What’s in that silver box?

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Eric Cooley eric.cooley@wisc.edu

If you have ever driven by a Discovery Farms water quality monitoring site, you have likely seen our signature silver box that often sticks out like a sore thumb on the landscape. Whether it is an edge-of-field, tile drainage or stream monitoring site, the equipment in the silver box is all the same. The Wisconsin Discovery Farms Program collaborates with the United States Geological Survey (USGS) to use these silver boxes to evaluate the soil and nutrient loss from diverse landscapes and farming systems throughout Wisconsin.

There are three main components to the monitoring equipment inside the silver box. The first is the water level sensing equipment that determines the water flow at the site we are monitoring. This device uses pressure detecting technology to determine water depth that is similar to the pressure you feel on your eardrum as you dive deeper into water. Air is continuously passed across one side of a diaphragm (like your eardrum) and increasing pressure is felt on the opposite side of the diaphragm as water depth increases. The pressure sensor that is used can determine water depth to an accuracy of 0.01 foot. If we know the depth of water that is flowing through a device of a given geometry, we can use rating curves to calculate the flow rate and volume of water.

The next component is the water sampler to evaluate the water quality.  A water sample is automatically triggered when water is flowing at the site. A rotating arm within the sampler turns to fill different bottles within the sampler so that different portions of the storm or multiple flow events can be sampled.

When water is running off a landscape, the flowrate can strongly influence the amount of soil and nutrients that are entrained in the water. During peak flow, the water is more turbulent and soil remains suspended in the water as compared to lower flowrates. The water sampler collects multiple samples throughout a single flow event to represent the differing flowrates during the event. This allows us to most accurately determine soil and nutrients that are moving throughout the entire flow event.

Once water samples are collected, a refrigeration unit on the sampler is activated to immediately cool the samples to just above freezing. Microbial activity can potentially change the speciation of nutrients that are being analyzed, therefore samples are refrigerated so that the water chemistry doesn’t change from the time that the samples are collected until the time they are analyzed at the lab. When water samples are collected at a site, they are covered in ice during transport to the laboratory and refrigerated until being analyzed.

The final component to monitoring equipment is the datalogger that operates the site, stores data and communicates information back and forth to the site. The data logger is a microcomputer that is programmed to measure flowrate and operate the water sampler and any other meteorological or soil condition measurement devices. Additionally, the datalogger stores collected data until it is retrieved from the site.

The data logger also has telemetry (cellular phone transmission) to allow for continuous communication with the site to both retrieve data that is being collected and change operational functions of the site on-the-fly. USGS real-time data for Discovery Farms current sites can be viewed here: http://www.uwdiscoveryfarms.org/usgs-real-time-data

Next time you drive by one of Discovery Farm’s mystical silver boxes in a field, now you can say, “I know what is in that silver box!” §

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