D3 Surface Flows and Energy Balance

Topic

The eddy covariance method measures turbulent fluxes of sensible heat, latent heat, and carbon dioxide as the covariance between turbulent fluctuations in vertical air velocity and turbulent fluctuations in temperature, specific humidity, or carbon dioxide concentration, depending on the flux being measured; this method requires very high temporal resolution—at least ten measurements per second—and relies on the hypothesis that the net heat or gas flux is almost entirely dominated by these turbulent eddies.

A global network of stations employing the eddy covariance method currently comprises hundreds of measurement towers distributed across various ecosystems worldwide, enabling comparisons between turbulent fluxes measured in a tropical forest and those measured in a grassland or an agricultural crop.

The surface energy balance relates the net radiation received by a surface to how that radiation is partitioned among three distinct pathways: latent heat, sensible heat, and heat flux transmitted into the soil; it is precisely this available net radiation that dictates that the sum of these three fluxes must, at all times, equal the energy that actually reached the surface.

The Bowen ratio quantifies precisely how this energy is partitioned between sensible heat and latent heat: surface moisture availability causes this ratio to be high over dry surfaces—where almost all energy is released as sensible heat—and low over wet surfaces, where the majority of the energy is consumed in evaporating water (latent heat).

The net radiation driving this entire balance is obtained, in part, by measuring the surface temperature with an infrared thermometer and converting that temperature into a longwave radiation flux using the Stefan-Boltzmann law; Heat flux into the soil, on the other hand, is derived from temperature gradients measured at various depths within the soil itself by applying Fourier's law of heat conduction.

Finally, flux chambers offer a more direct method for measuring gases such as carbon dioxide and methane: they enclose a portion of soil within an airtight chamber and determine the flux of these gases based on the rate at which their concentration changes inside that enclosed space. Thus, ranging from the eddy covariance method with which this document begins to the flux chambers with which it concludes—and encompassing the surface energy balance, the Bowen ratio, and the Stefan-Boltzmann and Fourier laws—the measurement of surface fluxes allows the energy and gases exchanged between the Earth's surface and the atmosphere, invisible to the naked eye, to be precisely quantified from a single measurement tower.

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