Surface flux measurements from two contrasting sites within the FIFE study area are analyzed. The seasonal variation of midday latent heat fluxes and surface conductances at the two sites was found to be very similar even though the ratio of their average total leaf area indices was greater than two to one. Concurrent measurements made at another six flat sites within the FIFE area confirmed the independence of latent heat flux with respect to leaf area index. The surface conductance stress function related to specific humidity deficit was the same for the two sites, whereas the solar radiation stress function showed a difference of 30% at a solar radiation input of 300 W m−2 and the soil moisture stress function showed a difference of 16% when half the extractable soil moisture had been taken up. However, when the stress functions were combined to determine the surface conductance, these differences virtually canceled out.

In 1987 the Surface Flux Group of the first ISLSCP Field Experiment (FIFE) operated 22 stations at 20 sites. In 1989, 13 sites were instrumented. A variety of sensors were employed to calculate the fluxes of mass and energy. An effort was made throughout the FIFE campaign to compare sensors. A series of papers in this special issue present these group studies and efforts. These papers principally report the 1987 campaign, although two papers report station intercomparison during 1989. Additional papers examine the time‐space variability of heat, moisture, and momentum fluxes, as well as analyses of the properties of the CO2 fluxes and their relationships to water stress. In this overview paper we describe the basic methodologies of the measurements, provide details on the sensor systems used by members of the Surface Flux Group, and provide a summary of the flux articles appearing in this special issue.

The underlying mean and variance properties of surface net radiation, soil heat flux, and sensible‐latent heat fluxes are examined over the densely instrumented grassland region encompassing the First ISLSCP Field Experiment (FIFE). Twenty‐two surface flux stations at 20 sites were deployed during the four 1987 intensive field campaigns (IFCs). Flux variability is addressed together with the problem of scaling up to area‐averaged fluxes. Successful parameterization of area‐averaged fluxes in atmospheric models is based on accounting for internal spatial and temporal scales correctly. Mean and variance properties of fluxes are examined in both daily and diurnally averaged frameworks. Results are compared and contrasted for clear and cloudy situations and checked for the influence of surface‐induced biophysical controls (burn and grazing treatments) and topographic controls (slope factors and aspect ratios). Examination of the sensitivity of domain‐averaged fluxes to different averaging procedures demonstrates that this may be an important consideration. The results reveal six key features of the 1987 surface fluxes: (1) cloudiness variability and ample rainfall throughout the growing season led to near‐consistency in flux magnitudes during the first three IFCs; (2) burn treatment, grazing conditions, and topography have clearly delineated influences on the diurnal cycle flux amplitudes but do not alter the evaporative fraction significantly; (3) cloudiness is the major control on flux variability in terms of both mean and variance properties but has little impact on the Bowen ratio or evaporative fraction; (4) spatial weighting of fluxes based on a biophysicaltopographical cross stratification generates a measurable bias with respect to straight arithmetic averaging (up to 20 W m−2 in available heating); (5) structure function analysis demonstrates significant underlying spatial autocorrelation structure in the fluxes, but the observed distance dependence is due to cloudiness controls, not surface controls; (6) Monte Carlo analysis of high resolution vegetation indices obtained from SPOT satellite measurements suggest that the mean domain amplitudes of the diurnal sensible and latent heat flux cycles can be biased up to 30–40 W m −2 by repositioning the 20 site locations within the experimental domain.

Mass and energy exchanges with the atmosphere were compared in two soybean (Glycine max L. Merr. cv. Harosoy) isolines differing in pubescence density. The study was conducted in a field with a Sharpsburg silty clay loam soil (fine, montmorillonitic, mesic Typic Argiudoll) during the summer of 1980 at Mead, Nebr. Mass and energy exchanges were determined by means of micrometeorological techniques. Evapotranspiration (reported in terms of latent heat flux) was reduced in the densely pubescent isoline. Canopy CO 2 exchange was unchanged on a per unit land area basis. Water use efficiency (reported in terms of the CO 2-water flux ratio) was, accordingly, greater in the densely pubescent isoline. The increase in pubescence did not significantly alter the net radiation balance, turbulent mixing, canopy CO 2 exchange, or plant water status. Observed differences in the partitioning of net radiation into latent and sensible heat can be explained by greater penetration of solar radiation into the densely pubescent canopy. Leaf pubescence appears to alter the spectral characteristics of the leaf and, thus, to facilitate the penetration of solar radiation into the canopy.________

Net all‐wave radiation was observed at 22 surface sites during the 1987 observation year of the First ISLSCP Field Experiment (FIFE). Eight groups of investigators employed seven different designs of net radiometer by five different manufacturers. To establish true differences in received net radiation among sites requires knowledge of differences due to instrument performance. After careful Sun/shade calibration, side‐by‐side comparison revealed daytime differences as large as 5 to 7% for instruments of the same manufacture and 10 to 15% between manufacturer. The largest differences are between instruments with so‐called “thin windows” and “thick windows” and between instruments of “double‐dome” and “single‐dome” design. Comparisons with four‐component reference net radiation observations reveal that the double‐dome and thick window instruments have substantially lower sensitivity to longwave (thermal) net radiation than to shortwave (solar) net radiation. The magnitude of the sensitivity difference is greater when the sky is clear than when cloudy. Observations with thin window instruments agreed more closely with the reference component net radiation. Field observations made with double‐dome radiometers can be corrected when net shortwave radiation is separately measured. Such a correction is shown to reduce the systematic root‐mean‐square differences among instruments to between one half and one quarter of those shown by uncorrected measurements. When net shortwave radiation is not available, correction according to a regression comparison against a “standard” net radiometer may be used. This reduced the systematic root‐mean‐square differences by up to one half of their uncorrected values. From this analysis it is estimated that the regression corrected daytime net radiation observations reported to the FIFE Information System include systematic‐root‐mean‐square instrumental differences of 15 to 35 W m−2.