TITLE: USDA LTAR Common Experiment measurement: Overland flow
AUTHORS: Kevin J Cole, Anthony R. Buda, Pamela J. Rice, Claire Baffaut

[DESCRIPTION]
Overland flow is water that flows over a land surface (sheet flow) or in rills and gullies (concentrated flow) in response to rainfall or melting snow. Measuring overland flow on croplands and rangelands is important because it represents a key water pathway that controls pollutant losses from agricultural fields. When overland flow is accurately measured along with other water inputs and outputs, scientists can develop budgets describing how water and associated pollutants traverse cropland and rangeland systems. Tracking these budgets over extended time frames (years, decades) helps quantify the beneficial effects of improved agricultural management. In many edge-of-field and small watershed studies, overland flow measurements typically involve control structures such as flumes or weirs. Several hydrology textbooks and agency manuals (USDA, USGS) provide detailed guidance on weir and flume development, installation, and operation. In general, weirs and flumes are located at the outlets of drainage areas where overland flow exits an agricultural field. The devices are sized based on the range of expected flows. In most instances, weirs and flumes have preset equations that calculate discharge (the volume of overland flow leaving a field over a given unit of time) based on measurements of water height, or stage. A wide variety of sensors are useful in measuring stage in weirs and flumes, including floats, bubblers, and pressure transducers. Stage data from these sensors are then recorded at frequent time steps (seconds to hours) to provide continuous estimates of overland flow.

[STEPS]
SECTION: 1.	Data collection
1. Measurement

Collect sensor data every 5 minutes. More frequent measurements may be desirable in areas of very rapid runoff response.

 

SECTION: 1.	Data collection
2. Site Maintenance
SECTION: 1.	Data collection
2.1. Visit weekly to ensure sensors (rain gauge and water level), flumes, and weirs are clear of debris and sediment.
SECTION: 1.	Data collection
2.2. Perform routine water level field verification and recalibration if needed. One method would be to physically block the primary control structure and then pump water above the structure to check stage readings.
SECTION: 1.	Data collection
2.3. Perform general site maintenance to prevent vegetation and debris from impeding site operation.
SECTION: Data processing and quality control
3. Screen the data for out-of-range values due to debris or ice buildup.

 

SECTION: Data file formats and metadata
6. Data format: Site identifier, datetime value, sensor values. Archive the data as raw or production (corrected).

Metadata: Unique site identifiers, spatial coordinates, description of primary control structure, drainage basin area, record of units, indication of accuracy, sensor type, data logger program reference. Record the data adjustments in the metadata.

 

SECTION: Data processing and quality control
4. Apply water level shifts because of field verification results.
SECTION: Data processing and quality control
5. Fill the data gaps and set quality flag. If gaps are detected during runoff events, use field notes and observations to try reconstructing data from manual stage readings or backup stage sensors. Set a quality flag for all data records with at least four levels, such as good, fair, estimated, and suspect.
SECTION: Recommendations for data collection
8. Table 1. Summary of recommendations for measuring overland flow.

 

SECTION: Illustrative media
7.