Champaign, Illinois.

During the summer of 1996, a second flux/meteorological system was installed just south of Champaign, Illinois on a farm owned and operated by John Reifsteck (Word Wide Web site is http://W3.ag.uiuc.edu/INFOAG/cyberfarm/reifstck/index/html). This farm (40º 00.366' N, 88º 17.512' W) is within the GEWEX/GCIP large scale area north central (NC), and just on the northeastern edge of the LSA southeast (SE) region. The site characteristics are typical of those found throughout the midwestern U.S., with most of the land in agricultural production. The soil (silt loam) has a bulk density of 1.5 g/cm3 with sand, silt, and clay fractions of 5%, 70%, and 25%, respectively. The farm has been in continuous no-till since 1986, alternating each year between corn and soybeans. In 1996, the field consisted of soybeans, corn in 1997 and again soybeans in 1998. Corn was planted in 1999 and soybeans will be grown in the year 2000. From measured leaf area index data and measures of visible reflectance, files containing leaf area index information for each day in the growing season have been generated for 1997, 1998 and 1999. The file names are BV97_LAI.prn, BV98_LAI.prn, BV99_LAI.prn

Additional information

Year	crop	Planting Date	Harvest Date

1996	Soybeans	--	10/25

1997	corn	04/18	10/19

1998	Soybeans	06/01	10/10

1999 Corn	05/10	10/23

The site is within 5 km of the NOAA's SURFRAD site, which provides measurements of direct and diffuse shortwave radiation and the incoming and outgoing longwave components. The flux/meteorological system is nearly identical to the Little Washita System with the exception that the instrumentation and solar panels are mounted on a scaffolding tower.

Site: Champaign, Illinois (near Bondville)

site	latitude 40 deg 00.366 min N

longitude 88 deg 22.373 min W

elevation approx 300 m

2. Methodology

Traditionally, the use of the eddy correlation method (Businger, 1987; Baldocchi et al., 1988) has been constrained to mainly short term intensive field campaigns. Improvements in instrument design, stability, and power requirements over the past decade now allow for nearly continuous measurements of sensible and latent energy fluxes using the eddy covariance technique. Using this technique, the average vertical turbulent eddy fluxes of sensible and latent heat (and other scalars) are determined as

where is the vertical velocity component of the wind vector, and is the scalar of interest (e.g. water vapor concentration). Here, the bracketed quantities denote an average or "mean" that is subtracted from the instantaneous values to obtain the fluctuating component. Average vertical turbulent fluxes ( ) are computed in real time using a digital recursive filter (200 s time constant) for the determination of a running "mean" from which the instantaneous values are subtracted. An averaging period of 30 minutes (denoted by the overbar) is used and is considered large enough for statistical confidence in the covariance quantity but is short enough to resolve the structure of the diurnal cycle.

Wind vector measurements made at experimental sites that are not perfectly flat can result in non- zero vertical wind velocities measured from the "vertical " coordinate system of the measurement platform. At the end of an average period, vertical turbulent fluxes perpendicular to the mean horizontal wind (which generally follows the contour of the land surface) are obtained by mathematically rotating the coordinate system of the measurement frame of reference (sonic anemometer) to obtain a zero mean vertical and transverse velocity (). Details of this procedure described by Wesely (1969) are outlined by Businger (1986) and Baldocchi et al., (1988).

The three components of the wind vector are determined with a sonic anemometer ( R2, Gill Instruments, Hampshire, England). The stable long-term operational characteristics of this instrument and its ability to continue measurements during cold weather and light rain events (Yellard et al., 1994), as well as its low power consumption, were important considerations in the selection of this anemometer. The symmetric head design of the R2 with its slender support structure produces little flow distortion (Grelle and Lindroth, 1994) and is well suited for measurements in the the relatively flat and open locations of the Little Washita Watershed and Champaign, Illinois sites Fast response water vapor and CO2 concentration measurement are made with an open-path, fast response infrared gas analyzer (Auble and Meyers, 1992). This sensor was used extensively for flux measurements during recent ARM (Doran et al., 1992) and BOREAS (Baldocchi et al., 1997) experiments. In a recent evaluation of open and closed path sensors for water vapor and CO2 concentrations, Leuning and Judd (1996) found that for the measurement of CO2, this sensor displayed minimal cross sensitivity to water vapor (see Leuning and Moncrieff, 1990).

Standard meteorological data collected at each site. The following are the variables, in order as they appear in the file.

jday	Julian Day

time	LST, half hour ending

w_speed propeller anemometer (10 meters, Bondville ISIS)

w_dir	wind direction (10 meters, Bondville ISIS)

Ta	air temperature (C), at 3 m

RH	relative humidity at 3 m

Pres	surface pressure in mb

Rg	incoming global radiation (W/m2)

Par_in	incoming visible radiation (0.4-0.7 um) in uE/m2/s

Par_out	outgoing or reflected visible light

Rnet	net radiation (W/m2)

GHF	soil or ground heat flux (W/m2)

rain	total rain for half hour (inches)

wet	wetness sensor (in voltage with higher values indicating wetness)

IRT	surface or skin temp (C)

2_cm	soil temp at 2 cm (C)

4_cm	soil tmep at 4 cm

8_cm	soil temp at 8 cm

16_cm	soil temp at 16 cm

32_cm	soil temp at 32 cm

64_cm	soil temp at 64 cm

u_bar	average wind vector speed (m/s)

u'w'	kinematic shear stress (m2/s2)

u'2	streamwise velocity variance (m2/s2)

v'2	crosswind velocity variance (m2/s2)

w'2	vertical velocity variance (m2/s2)

H	sensible heat flux (W/m2)

LE	latent energy flux (W/m2)

CO2	CO2 flux (mg CO2/m2/s)

LW_in	downwelling longwave from sky (W/m2)

sm_5	soil volumetric water content at 5 cm zone (after November 19 1997)

sm_20	soil volumetric water content at 20 cm zone (after November 19 1997)

sm_60	soil volumetric water content at 60 cm zone (after November 19 1997)

The eddy covariance sensors are located at 6 m AGL. The bulk density of

the soil is 1.4 gm/cm3. The site is currently in corn stubble (like it would look

after combining). I am currently working up more soils data, and will have that

information available shortly.

The standard meteorological sensors (Table 1) are sampled every 2 s with a datalogger and multiplexor (CR21x, Campbell Scientific, Inc.) and averages are computed every 30 minutes, coincident with the eddy covariance data.

Data Acquisition

A laptop computer is configured in a mulitasking mode to simultaneously perform three operations. For the first and foremost task, measurements of the three componenets of the wind vector along with the speed of sound (from which the virtual temperature can be derived) are digitally sent from the sonic anemometer (which includes the digitized H2O and CO2 signals from the IRGA) to the laptop computer, which is housed in a small environmental enclosure. In the second task, the computer retrieves the standard meteorological data from the CR21X datalogger every 30 minutes and appends the data to an existing file. After midnight, the covariance data and standard meteorological data are copied to separate files with a name, year and calendar day header. The computer is equipped with a modem and cellular phone in order to retrieve the data and conduct occasional system checks. On average, data are retrieved from the laptop computers about once every two days.

Power Operation and Management

To avoid the constraints of using standard line power, the entire flux/meteorological system is designed to operate on 12 volts DC making it truly remote and portable. The entire flux system, including all the instruments and data logging devices are powered by nine deep-cycle 12 volts DC batteries that are charged daily with solar panels ( M75, Siemans, Inc.). Each solar panel is capable of producing 3 amperes at 12 volts in full sunlight. The batteries are enclosed in an insulated container that is 2/3 submerged into the ground near the base of the tower. Ten solar panels are required at the Illinois site while eight are used at the Little Washita location. Each charging system is controlled with a 30 ampere regulator. The regulator is equipped with a low voltage disconnect option that disconnects electronic devices with the largest power consumption when the battery voltage falls below 11.5 VDC. Table (2) lists the major instruments and corresponding power requirements. The entire system continuously draws about 3 amperes at 12 VDC. The 21X datalogger and cellular phone are connected directly to the batteries and operate continuously since they have low power demands. When the regulator disconnects the computer because of low voltage (i.e. long periods of cloudy weather), only standard meteorological data are logged. After the batteries are charged to 12.5 VDC, the computer, IRGA and sonic are reconnected and logging of the flux data resumes. As will be discussed later, this happens infrequently and only during the winter months when cloudy conditions persist during the relatively short daylight hours.

REFERENCES

Auble, D. L, T. P. Meyers, 1992. An open path, fast response infrared absorption gas analyzer for H2O and CO2, Boundary-Layer Meteorology, 59, 243-256.

Atlas, R., N. Wolfson and J. Terry, 1993. The effect of SST and soil moisture anomalies on GLA model simulations of the 1988 U.S. summer drought, J. of Climate, 2034-2048.

Baldocchi, D. D. and T. P. Meyers, 1991. Trace gas exchange at the floor of a deciduous forest: I Evaporation and CO2 efflux, Journal of Geophysical Research, Atmospheres, 96, 7271-7285.

Baldocchi, D. D., B. B. Hicks and T. P. Meyers, 1988: Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology 69:1331-1340.

Chen. T. H., A. Henderson-Sellers, and A. J. Pitman, 1994. Recent progress in the Project for

Intercomparison of Land Surface Parameterization Schemes (PLIPS), GEWEX News, 4, 8-9.

Dolske, D. A. and D. F. Gatz, 1985. A field intercomparison of methods for the measurement of article and gas dry deposition, J. Geophysical Research, 90, 2076-2084.

Doran, J. C., F. J. Barnes, R. L. Coulter, T. L. Crawford, D. D. Baldocchi, L Ballick, D. R. Cook, D. Cooper, R. J. Dobosy, W. A. Dugas, L. Fritschen, R. L. Hart, L Hipps, J. M. Hubbe, W. Gao,	R. Hicks, R. R. Kirkham, K. E. Kunkel, T. J. Martin, T. P. Meyers, W. Porch, J. D.	Shannon, W. J. Shaw, E. Swiatek, and C. D. Whiteman, 1992. The Boardman Regional	Flux Experiment, Bulletin of the American Meteorological Society, 73, 1785-1795.

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layer treatments - a review, J. of Climate, 6, 419-449.

Grelle, A. and A. Lindroth, 1994. Flow distortion by a Solent Sonic anemometer: wind tunnel calibration and its assessment for flux measurements over forest and field, Journal of Atmospheric and Oceanic Technology, 11, 1529-1542.

Henderson-Sellers, A., 1993. A factorial assessment of the sensitivity of the BATS land-surface	parameterization scheme, J. of Climate, 6, 227-247.

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Leuning, R. and J. Moncrieff, 1990. Eddy covariance CO2 flux measurements using openpath and closed-path CO2 analyzers-corrections for analyzer water vapor sensitivity and damping of fluctuations in air sampling tubes, Boundary-Layer Meteorology, 53, 63-76.

Leuning, R. and M. J. Judd, 1996. The relative merits of open- and closed-path analyzers for the measurement of eddy fluxes, Global Change Biology, 2, 241-253.

Meehl, G. A. and W, M. Washington, 1988. A comparison of soil-moisture sensitivity in two	global climate models, J. Atmospheric Sciences, 45, 1476-1492.

Meyers, T. P. and D. D. Baldocchi, 1993. Trace gas exchange at the floor of a deciduous forest:	II O3 and SO2 deposition rates, Journal of Geophysical Research, Atmospheres,98,2519-
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Table 1. Meterological variables measured at NOAA Energy Flux Monitorin Sites along with model number and manufacturer of instrumentation used.

Meteorological Variable	Manufacturerer	model number
Air Temperature and RH	Vaisala	50Y
Net Radiation	Radiation and Energy Balance Systems (REBS)	Q*7
Global Radiation	LI-COR	LI-200 SB
Precipitation	Texas Instrument
Wetness	ATDD	-
Soil Heat Flux	REBS
PAR	LI-COR	LI-190 SB
Atmospheric Pressure	Vaisala	PTB101B
Surface Temperature	Everest	4000A
Soil Temperature	ATDD	-
Soil Moisture	Vitel	hydra