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EDDY CORR. SURFACE FLUX: UK (FIFE)
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Eddy Corr. Surface Flux: UK (FIFE)

Summary:

Surface flux measurements were made at selected sites within the FIFE study area. Each surface flux station was capable of measuring the fluxes of net radiation, sensible heat, and latent heat. The Eddy Correlation Surface Flux Observations (UK) Data Set contains surface flux and micrometeorological measurements collected from one location in the southwest quadrant of the FIFE study area. This location was grazed and had a gentle downhill slope to the southwest. Data were collected daily from May 14 - October 18, 1987, and from July 21 - August 16, 1989.

The output from the Hydra sensors is sampled at 10 Hz and processed in real time to give hourly averages of sensible and latent heat flux and the friction velocity. The hourly mean values of net radiation, temperature, and of vapor pressure, provided in this data are a synthesis of the best measurements available for this site.

The temperature measurement provided here is the preferred value for this site. This temperature was used to calculate the fluxes and some standard deviations. This is necessary because the sonic anemometer has a slightly temperature dependent calibration.

The average soil heat flux measured at 5 mm depth is a weighted average value over three sample positions with dense, medium and sparse vegetation. A vegetation survey was made to assign weights to these three classes at this site. The spatial variability in this measurement at this (over) grazed site is particularly high and the three individual sensors commonly measure soil heat fluxes differing by factors of two or three. Some evidence suggests these data are providing a measurement of this component of the energy budget for this site which is biased low. Presumably this is because the limited number of sensors inadequately samples the points with low canopy density for this sparse crop cover.

Table of Contents:

  1. Data Set Overview
  2. Investigator(s)
  3. Theory of Measurements
  4. Equipment
  5. Data Acquisition Methods
  6. Observations
  7. Data Description
  8. Data Organization
  9. Data Manipulations
  10. Errors
  11. Notes
  12. Application of the Data Set
  13. Future Modifications and Plans
  14. Software
  15. Data Access
  16. Output Products and Availability
  17. References
  18. Glossary of Terms
  19. List of Acronyms
  20. Document Information

1. Data Set Overview:

Data Set Identification:

Eddy Corr. Surface Flux: UK (FIFE)
(Eddy Correlation Surface Flux Observations (UK)).

Data Set Introduction:

The Eddy Correlation Surface Flux Observations (UK) Data Set contains surface flux and micrometeorological measurements collected from one location in the southwest quadrant of the FIFE study area. Data were collected daily from May 14 - October 18, 1987, and from July 21 - August 16, 1989.

Objective/Purpose:

The combined aim of the surface flux group was to use a network of ground based observing systems to measure fluxes of heat, water vapor and radiation at a number of points within the FIFE study area. The specific objectives were:

  1. To use measurements of surface energy and momentum fluxes to understand and model the physical and biological conductance controlling the evaporation from sparse prairie grass.

  2. To determine areal average evaporation by combining remotely sensed and ground-based measurements of surface temperature with surface flux data.

Summary of Parameters:

Net radiation, latent and sensible heat flux, soil heat flux, surface radiant temperature, soil temperature, wind speed, air temperature, vapor pressure, friction velocity, and carbon dioxide flux.

Discussion:

Surface flux measurements were made at selected sites within the FIFE study area. The major data collection effort was conducted in 1987 when 6 sites were equipped with eddy correlation instrumentation operated by several different groups. In 1989, eddy correlation surface flux stations were installed at 3 locations within the FIFE study area. Each surface flux station was capable of measuring the fluxes of net radiation, sensible heat, and latent heat.

The surface flux and micrometeorological measurements available in this data set were collected from one location within the FIFE study area, in two different years, 1987 and 1989. This site was located in the southwest quadrant of the study area (station 26, sitegrid 8739). This location was grazed and had a gentle downhill slope to the southwest. Data were collected daily from May 14 - October 18, 1987, and from July 21 - August 16, 1989.

The output from the Hydra sensors is sampled at 10 Hz and processed in real time to give hourly averages of sensible and latent heat flux and the friction velocity. The hourly mean values of net radiation, NET_RADTN, of temperature AIR_TEMP_MEAN, and of vapor pressure, VAPOR_PRESS_MEAN, provided in this data are a synthesis of the best measurements available for this site.

As in the previous notes, the temperature measurement provided here is again the preferred value for this site and is, in general, that from the (super) PAM station. This temperature was used to calculate the fluxes and some standard deviations. This is necessary because the sonic anemometer has a slightly temperature dependent calibration. Again a recalibration/interpolation procedure was used (see above) to provide a more consistent PAM-based measurement of temperature, with the thermocouple thermometer (which is part of the eddy-correlation device) defined as the inferior interpolation sensor. In practice the accuracy of this device rivals that of the sensor on the (super) PAM.

The mean vertical wind speed, WIND_SPEED_VERT_MEAN, if/when provided is more correctly a measure of the offset drift in the sensor.

The average soil heat flux measured at 5 mm depth, G1, is a weighted average value over three sample positions with dense, medium and sparse vegetation. A vegetation survey was made to assign weights to these three classes at this site. The weights assigned were 0.07, 0.25 and 0.68 respectively. The spatial variability in this measurement at this (over) grazed site is particularly high and the three individual sensors commonly measure soil heat fluxes differing by factors of two or three. Some evidence, e.g. comparison with other FIFE sites and inspection of the local energy budget, suggests these data are providing a measurement of this component of the energy budget for this site which is biased low. Presumably this is because the limited number of sensors inadequately samples the points with low canopy density for this sparse crop cover.

Related Data Sets:

FIS Data Base Table Name:

SURFACE_FLUX_30MIN_DATA.

2. Investigator(s):

Investigator(s) Name and Title:

Dr. J. B. Stewart
British National Space Center

Title of Investigation:

A Study of Areal Average Evapotranspiration by Measuring and Modeling Its Surface Controls.

Contact Information:

Contact 1:
Dr. J.B. Stewart
Institute of Hydrology
United Kingdom
Telephone: (44) 491-38800
Fax: (44) 491-32256
Email: JBS%IBMA.NERC-WALLINGFORD.AC.UK%NSFNET-RELAY.AC.UK@DFTSRV

Contact 2:
Prof. J.W. Shuttleworth
University of Arizona
Tucson, Arizona
Telephone: (602) 621-8787
Fax: (602) 621-1422
Email Bitnet: Shuttle@arizvms
Internet: Shuttle@hwr.arizona.edu

Requested Form of Acknowledgment.

Eddy Correlation Surface Flux Observations (UK) were acquired by W.J. Shuttleworth, K. Blyth, C.J. Holwill and C.R. Lloyd of Institute of Hydrology, Wallingford, United Kingdom. Their contribution of these data is gratefully acknowledged. This research was funded by the Natural Environment Research Council (NERC) and the British National Space Center (BNSC).

3. Theory of Measurements:

Eddy correlation is a well-established technique that has the primary advantage of measuring turbulent diffusive fluxes directly across a near-horizontal plane above the surface. It requires a rigid platform unencumbered by significant aerodynamic obstacles. The fluxes are computed as covariance's of the fluctuations of vertical wind velocity with fluctuations of concentration or mixing ratio at the same point and time. Perhaps the most difficult requirement in using eddy-correlation methods to measure fluxes of trace substances is the need for fast-response sensors. For example, suitable sensors have yet to be fully developed for nonmethane hydrocarbons, organic sulfur compounds, N2O, NH3, and many other species. Substances for which fast-response sensors are currently available include O3, CO, SO2, CH4, sulfate particles, NO, NO2, and CO2, in addition to wind components, temperature, and water vapor. Some recent technological advancements offer promise for other species.

There is a typical variability of 10% to 20% in flux estimates from half-hour to half-hour for sensors not significantly affected by noise. Considering estimates of some sensor requirements to achieve specified levels of accuracy in eddy-correlation flux estimates from towers (neutral atmospheric stability is assumed and height z is between 1 and 50 m), often the most demanding requirement is for fast response. If we assume a perfectly responding vertical wind sensor co-located with a chemical sensor, and as a rough approximation, we assume that the ratio of horizontal wind speed U to height z is 1 m/s per meter, then, the time constant t (c) should be <0.15 s in order to ensure an accuracy of 10% or better. (A sensor with t (c) = 0.1 s will measure 72% of the variance of the input signal, and its phase will be shifted by 30 deg. at 1 Hz.) Alternatively, the calculated amount of chemical flux found by Eddy correlation, in which the chemical sensor has a first-order response time constant of t (c) and delay time of dt, and otherwise ideal characteristics, can be used to correct the measured flux due to the sensor time constant. This procedure becomes increasingly unreliable, however, when attenuation becomes more than about 30%.

A delay time dt, such as might be associated with ducting the air sample from an intake to a sensing chamber, and a separation (d) between the species sensor and the vertical velocity sensor can cause significant flux underestimates. The effects of a delay time can be counteracted by shifting one time series with respect to the other in the analysis procedures. The effects of sensor separation can be minimized by mounting sensors as closely as possible, but this requires that the sensors be small; otherwise aerodynamic flow distortion can result. Small, aerodynamically streamlined sensors also have the advantage of reducing the effects of flow distortion on the measured flux. Poor sensor signal-to-noise ratios also have significant effect on half-hour eddy correlation estimates of vertical flux.

4. Equipment:

Sensor/Instrument Description:

Summary of Eddy Correlation System used by BNSC:

The sensible and latent heat fluxes were measured by the eddy correlation technique using equipment, 'the Hydra', developed at the Institute of Hydrology. Hydra consists of a vertical component sonic anemometer, a fine wire thermocouple thermometer, an infrared absorption hydrometer and a fast response cup anemometer. Surface temperature was measured by an unchopped infrared thermometer.

Collection Environment:

Ground-based.

Source/Platform:

The instrument was mounted on a vertical pole at a height of 2.5 m above the ground.

Source/Platform Mission Objectives:

The Hydra based on the eddy correlation technique, consists of sensors, and real-time and off-line computers. It is a complete instrumentation system. It was designed specifically to provide routine measurements of the surface energy fluxes with the minimum supervision.

Key Variables:

Surface variables measured: latent heat flux, net radiation, sensible heat flux, soil heat flux, soil temperature, total wind speed, fluctuations of air temperature and humidity, surface temperature, and carbon dioxide flux.

Principles of Operation:

Usually time average of correlations between vertical wind speed and the fluctuation quantities are used for parameter estimation.

The Didcot net radiometer used in conjunction with the eddy-correlation device was designed as a robust all-weather device, to provide a daily integral of net radiation for hydrological applications, and is only good to systematic errors of order 5 percent. The preferred net radiation measurement for site ECUK is therefore that provided by the adjacent (super) PAM station, and this is, in general, the value provided in these data. However, this measurement is prone to intermittence and the Didcot instrument was used to interpolate through gaps in the PAM data. Most gaps are less than 3 hours, and for such gaps the measurements immediat Didcot instrument, which was then used to provide interpolated measurements using this calibration. For the occasional gaps greater than three hours the Didcot instrument was again used for interpolation, but in this case with a calibration which was entirely empirical, second-order polynomial fit between its output and that of the PAM net radiometer using data over the whole season (May - October 1987). In this way the net radiation measurement here provided retains a calibration linked at all times to that of the (super) PAM station, but draws on the Didcot radiometer to provide a (recalibrated) interpolation through periods where PAM data are not available.

The humidity measurement provided by the eddy-correlation system is also only approximate, and is made as an RH measurement using a Lee-Dickens meter. Its purpose is merely to allow better calibration of the fast-response infrared humidiometer (used to measure latent-heat flux) whose calibration has a weak dependence on absolute humidity. In this experiment this field calibration was superseded with recalibration based on the preferred humidity measurement at the site, this, in general, being that available from the adjacent (super) PAM. It is this preferred, hourly-average value of humidity which is provided in these data, but in order to mitigate the inconsistency in the PAM data a recalibration/interpolation procedure is used, identical to that described for net radiation in the previous note, but in this case using the inferior Lee-Dickens RH measurement to provide empirical interpolation through gaps in the superior PAM measurement.

Measurement of radiometric surface temperature is made with an eight-headed Everest Interscience Model 4000 Infrared Thermometer (IRT). The data are quoted as the mean and standard deviation of the available measurements; 6 heads during IFC-2, 8 heads during IFC's 1, 3 and 4. An attempt was made to provide calibration for each head at least once during each IFC, but it would be naive to assume an accuracy better than 0.5 degree C in the mean value.

Sensor/Instrument Measurement Geometry:

Hydra was mounted at a height of 2.5 m above the ground with the sensors in north-south plane with sonic anemometer to north of other sensors. The sensor support partly affects the airflow when the wind-direction is between 150 and 210 degrees.

The infrared thermometer had eight sensor heads. The individual heads were mounted up to 8 m apart at a height of about 3.5 m above the ground, looking vertically downwards. Each head had a field of view of 15 degrees, hence sampling an area of 0.66 m .

Manufacturer of Sensor/Instrument:

Hydra 201 was custom built in 1987 by:

Institute of Hydrology.

Sonic Transducers:

Murata Manufacturing Co. Ltd.
6-10, 2-Chome
Tnejin, Nagaokakyo-Shi
Kyoto 617 Japan

Hygrometer Detector:

New England Photoconductors
7 Lawrence St.
Taunton, MA O2780, U.S.A.

Thermocouples:

Lab Facility Ltd
99, Waldegrave Road, Teddington
Middlesex, TW11 8LR, UK

Thermocouple Reference:

Consolidated Ohmic Devices Inc
Junctions 25, Allen St
Bridgeport, CT 06604 U.S.A.

Relative Humidity Sensor:

Lee-Dickens Ltd.
Desborough, Kettering
NORTHANTS, NN14 2QW, UK

Cup Anemometer:

Vector Instruments
113, Marsh Road, Rhyl
Clwyd, LL18 2AB UK

Net Radiometer:

Didcot Instrument Co., Ltd.
Thames View Ind. Park Station Road
Abingdon, Oxon OX14 3LD UK

Data Storage:

GK Instrument Ltd.
Simpson Road, Fenny Stratford
Milton Keynes, Bucks MK1 1LN UK.

Solar Power:

Arco Solar Inc.
PO Box 6032
Camarillo, CA 93010 U.S.A.

Soil Heat Flux Plates:

CW Thornthwaite Assocaites
Route 1, Centerton
Elmer, NJ 08318 U.S.A.

Infrared Thermometer: Model 4000

Everest Interscience Inc.
P.O. BOX 3640
Fullerton, CA 92634-3640, U.S.A.

Calibration:

The sonic anemometer has its electronics calibrated regularly with an electronic phase shifting device. As a sample, its overall cosine response has been checked in a wind tunnel and its general performance checked against a Kaijo-Denki sonic anemometer.

The infrared hygrometer detectors are individually calibrated at known temperatures measured by a precision thermometer in an environment cabinet and simultaneously at known humidities, produced by a water vapor generator, made by The Analytical Development Co Ltd. This maintains a constant partial pressure of water vapor, which is subsequently mixed with dry air in various proportions using orifices. The resultant humidity is checked with a dew point hygrometer.

The thermocouple thermometer reference junctions and electronics are individually set up against a precision thermometer in an environment cabinet, and subsequently calibrated. The thermocouples are inter-changeable.

The relative humidity sensors are individually calibrated against desiccants in an environment cabinet. This method has been assessed as tending to occasionally be irreproducible and so has been changed since 1987. However this has not caused too many problems, as the device is not a primary sensor anyway and can easily be cross checked with wet and dry bulb systems.

The cup anemometers and rotors are supplied with calibrations. They have only minor difference between units. As this is not a primary sensor a single average calibration is used to allow interchangeability.

The net radiometers are supplied with calibrations. Samples have been checked against an array of radiation balance instruments.

Soil heat flux plates are supplied with calibrations. At present they are not sampled by the Hydra but by a separate logging system.

Specifications:

The Hydra is a dry weather device whose performance is reduced while rain drops are falling on the sensors and while the sensors are very wet. Automatic status signals warn of this state of affairs in the data. It is also designed to operate perpendicular to the average streamlines of wind.

The Hydra is expected to produce data when temperatures are between 0-50 degrees C. It will work below freezing but as it is primarily an evaporation measuring device, so very low temperatures are not considered important. It is also calibrated typically from 2-30 [gm][m^-1] absolute humidity. The sonic anemometer is limited to +/-3.5 [m][s^-1] vertical wind speed, and the cup anemometer is limited to 20 [m][s^-1] in its current set up to allow higher precision. Output is only considered accurate to 10%.

Tolerance:

Not available at this revision.

Frequency of Calibration:

The Hydra is calibrated typically once a year if it is not overseas for longer than this. A constant check of its performance is made on a 24 hour basis by checking that the sum of sensible and latent heat fluxes is equivalent to the available energy for this same period. +/-10% is considered good although +/-20% is quite common especially when weather conditions are changeable. If errors become consistent or if a failure occurs a spare set of sensors and electronics are carried with each Hydra which is designed to accept changeover in the field easily.

Other Calibration Information:

There are a large number of calibration coefficients that may have to be used depending on which combination of sensors and boards are in use. These calibration factors are stored in the data processing program. It outputs a relevant set with each analysis. It is not possible to calibrate the Hydra as a whole unit. Its parts have been calibrated and checked against accepted standards and methods. In general terms it has been found to be an acceptably accurate instrument of its type. It was first produced in 1980 after three years of research. The use of the Mark 1 version over the next four years revealed unacceptable errors in arid high radiation conditions. The Mark 2 was produced in 1985 to overcome these problems and has only been changed in minor ways since. A net radiometer calibration was accomplished using a transfer pyheliometer standard on loan from the Solar Energy Research Institute.

5. Data Acquisition Methods:

The Hydra has to be set up so that the sonic anemometer is at right angles to the local stream lines of the flow.

The 8 heads of the IRT system were positioned so that collectively they made measurements over a representative sample of the vegetation. The positions of 8 heads were regularly changed to reduce sampling errors. Similarly the soil heat flux plates were installed just below the soil/ vegetation surface to make representative measurements. Since soil heat flux plates must not be disturbed, it is not possible to reduce sampling errors by altering their positions during the season.

6. Observations:

Data Notes:

Not available.

Field Notes:

Sunday 17th May (day number 137) - Calibration of IRT system checked before being installed in northern half of site.

Friday 29th May (day number 149) - Heavy overnight rain leaving grass very wet, drying throughout morning.

Saturday 30th May (day number 150) - Heavy dew after clear night.

Monday 1st June (day number 152) - Clear night, but no dew noted at 08.30.

Tuesday 2nd June (day number 153) - Moved IRT system to southern half of site.

Saturday 6th June (day number 157) - IRT system dismantled and electric fence removed to allow cattle to graze area where IRT system had been located.

Tuesday 23rd June (day number 174) - Calibration of IRT system checked.

Wednesday 24th June (day number 175) - IRT system installed in northern half of site, with 8 heads sampling different locations from those sampled during first part of IFC1. Only northern half of site was protected by electric fence.

Thursday 25th June (day number 176) - strong winds overnight, knocked over two of the IRT tripods.

Sunday 28th June (day number 179) - Severe storm overnight.

Thermocouple on Hydra bent through 90 degrees by the wind.

Saturday 4th July (day number 185) - IRT system moved to southern half of site, with 8 heads sampling different locations from those sampled during second half of IFC1.

Sunday 5th July (day number 186) - Net radiometer moved to new position outside existing electric fence and electric fence repositioned. However not much visible difference in vegetation height/appearance inside and outside fence.

Wednesday 8th July (day number 198) - Vegetation in soil heat flux area selectively clipped: fine grass shortened, coarse grasses less so and coarse weeds left alone.

Monday 13th July (day number 194) - Calibration of IRT system checked.

Saturday 15th August (day number 227) - IRT system, two heads out of 8 had failed.

Sunday 16th August (day number 228) - IRT system installed in northern half of site.

Saturday 22nd August (day number 234) - IRT system moved to southern half of site.

Tuesday 25th August (day number 237) - Calibration of IRT system checked.

Saturday 3rd October (day number 276) - leveled Hydra, had been leaning to NW.

Sunday 4th October (day number 277) - changed anemometer cups. Changed net radiometer domes and positioned over new representative site.

Monday 5th October (day number 278) - IRT system installed.

Tuesday 6th October (day number 279) - 13.00 to 13.30, interference upwind of Hydra.

Wednesday 7th October (day number 280) - Frost on ground at 8.15.

7. Data Description:

Spatial Characteristics:

The FIFE study area, with areal extent of 15 km by 15 km, is located south of the Tuttle Reservoir and Kansas River, and about 10 km from Manhattan, Kansas, USA. The northwest corner of the area has UTM coordinates of 4,334,000 Northing and 705,000 Easting in UTM Zone 14.

Spatial Coverage:

These data were obtained at the following location within the FIFE study area:

SITEGRID   STN   LATITUDE   LONGITUDE   EASTING   NORTHING   ELEV   SLOPE   ASPECT
(ft)      (deg)
--------   ---   --------   ---------   -------   --------   ----   -----   ------
8739-ECB    26   38 58 31   -96 32 35    712845   4316699     442     1      TOP
926

The infrared thermometer and soil heat flux plates were in a fenced area (ungrazed) 30 m to the south-south-east of the Hydra eddy correlation system. The location of the center of the site was 43, 16, 702 North, 7, 12, 898 East.

Spatial Coverage Map:

Not available.

Spatial Resolution:

These are point data except that the Hydra eddy correlation system effectively samples fluxes during the daytime from an area about 100 m upwind of the sensor (Leclerc and Thurtell, 1989, and Schmed and Oke, 1990).

The 8 heads of infrared thermometer sample a total area of 5 square m.

The three soil heat flux plates sample a total area of 4 x 10E-3 square m.

Projection:

Not available.

Grid Description:

Not available.

Temporal Characteristics:

Temporal Coverage:

The data in this data set were collected between May 14, 1987 and August 16, 1989. During this period there are 185 days of data. Data are available every day from May 14 - October 18, 1987 and from July 21 - August 16, 1989.

Surface flux measurements are available for May 14 - July 20 and August 14 - October 17 in 1987, and from July 21 - August 16 in 1989. Surface temperatures and soil heat fluxes are available for May 28 - June 6, June 24 - July 12, August 17 - 24, and October 6 - 17 in 1987, and for July 21 - August 16 in 1989.

Temporal Coverage Map:

Not available.

Temporal Resolution:

Surface fluxes were hourly averages, while surface and soil temperatures and soil heat fluxes were 30 minute averages. Measurements are made daily during the periods listed above.

Data Characteristics:

The SQL definition for this table is found in the SF_30MIN.TDF file located on the FIFE CD-ROM Volume 1. The following chart lists the four key surface flux variables and only those other variables that are contained in the data set described in this document.


Parameter/Variable Name
Parameter/Variable Description Range Units Source
SITEGRID_ID This is a FIS grid location code. Site grid codes (SSEE-III) give the south (SS) and the east (EE) cell number in a 100 x 100 array of 200 m square cells. The last 3 characters (III) are an instrument identifier.
STATION_ID The station ID designating the location of the observations.
OBS_DATE The date of the observations, in the format (DD-mmm-YY).
OBS_TIME The time that the observation was [GMT] taken, in GMT. The format is HHMM.
LATENT_HEAT_FLUX The latent heat flux, the flux of [Watts] the energy due to the evaporation [meter^-2] of water.
NET_RADTN The net radiation, including both [Watts] downward and upward energy. [meter^-2]
SENSIBLE_HEAT_FLUX The sensible heat flux, the flux [Watts] of the energy due to temperature [meter^-2] differences.
SOIL_HEAT_FLUX The surface soil heat flux, the [Watts] flux of energy into the soil. [meter^-2]
SOIL_HEAT_FLUX_0_TO_5CM The soil heat flux recorded [Watts] somewhere between 0 and 5 cm in [meter^-2] depth. Recorded at 5 mm.
SOIL_HEAT_FLUX_5_TO_10CM The soil heat flux recorded [Watts] somewhere between 5 and 10 cm [meter^-2] in depth. Recorded at 10 cm.
SURF_RADIANT_TEMP The surface radiant temperature. [degrees Celsius]
SURF_RADIANT_TEMP_SDEV The standard deviation of the [degrees surface radiant temperature. Celsius]
SOIL_TEMP_0_TO_25MM The soil temperature recorded [degrees somewhere between 0 and 25 mm in Celsius] depth. Recorded at 5 mm.
SOIL_TEMP_25MM_TO_5CM The soil temperature recorded [degrees somewhere between 25 mm and 5 cm Celsius] in depth. Recorded at 3 cm.
SOIL_TEMP_5_TO_10CM The soil temperature recorded [degrees somewhere between 5 and 10 cm in Celsius] depth. Recorded at 6 cm.
SOIL_TEMP_10_TO_20CM The soil temperature recorded [degrees somewhere between 10 and 20 cm in Celsius] depth. Recorded at 10 cm.
WIND_SPEED_HOR_MEAN The mean horizontal wind speed [meters] in this 30 minutes. [sec^-1]
WIND_SPEED_VERT_MEAN The mean vertical wind speed [meters] in this 30 minutes. [sec^-1]
WIND_SPEED_HOR_SDEV The standard deviation for the [meters] horizontal wind speed. [sec^-1]
WIND_SPEED_VERT_SDEV The standard deviation for the [meters] vertical wind speed. [sec^-1]
AIR_TEMP_MEAN The mean air temperature in [degrees this 30 minutes. Celsius]
AIR_TEMP_MEAN_SDEV The standard deviation for [degrees the mean air temperature. Celsius]
VAPOR_PRESS_MEAN The mean vapor pressure in [kiloPascals] this 30 minutes.
VAPOR_PRESS_SDEV The standard deviation for [kiloPascals] the vapor pressure.
FRICTION_VELOC The friction velocity of the wind. [meters] [sec^-1]
CO2_FLUX The carbon dioxide flux. [mg] [meter^-2] [sec^-1]
FIFE_DATA_CRTFCN_CODE The FIFE Certification Code for * the data, in the format: CGR (Certified by GRoup), CPI (Certified by PI), CPI-??? (CPI - questionable data).
LAST_REVISION_DATE data, in the format (DD-MMM-YY).

Footnotes:

* Valid levels

The primary certification codes are:

EXM Example or Test data (not for release) PRE Preliminary (unchecked, use at your own risk) CPI Checked by Principal Investigator (reviewed for quality) CGR Checked by a group and reconciled (data comparisons and cross checks)

The certification code modifiers are:

PRE-NFP Preliminary - Not for publication, at the request of investigator. CPI-MRG PAMS data which is "merged" from two separate receiving stations to eliminate transmission errors. CPI-??? Investigator thinks data item may be questionable.

** There are several missing value indicators in each column. The values may be positive or negative 9.9, 9.99, 99.99, 999, 999.99, 9999 or 99999.99.

Sample Data Record:

The following sample record contains all the fields in the surface flux record but only those fields that are described here (i.e., reported by J.B. Stewart) contain data.

SITEGRID_ID   STATION_ID   OBS_DATE     OBS_TIME    LATENT_HEAT_FLUX
-----------   ----------   ---------   ----------   ----------------
8739-ECB        926        28-JUL-89      1315           -9999
8739-ECB        926        28-JUL-89      1345           -9999
8739-ECB        926        28-JUL-89      1415           -9999
8739-ECB        926        28-JUL-89      1445           -9999
8739-ECB        926        28-JUL-89      1515            -90
NET_RADTN   SENSIBLE_HEAT_FLUX   SOIL_HEAT_FLUX   DIFFUSE_SOLAR_RADTN_DOWN
----------  ------------------   --------------  ------------------------
139.46           -12
139.46           -12
261.08           -63
261.08           -63
364.64          -107
SOLAR_RADTN_DOWN   SOLAR_RADTN_UP   SOLAR_RADTN_NET   SOLAR_RADTN_DOWN_SDEV
----------------   --------------   ---------------   ---------------------
SOLAR_RADTN_UP_SDEV    PAR_DOWN      PAR_UP     SURF_ALBEDO
-------------------   ----------   ----------   -----------
LONGWAVE_RADTN_DOWN   LONGWAVE_RADTN_UP   LONGWAVE_RADTN_NET
-------------------   -----------------   ------------------
BB_TEMP_LONGWAVE_DOWN   BB_TEMP_LONGWAVE_UP   TOTAL_RADTN_DOWN
---------------------   -------------------   ----------------
TOTAL_RADTN_UP   SOIL_HEAT_FLUX_0_TO_5CM   SOIL_HEAT_FLUX_5_TO_10CM
--------------   -----------------------   ------------------------
-23.4                       6.22
-23.4                       6.22
-54                         1.79
-54                         1.79
-86.5                      -5.31
SOIL_HEAT_FLUX_10_TO_20CM   HEAT_STORAGE   SOIL_WATER_POTNTL_0_TO_5CM
-------------------------   ------------   --------------------------
SOIL_WATER_POTNTL_5_TO_20CM   SURF_RADIANT_TEMP   SURF_RADIANT_TEMP_SDEV
---------------------------   -----------------   ----------------------
28.18                  1.03
28.18                  1.03
34.01                  1.64
34.01                  1.64
39.84                  2.35
SOIL_TEMP_0_TO_25MM   SOIL_TEMP_25MM_TO_5CM   SOIL_TEMP_5_TO_10CM
-------------------   ---------------------   -------------------
25.91                  24.94                  25.58
25.91                  24.94                  25.58
28.86                  25.82                  25.68
28.86                  25.82                  25.68
32.52                  27.41                  26.16
SOIL_TEMP_10_TO_20CM   SOIL_TEMP_20_TO_50CM    RAINFALL    BOWEN_RATIO
--------------------   --------------------   ----------   -----------
WIND_SPEED    WIND_DIR   WIND_SPEED_MIN   WIND_SPEED_MAX   WIND_SPEED_SDEV
----------   ---------   --------------   --------------   ---------------
WIND_DIR_SDEV   TIME_WIND_SPEED_MIN   TIME_WIND_SPEED_MAX
-------------   -------------------   -------------------
TIME_WIND_DIR_MIN   TIME_WIND_DIR_MAX   WIND_SPEED_HOR_MEAN
-----------------   -----------------   -------------------
2.6
2.6
2.8
2.8
3
WIND_SPEED_LAT_MEAN   WIND_SPEED_VERT_MEAN   WIND_SPEED_HOR_SDEV
-------------------   --------------------   -------------------
-.08                    .14
-.08                    .14
-.09                    .17
-.09                    .17
-.13                    .18
WIND_SPEED_LAT_SDEV   WIND_SPEED_VERT_SDEV   AIR_TEMP_LOW   AIR_TEMP_HIGH
-------------------   --------------------   ------------   -------------
.31
.31
.33
.33
.37
AIR_TEMP_OTHER   AIR_TEMP_MEAN   AIR_TEMP_MEAN_SDEV   AIR_TEMP_OTHER_SDEV
--------------   -------------   ------------------   -------------------
22.6            -9999
22.6            -9999
25                   .48
25                   .48
27                   .72
DELTA_TEMP   WET_BULB_TEMP_LOW   WET_BULB_TEMP_HIGH   VAPOR_PRESS_LOW
----------   -----------------   ------------------   ---------------
VAPOR_PRESS_HIGH   VAPOR_PRESS_MEAN   VAPOR_PRESS_SDEV   REL_HUMID_LOW
----------------   ----------------   ----------------   -------------
REL_HUMID_HIGH   REL_HUMID_SDEV   SURF_AIR_PRESS   FRICTION_VELOC
--------------   --------------   --------------   --------------
.22
.22
.26
.26
.25
W_T_MEAN     W_E_MEAN    CO2_CONTENT   OZONE_CONTENT   CO2_CONTENT_SDEV
----------   ----------   -----------   -------------   ----------------
OZONE_CONTENT_SDEV    CO2_FLUX    OZONE_FLUX   FIFE_DATA_CRTFCN_CODE
------------------   ----------   ----------   ---------------------
CPI
CPI
CPI
-.23                           CPI
-.14                           CPI
LAST_REVISION_DATE
------------------
10-DEC-91
10-DEC-91
10-DEC-91
10-DEC-91
10-DEC-91

8. Data Organization:

Data Granularity:

These are point data except that the Hydra eddy correlation system effectively samples fluxes during the daytime from an area about 100 m upwind of the sensor (Leclerc and Thurtell, 1989, and Schmed and Oke, 1990). Surface fluxes were hourly averages, while surface and soil temperatures and soil heat fluxes were 30 minute averages.

A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.

Data Format:

The CD-ROM file format consists of numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe. There are no spaces between the fields. Each file begin with five header records. Header records contain the following information:

Record 1 Name of this file, its table name, number of records in this file, and principal investigator name.

Record 2 Path and filename of the previous data set, and path and filename of the next data set. (Path and filenames for files that contain another set of data taken at the same site on the same day.)

Record 3 Path and filename of the previous site, and path and filename of the next site. (Path and filenames for files of the same data set taken on the same day for the previous and next sites, sequentially numbered by SITEGRID.)

Record 4 Path and filename of the previous date, and path and filename of the next date. (Path and filenames for files of the same data set taken at the same site for the previous and next date.)

Record 5 Column names for the data within the file, delimited by commas.

Record 6 Data records begin.

Each field represents one of the attributes listed in the chart in the Data Characteristics Section and described in detail in the TDF file. These fields are in the same order as in the chart.

9. Data Manipulations:

Formulae:

Derivation Techniques and Algorithms:

Upper heat layer storage was calculated by averaging 15 second samples from three plates at 1 cm.

Data Processing Sequence:

Processing Steps:

Not provided by Principal Investigator.

Processing Changes:

Not provided by Principal Investigator.

Calculations:

Special Corrections/Adjustments:

Not provided by Principal Investigator.

Calculated Variables:

Not available.

Graphs and Plots:

None.

10. Errors:

Sources of Error:

There are two main sources of error - instrumental and sampling errors.

Instrumental errors of Hydra and IRT system have been described in the following papers: Moore 1983, Moore 1986, Shuttleworth 1988, and Wright 1990.

The occurrence of rain drops on the sonic anemometer causes large errors in the measurement of the vertical component of the wind speed. Under these conditions the fluxes should not be used and have been removed from the data set.

Sampling errors primarily effect the sensors which sample a small area, mainly from soil heat flux plates and IRT heads. Sampling is particularly difficult at a heavily grazed site which exhibits a highly variable surface consisting of grasses, forbs and bare soil.

Quality Assessment:

It was recognized early in the study that standardization of "constants" (e.g., physical constants of the air, psychrometric constant, etc.), methods of computation, integration and reporting time, etc. were necessary. These were agreed upon in planning sessions. Preliminary data sets were compared among stations and instruments from different manufacturers for estimating net radiation, soil heat flux, water vapor density, temperature, solar radiation, and wind speed, it was necessary to have confidence that differences in observations were due to site differences and not due to instrumentation.

Data Validation by Source:

The following measures were taken to validate the data:

  1. Determine the recovery ratio using daily totals. The recovery ratio is the sum of the sensible and latent heat fluxes divided by the sum of net radiation and soil heat flux. Values were found to vary between 0.8 and 1.1.

  2. The air temperature and humidity were checked against the equivalent data from the PAM at the same site.

  3. The net radiation was compared to the net radiation from the PAM at the same site. The diurnal variation of the variables was checked for consistency and inter-day comparisons were carried out.

The Hydrological Sciences Branch at NASA Goddard Space Flight Center was also given the responsibility to compare flux data from all flux stations. This served two purposes: 1) as a data quality check, and 2) a preliminary analysis of site differences.

Confidence Level/Accuracy Judgment:

The following are the best estimates of accuracy for a single flux estimate:

None of these estimates addresses the variability of flux estimates from site to site.

Measurement Error for Parameters:

Checks of the calibration of the individual heads of IRT system showed that the differences in temperature were less than 0.3 degree C, which was considered to be the accuracy of the calibration system. Consequently, none of the calibrations were altered and the measurements of surface temperature were considered to be accurate to 0.3 degrees C, if the emissivity of the surface is unity. If the emissivity was 0.9 then the true surface temperature would be 2.3 degree C higher than the measured temperature of 30 degree C, if there was no longwave radiation emitted from the atmosphere to be reflected at the surface. Using typical values of the longwave radiation emitted by the atmosphere indicates that the error in the measured surface temperature will be less than 1 degree C.

No quantitative error estimates can be given for the surface fluxes determined by the Hydra.

Additional Quality Assessments:

Much of the data has been subjected to further analysis, for example, calculation of surface and aerodynamic conductance, comparisons between radioactive and aerodynamic surface temperatures. These derived quantities have been used in modeling studies, which have shown that generally the data are of high quality, with very few outliers.

Several of the key surface flux parameters have undergone extensive intercomparison and examination for spikes in the data. Details of these analyses are described in the Surface Flux Baseline 1992 document on FIFE CD-ROM Volume 1.

FIS staff applied a general Quality Assessment (QA) procedure to some of the fields in this data set to identify inconsistencies and problems for potential users. As a general procedure, the FIS QA consisted of examining the maximum, minimum, average, and standard deviation for numerical field. An attempt was made to find an explanation for unexpected high or low values, values outside of the normal physical range for a variable, or standard deviations that appeared inconsistent with the mean. In some cases, histograms were examined to determine whether outliers were consistent with the shape of the data distribution.

The discrepancies, which were identified, are reported as problems in the Known Problems with the Data Section.

Data Verification by Data Center:

The data verification performed by the ORNL DAAC deals with the quality of the data format, media, and readability. The ORNL DAAC does not make an assessment of the quality of the data itself except during the course of performing other QA procedures as described below.

The FIFE data were transferred to the ORNL DAAC via CD-ROM. These CD-ROMs are distributed by the ORNL DAAC unmodified as a set or in individual volumes, as requested. In addition, the DAAC has incorporated each of the 98 FIFE tabular datasets from the CD-ROMs into its online data holdings. Incorporation of these data involved the following steps:

Each distinct type of data (i.e. "data set" on the CD-ROM), is accompanied by a documentation file (i.e., .doc file) and a data format/structure definition file (i.e., .tdf file). The data format files on the CD-ROM are Oracle SQL commands (e.g., "create table") that can be used to set up a relational database table structure. This file provides column/variable names, character/numeric type, length, and format, and labels/comments. These SQL commands were converted to SAS code and were used to create SAS data sets and subsequently to input data files directly from the CD-ROM into a SAS dataset. During this process, file names and directory paths were captured and metadata was extracted to the extent possible electronically. No files were found to be corrupted or unreadable during the conversion process.

Additional Quality Assurance procedures were performed as follows:

As errors are discovered in the online tabular data by investigators, users, or DAAC staff, corrections are made in cooperation with the principal investigators. These corrections are then distributed to users. CD-ROM data are corrected when re-mastering occurs for replenishment of CD-ROM stock.

11. Notes:

Limitations of the Data:

Not available.

Known Problems with the Data:

See the Field Notes Section.

Several different missing values are used within each column. There can be positive or negative 9.9, 9.99, 99.99, 999.99, 9999 or 99999.99.

Unexpected negative CO2 flux values of -10 are reported for station 926 (SITEGRID_ID = 8739) on the following days:

OBS_DATE    STATION_ID   CO2_FLUX
---------   ----------   ----------
24-JUL-89      926          -10
25-JUL-89      926          -10
27-JUL-89      926          -10
28-JUL-89      926          -10
02-AUG-89      926          -10
04-AUG-89      926          -10
07-AUG-89      926          -10
08-AUG-89      926          -10
09-AUG-89      926          -10
10-AUG-89      926          -10
11-AUG-89      926          -10
12-AUG-89      926          -10

The missing value indicators in the following fields may have been inadvertently converted to 1000. Use these data with caution.

         Name                      Name
------------------------   ---------------------
DIFFUSE_SOLAR_RADTN_DOWN   TOTAL_RADTN_DOWN
SOLAR_RADTN_DOWN           TOTAL_RADTN_UP
SOLAR_RADTN_UP             HEAT_STORAGE
SOLAR_RADTN_NET            RAINFALL
SOLAR_RADTN_DOWN_SDEV      WIND_DIR_MIN
SOLAR_RADTN_UP_SDEV        WIND_DIR_MAX
LONGWAVE_RADTN_DOWN        CO2_CONTENT
LONGWAVE_RADTN_UP          O3_CONTENT
LONGWAVE_RADTN_NET         CO2_STDEV
BB_TEMP_LONGWAVE_DOWN      O3_STDEV
BB_TEMP_LONGWAVE_UP

Usage Guidance:

Caution should be exercised when using flux data for several hours surrounding dawn and dusk since these are periods of unsteady conditions. In addition, nighttime data should be closely scrutinized.

Any Other Relevant Information about the Study:

None.

12. Application of the Data Set:

The measurements of surface energy and momentum fluxes in this data set can be used to understand and model the physical and biological conductance controlling the evaporation from sparse prairie grass.

13. Future Modifications and Plans:

The FIFE field campaigns were held in 1987 and 1989 and there are no plans for new data collection. Field work continues near the FIFE site at the Long-Term Ecological Research (LTER) Network Konza research site (i.e., LTER continues to monitor the site). The FIFE investigators are continuing to analyze and model the data from the field campaigns to produce new data products.

14. Software:

Software to access the data set is available on the all volumes of the FIFE CD-ROM set. For a detailed description of the available software see the Software Description Document.

15. Data Access:

Contact Information:

ORNL DAAC User Services
Oak Ridge National Laboratory

Telephone: (865) 241-3952
FAX: (865) 574-4665

Email: ornldaac@ornl.gov

Data Center Identification:

ORNL Distributed Active Archive Center
Oak Ridge National Laboratory
USA

Telephone: (865) 241-3952
FAX: (865) 574-4665

Email: ornldaac@ornl.gov

Procedures for Obtaining Data:

Users may place requests by telephone, electronic mail, or FAX. Data is also available via the World Wide Web at http://daac.ornl.gov.

Data Center Status/Plans:

FIFE data are available from the ORNL DAAC. Please contact the ORNL DAAC User Services Office for the most current information about these data.

16. Output Products and Availability:

Eddy Correlation Surface Flux Observations (UK) data are available on FIFE CD-ROM Volume 1. The CD-ROM filename is as follows: \DATA\SUR_FLUX\30_MIN\GRIDxxxx\YyyMmm\ydddgrid.ECV or \DATA\SUR_FLUX\30_MIN\GRIDxxxx\Yyyyy\ydddgrid.ECV

Where xxxx is the four digit code for the location within the FIFE site grid, yy is the last two digits of the year (e.g., Y87 = 1987), yyyy is the four digits for the century and year (e.g., Y1987 = 1987), mm is the month of the year (e.g., M12 = December), and ddd is the day of the year, (e.g., 061 = sixty-first day in the year). Note: capital letters indicate fixed values that appear on the CD-ROM exactly as shown here, lower case indicates characters (values) that change for each path and file.

The format used for the filenames is: ydddgrid.sfx, where grid is the four-number code for the location within the FIFE site grid, y is the last digit of the year (e.g., 7 = 1987, and 9 = 1989), and ddd is the day of the year. The filename extension (.sfx), identifies the data set content for the file (see the Data Characteristics Section) and is equal to .ECV for this data set.

17. References:

Satellite/Instrument/Data Processing Documentation.

Everest Interscience Inc. Model 4000 Infrared Thermometer Operating Manual. Field, R.T., L.J. Fritschen, E.T. Kanemasu, W.P. Kustas, E.A. Smith, J.B. Stewart, and S.B. Verma. 1992. Calibration, comparison and correction of net radiometer instruments used during FIFE. J. Geophys. Res. 97:18,681-18,695.

Kalma, J.D., H. Alksins, and G.P. Laughlin. 1988. Calibration of small infrared temperature transducers. Agric. For. Meteorol. 43:83-98.

Leclerc, M.Y. and G.W. Thurtell. 1989. Foolprint prediction of scale fluxes using a Marksvian Analysis. Boundary-Layer Meteorology. 52:247-258.

Schmid, H.P. and T.R. Oke. 1990. A model to estimate the source area contributing to surface layer turbulance at a point over patchy terrain. Quart. J. Royal Meteorol. Soc. 116:965-988.

Shuttleworth, W.J., J.H.C. Gash, C.R. Lloyd, D.D. McNeil, C.J. Moore, and J.S. Wallace. 1988. An integrated micrometeorological system for evaporation measurement. Agric. and For. Meteorol. 43:295-317.

Journal Articles and Study Reports.

Lloyd, C.R., W.J. Shuttleworth, J.H.C. Gash, and M. Turner. 1984. A microprocessor system for eddy correlation. Agric. and Forest Meteorol. 33:67-80.

Moore, C.J. 1983. On the calibration and temperature behaviour of single-beam infrared hygrometers. Boundary-Layer Meteorol. 25:245-269.

Moore, C.J. 1986. Frequency response corrections for eddy correlation systems. Boundary-Layer Meteorol. 37:17-45.

Shuttleworth, W.J., D.D. McNeil, and C.J. Moore. 1982. A switched continuous wave sonic anemometer for measuring surface heat fluxes. Boundary-Layer Meteorol. 23:425-448.

Shuttleworth, W.J. 1988. Corrections for the effect of background concentration change and sensor drift in real-time eddy correlation systems. Boundary-Layer Meteorol. 42:167-188.

Shuttleworth, W.J., J.H.C. Gash, C.R. Lloyd, D.D. McNeil, C.J. Moore, and J.S. Wallace. 1988. An integrated micrometeorological system for evaporation measurement. Agric. and Forest Meteorol. 43:295-317.

Stewart, J.B., W.J. Shuttleworth, K. Blyth, and C.R. Lloyd. 1989. FIFE: A comparison between aerodynamic surface temperature and radiometric surface temperature over sparse prairie grass. Proc. 19th Agric. and For. Meteor. and 9th Conf. Biometeor. and Aerobiology, March 7-10, 1989. Charleston, S. Carolina. Pub. Amer. Meteor. Soc. pp. 144-146.

Stewart, J.B., and S.B. Verma. 1992. Comparisons of surface fluxes and conductances at two contrasting sites within the FIFE area. J. Geophys. Res. 97:18,623-18,628.

Archive/DBMS Usage Documentation.

Contact the EOS Distributed Active Archive Center (DAAC) at Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee (see the Data Center Identification Section). Documentation about using the archive and/or online access to the data at the ORNL DAAC is not available at this revision.

18. Glossary of Terms:

A general glossary for the DAAC is located at Glossary.

19. List of Acronyms:

AMS Automatic Meteorological Station BNSC British National Space Center CD-ROM Compact Disk (optical), Read-Only Memory DAAC Distributive Active Archive Center EOSDIS Earth Observation System and Information System FIFE First ISLSCP Field Experiment FIS FIFE Information System IRT Infrared Thermometer ISLSCP International Satellite Land Surface Climatology Project LAI Leaf area index NERC Natural Environment Research Council (UK) ORNL Oak Ridge National Laboratory PAMS Portable Automatic Mesonet REBS Radiation and Energy Balance Systems SAMS Super AMS SQL Structured Query Language URL Uniform Resource Locator UTM Universal Transverse Mercator

A general list of acronyms for the DAAC is available at Acronyms.

20. Document Information:

April 28, 1994 (citation revised on October 15, 2002).

This document has been reviewed by the FIFE Information Scientist to eliminate technical and editorial inaccuracies. Previous versions of this document have been reviewed by the Principal Investigator, the person who transmitted the data to FIS, a FIS staff member, or a FIFE scientist generally familiar with the data. It is believed that the document accurately describes the data as collected and as archived on the FIFE CD-ROM series.

Document Review Date:

October 24, 1996.

Document ID:

ORNL-FIFE_SF30_ECB.

Citation:

Cite this data set as follows:

Stewart, J. B. 1994. Eddy Corr[elation]. Surface Flux: UK (FIFE). Data set. Available on-line [http://www.daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A. doi:10.3334/ORNLDAAC/32. Also published in D. E. Strebel, D. R. Landis, K. F. Huemmrich, and B. W. Meeson (eds.), Collected Data of the First ISLSCP Field Experiment, Vol. 1: Surface Observations and Non-Image Data Sets. CD-ROM. National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland, U.S.A. (available from http://www.daac.ornl.gov).

Document Curator:

DAAC Staff

Document URL:

http://daac.ornl.gov