Instrument : Eddy Correlation Flux Measurement System (ECOR)

Background. The extent of the SGP domain was originally established to take advantage of the local seasonal variation (hot, dry summers to moderately cold, wet winters), the climatic variation across the site (significant north-south gradient of temperature and east-west gradient of humidity and soil moisture), and the proximity of the densely instrumented Oklahoma Mesonet.

The SGP site's Extended Facilities (EFs) were intentionally chosen to provide surface measurements representative of the dominant land-use types common to the geographic area: cropland, pastureland (grassland), and wooded areas. ECOR systems were placed at the north edge of primarily cropland because of the fairly long wind fetch required for the ECOR flux measurements (and prevailing southerly wind directions) and so that they would not interfere with farming practices. At one one site, instruments are located on a platform in a forest canopy. Conversely, the EBBR stations were placed in the middle of grasslands (the species mix of which varies from EF to EF) to take advantage of the fairly short wind fetch required for the EBBR flux measurements.

Variable land-uses throughout the SGP site result in differences in surface flux measurements from one side of the site to the other (both north-south and east-west). Therefore, a straight average of the surface fluxes across the entire SGP site can't properly describe the variation in surface fluxes across the site.

Recommendation. The recommended approach for using the surfaces fluxes from the ECOR and EBBR is to divide the SGP site into northwest, northeast, southwest, and southeast climatological quadrants, determine the land-use percentage of each vegetation surface in each quadrant, and then average the flux for each land-use as measured by the ECOR and EBBR systems within each quadrant. While the average flux in the four quadrants may be quite different, the average of the four quadrant averages may provide a more realistic average for the entire site than a simple straight average of all EF fluxes.

The following are some details concerning the ECOR locations, land-use type (vegetation), wind direction dependencies of the surface fluxes, uncertainties in the measurements, energy balance closure considerations, and potential data problems. Vegetation type and height is recorded every two weeks in the SGP OMIS database, which can be accessed after you obtain an account from SGP Site Operations.

More detail is available in the ECOR Handbook

1. Half-hour Flux Measurements:
   • momentum (M), sensible heat (H), latent heat (LE), carbon dioxide (CO2)
2. Locations (SGP ACRF Extended Facilities):
   • 1, 3, 5, 6, 10, 14, 16, 21, 24
3. Vegetation Viewed for Wind Directions:
   (data for directions not shown is not representative of a single vegetation surface); refer to the SGP Site Operations OMIS vegetation reports for vegetation type and height.)
   • ECOR (north edge of field of vegetation of primary interest):
     • EF1: 0-53, 120-360 wheat or wheat stubble
     • EF3: 0-48 pasture, 132-260 soybeans, wheat
     • EF5: 80-154 sorghum or wheat, 155-260 wheat or wheat stubble
     • EF6: 0-90 grazed pasture, 91-360 alfalfa and brome grass
     • EF10: 0-90, 270-360 grazed, 91-269 grass
     • EF14: 129-265 wheat (all years but 2005 and 2006), corn (2005), soybeans (2006), 352-85 ungrazed grass
     • EF16: 134-269 pasture, 334-360 ungrazed grass
     • EF21: 0-360 mixed deciduous forest (note that for the direction from the tower, 0-30, the data may be suspect)
     • EF24: 80-280 wheat or wheat stubble

4. Uncertainties of Fluxes from Instrument Limitations:
   • H (6%), LE (5%), M (5%), CO2 (4%)
5. Energy Balance Closure:
   • 75% to 90% typical
6. Potential Data Problems Under the Following Conditions:
   1. H underestimated at low temperatures.
   2. Non-closure of energy budget (H+LE) due to:
      1. canopy energy storage
      2. sonic anemometer frequency measurement limitations
      3. non-steady atmospheric conditions
      4. horizontal or vertical advection
   3. Precipitation, fog, and dew (frost) effects on LE, CO2
   4. Large spikes (positive and negative) in CO2 flux when the flux is essentially zero
   5. Sudden shifts in wind direction during a half hour
   6. Wind speed less than 1.5 m/s