Correcting eddy-covariance flux underestimates over a grassland

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Abstract

Independent measurements of the major energy balance flux components are not often consistent with the principle of conservation of energy. This is referred to as a lack of closure of the surface energy balance. Most results in the literature have shown the sum of sensible and latent heat fluxes measured by eddy covariance to be less than the difference between net radiation and soil heat fluxes. This under-measurement of sensible and latent heat fluxes by eddy-covariance instruments has occurred in numerous field experiments and among many different manufacturers of instruments. Four eddy-covariance systems consisting of the same models of instruments were set up side-by-side during the Southern Great Plains 1997 Hydrology Experiment and all systems under-measured fluxes by similar amounts. One of these eddy-covariance systems was collocated with three other types of eddy-covariance systems at different sites; all of these systems under-measured the sensible and latent-heat fluxes. The net radiometers and soil heat flux plates used in conjunction with the eddy-covariance systems were calibrated independently and measurements of net radiation and soil heat flux showed little scatter for various sites. The 10% absolute uncertainty in available energy measurements was considerably smaller than the systematic closure problem in the surface energy budget, which varied from 10 to 30%. When available-energy measurement errors are known and modest, eddy-covariance measurements of sensible and latent heat fluxes should be adjusted for closure. Although the preferred method of energy balance closure is to maintain the Bowen–ratio, the method for obtaining closure appears to be less important than assuring that eddy-covariance measurements are consistent with conservation of energy. Based on numerous measurements over a sorghum canopy, carbon dioxide fluxes, which are measured by eddy covariance, are underestimated by the same factor as eddy covariance evaporation measurements when energy balance closure is not achieved.

Introduction

A better understanding of how energy and mass are partitioned at the earth’s surface is necessary for improving regional weather and global climate models. Because measurements of scalar fluxes can only be made at a few locations, these weather and global climate models will be used to assess the impact of societal choices, such as abiding by the Kyoto Protocol for carbon sequestration. Usually surface flux models are only as accurate as the measurements used to validate them; therefore, accurate measurements of surface energy components are imperative for accurate modeling of surface energy and mass balances. The importance of accurate micrometeorological measurements of surface fluxes is a justification for long-term flux measurement networks (Baldocchi et al., 1996). Unfortunately, the micrometeorological literature contains numerous anecdotal references to possible systematic underestimates of surface scalar fluxes by the preferred measurement system; namely eddy covariance (Dugas et al., 1991, Nie et al., 1992, Fritschen et al., 1992, Goulden et al., 1997, McCaughey et al., 1997, Mahrt, 1998). The potential problems that systematic errors can create in long-term surface flux measurements, particularly selective systematic errors (different daytime errors from night-time errors), are considered by Moncrief et al. (1996) and can be serious. Therefore, dealing with lack of energy-balance closure should be considered in the standards for long-term, flux-measurement networks even though it has received little attention (Baldocchi et al., 1996).

All models of surface energy and mass exchange are based on the fundamental conservation principles; namely, conservation of energy and conservation of mass. The major components of the conservation of energy equation, which we often refer to as ‘energy-balance closure’, can be depicted asRn=H+LE+G+S+ε,where Rn is net radiation, H is convective sensible heat exchange, LE is latent heat exchange or evapotranspiration, G is the soil-surface heat conduction flux, S is the heat storage in the canopy and ε is any residual flux associated with errors. This equation neglects energy partitioned to photosynthesis, which is less than a few percent of the net radiation. If field measurements of surface fluxes are not consistent with Eq. (1), then modelers will have to make adjustments to the measured fluxes or accept uncertainties in their models that are of the same magnitude as the measured energy conservation discrepancy. Because the discrepancy in energy-balance closure (D=[H+LE]/[Rn−GS]) is a bias that varies from 0 to −30% (0.7<D<1), this problem is serious if the cause for this discrepancy is not known. With D<0.7, the utility of the sensible and latent heat flux measurements for model validation or calibration is greatly reduced (Kustas et al., 1999). Operationally, a systematic error that underestimates the evapotranspiration component of a water budget of a crop by 25% is intolerable to an irrigation scheduler. By half-way through a growing season, the underestimated portion of the evaporation could accumulate to an amount of water equivalent to half the total soil moisture available to the crop and result in erroneous predictions of severe yield reductions if not corrected. Likewise, the underestimation of a daytime net CO2 flux by 25% could easily lead one to conclude that a forest site was a net source of carbon when in fact it was a significant sink of carbon; because night-time fluxes are estimated by alternative methods that may not underestimate respiratory fluxes (Moncrief et al., 1996).

The micrometeorological measurement community should resolve these serious discrepancies in the energy-balance closure to provide guidance to the modeling community on how to interpret flux measurements that do not appear to be consistent with the conservation principles (Kustas et al., 1999). The objective of this paper is to suggest a method for treating micrometeorological measurements of surface fluxes that do not appear to be consistent with conservation of energy. Extensive measurements from the Southern Great Plains 1997 Hydrology Experiment in Oklahoma in 1997 (SGP97) are used to investigate closure of the energy balance. The accuracy of Rn and G will be evaluated and a procedure proposed for closing the energy budget. The measurement strategy employed here was to make extensive measurements simultaneously at one ideal collocation site (ER01) using as many instruments as possible with independent calibrations on all the individual sensors. Then one of the instruments from this collocation site was mounted next to instrumentation at other permanent sites to obtain paired comparisons at different sites with sensors from various manufacturers and research groups.

Section snippets

Review of surface flux measurements and energy-balance closure

Two primary micrometeorological systems for measuring surface scalar fluxes are in wide-spread use; the energy-balance-Bowen-ratio (EBBR) method and the eddy-covariance (EC) method (Dabberdt et al., 1993). The EBBR method uses direct measurements of Rn, G, and gradients of temperature and water vapor in the atmosphere to estimate LE and H by assuming similarity between heat and water vapor transport and conservation of energy. The EC method is based on direct measurements of the product of

Methods for energy-balance closure

The use of surface flux data to validate land surface models requires that conservation of energy be satisfied; therefore, the measured energy budget must be closed by some method. As we will discuss later, the net radiation is probably the most accurate measurement (accurate to about 5–7%) of the major components of the surface energy balance for large homogeneous sites even though some studies have reported otherwise based on past measurements with particular instruments (Field et al., 1992,

Field measurement methods

The Southern Great Plains 1997 Hydrology Experiment (SGP97), sponsored by NASA and USDA, took place in Oklahoma during June and July of 1997 (Jackson, 1997). The main objectives of SGP97 were to study the remote sensing of soil moisture and the effect of soil moisture on the development of the atmospheric boundary layer and clouds over the SGP97 region during the warm season (Jackson, 1997). One aspect of the project was the measurement of surface fluxes at numerous locations across the SGP97

Results

If the measurements of the components of the surface energy budget given by Eq. (1) do not balance, the discrepancy may arise from errors in any flux component. Identifying the flux components most likely to be responsible for the lack of energy-balance closure requires establishing the absolute accuracy of as many flux components as possible. Therefore, the results address the absolute accuracy of radiation and heat storage measurements and then consider comparisons among eddy-covariance

Concluding remarks

Energy into and out of a region of measurement must be conserved, but eddy-covariance systems do not always satisfy conservation of energy. Surface fluxes can be under-measured for a number of reasons including mismatched sources of LE and H, inhomogeneous surface cover and soil characteristics, flux divergence or dispersion, non-stationarity of the flow, lack of a fully developed turbulent surface layer, flow distortion, sensor separation, topography and instrument error.

During the Southern

Acknowledgements

This research was supported by NASA Grant NAGW-4138, USDA Cooperative Agreement No. 58-1270-7-008, and the University of Wisconsin Experiment Station. The authors appreciate the efforts of Dr. Tom Jackson of USDA Hydrology Laboratory in Beltsville, MD for his leadership of the SGP97 experiment.

Jerry L. Hatfield from USDA-ARS National Soil Tilth Laboratory and Tom J. Sauer from USDA-ARS Biomass Research Center provided valuable assistance in the maintenance of the four flux towers operating at

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