions, take preventive measures in autumn to avoid freeze damage to equipment and lines.
It is good practice to visit a site after a large precipitation event to check for damage and download data.
SECTION: Data processing and quality control
3. Download data from datalogging equipment at an interval compatible with the importance of the project and the consequences of missing data.
SECTION: Data file formats and metadata
9. Keep data in a .txt, .csv, spreadsheet, or database format. Good practice necessitates maintaining backups physically in a remote location and/or in the cloud. 

Suggested metadata to record, depending on the situation, include: tile diameter, tile slope, drainage area, tile spacing, weir/flume size, stilling pipe size, equipment manufacturers and model numbers, and soil characteristics.
SECTION: Data processing and quality control
6. Calculate the subsurface discharge (the flow volume over a given unit of time) by inserting the stage measurements into a rating equation for the particular weir or flume in use.
SECTION: Data processing and quality control
5. Check stage data for missing values or potentially erroneous readings by visually inspecting time series plots of stage and discharge. 
SECTION: Data processing and quality control
4. Calibrate and check field equipment for accuracy regularly, e.g., from weekly to bi-annually, depending on the site visit interval and the propensity for issues with the equipment and site. Keep detailed notes of instrument calibration for subsequent data correcting. 
SECTION: Data processing and quality control
8. Refer to the section “Quality control” in the USDA LTAR Common Experiment measurement: Best practices for collection, handling, and analyses of water quantity measurements (Baffaut et al., 2024) for additional recommendations.
SECTION: Recommendations for data collection
10. Table 1. Summary of recommendations for measuring subsurface tile drain discharge.

 

SECTION: Data processing and quality control
7. Express discharge in units of volume per time, m3 s-1.