Edge-of-field monitoring

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Edge-of-field (EOF) monitoring sites are installed at the edge of agricultural fields, either on the field surface or using subsurface tiles, where runoff can be intercepted and channeled through monitoring equipment before it enters the natural stream system. EOF sites monitor both runoff quantity and quality.

EOF monitoring often uses a nested-basin study design where both individual field runoff and larger subbasin streams are monitored. By monitoring at multiple scales, both the immediate effect of a conservation practice on a single field and the cumulative effect on the larger watershed can be evaluated. Year-round monitoring utilizes natural rainfall/snowmelt conditions to measure not only the quantity of nutrient loss but also the timing. This information allows event-timing-based options for strategic nutrient application and adaptive implementation to be evaluated.

WHY DO EOF MONITORING?

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Historically, monitoring for conservation-practice assessment has been done at the watershed scale, but those evaluations are often complicated by land-use variability and in-stream processes, which often require a long study duration. By monitoring runoff from an individual field, we can evaluate the direct impacts of agricultural activities and the effectiveness of conservation practices. The benefits of EOF monitoring include:

• Direct relations of nutrient sources and transport
• Quantification of the effects of field activities and conservation practice implementation
• Improved nutrient loss and conservation-practice impact modeling
• Shorter study duration due to field-focused monitoring scale
• Increased producer involvement
• Provides information for management decisions, adaptive management, and outreach

WHAT DOES EOF MEASURE?

Data collected at edge-of-field sites includes:

• Runoff quantity and flow - determines how much water passes by each site
• Meteorological data - precipitation, air temperature, relative humidity, solar radiation, soil temperature, and soil moisture
• In-situ water quality - temperature, pH, specific conductance, dissolved oxygen, turbidity, nitrate and phosphate (via sensors)
• Sediment and nutrients - samples are analyzed for suspended sediment, chloride, nitrate plus nitrite, ammonium, total Kjeldahl nitrogen, orthophosphate, and total phosphorus

Loads and yields are calculated by combining sample concentrations and runoff quantity to determine the amount of each consituent leaving the field. This is critical for evaluating the effectiveness of conservation practices.

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SURFACE SITES

Surface EOF monitoring sites are located in areas where runoff exits agricultural fields or in nearby streams where the fields directly drain. A typical EOF surface site consists of:

• Wingwalls - plywood or steel sheet piling combined with earthen berms are used to direct runoff
• H flume - runoff control structure
• Water quantity monitoring equipment - records water levels
• Refrigerated water-quality sampler - collects and stores water samples during runoff events
• Two-way communication and datalogger capabilities - provides direct control over the monitoring equipment for flexible, year-round at varying scales, locations, setting, and conditions
• Power - power is needed for year-round operation, including sample refrigeration in the summer and heat tape in the winter. Power is provided by A/C, if available, or through the use of solar equipment at remote sites
• Digital camera - remotely captures field conditions, records staff readings to verify flow data, and communicates site conditions to reduce personnel time and improve data accuracy

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SUBSURFACE TILE SITES

Subsurface tile EOF monitoring sites are typically located at drainage tile outlets, in a ditch or stream, or in-line with the subsurface tile system. A typical EOF subsurface tile site consists of the same equipment as the surface sites with some differences due to the requirements of the subsurface location:

• Flow control structure - a modified commercially available product to include a sharp-crested V-notch weir to measure subsurface tile runoff. Water depth measurements and water-quality samples are also taken inside the flow control structure
• Subsurface tile - artificial drainage placed within the field 3 to 4 feet below the surface
• Velocity meter - instrument used to measure velocity in the subsurface tile when backwater affects the flow control structure

FLEXIBLE MONITORING STRATEGIES

Like any monitoring effort, EOF has its challenges, and the USGS collaborates with producers and partners to develop rigorous monitoring strategies that are both flexible and adaptable. Each site brings its own complexities and opportunities, such as:

• Sites are usually located on private farms and are often inaccessible by road, so equipment must be easily transportable and minimally invasive while meeting unique project goals, landowner concerns, and limited budgets.
• EOF monitoring occurs year-round under all weather conditions, so the ability to adjust to changing runoff conditions and weather while minimizing the need for on-site personnel trips is key.
• Utilizing multiple monitoring designs (before/after; paired basin; surface/subsurface)
• Alternative monitoring methods (such as the use of real-time water-quality sensors, different sample collection strategies, or a depth-integrated sampling arm) can potentially improve data quality or reduce costs.

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