Environmental Engineering Reference
In-Depth Information
represented as a covariance between measurements of vertical velocity and concen-
tration of either humidity (vapor flux) or temperature (sensible heat flux).
Mathematically, the process can be presented for latent heat flux (
λρ w E ) and
sensible heat flux ( H ) as:
λρ w E
¼ λ
w 0 ρ 0 v
(3.8)
¼ ρ a c p w 0 T 0
H
(3.9)
where overbars indicate an average (typically a 30-min average), primes indicate
departures from the mean,
is the latent heat of vaporization (J kg 1 ),
ρ w is the
density of water (kg m 3 ), E is evapotranspiration flux (m s 1 ), w is the vertical
wind speed (m s 1 ),
λ
ρ v is the absolute humidity (also called vapor density)
(kg m 3 ),
ρ a is the density of air (kg m 3 ), c p is the specific heat capacity of air
(J kg 1 C 1 ), and T is the air temperature ( C). Measurements are typically made
10-20 times a second. For both latent and sensible heat fluxes, units are in J m 2 s 1
or W m 2 . E rather than ET is used in Eq. 3.8 to be consistent with other literature
that describes the evaporation process. The process is identical in wetland settings,
although the source for some of the water is via the stomates of leaves. In this
chapter, ET refers to the process of evapotranspiration and E refers to evapotrans-
piration flux in units of distance per time.
The above description is suitable for flat, open areas with uniform vegetative
cover over long distances upwind of the sensors. The process is a bit more complex
on sloping land surfaces or where air streams converge or diverge upwind of the
sensors, in which case covariances are determined on three axes and coordinate
rotations to the data may need to be performed before E is determined (Wilczak
et al. 2001 ). A krypton hygrometer or infra-red gas analyzer, and sonic anemome-
ter, are typically used to measure humidity and wind speed, respectively. Tempera-
ture is provided either as a by-product of the sonic anemometer or from a separate
sensor. Instrumentation has improved rapidly in this field; newer sensors are much
more robust and are now capable of being deployed during rain events. This method
is sensitive to misalignment; sensors need to be deployed and maintained on a
stable platform and at a constant orientation (Wilczak et al. 2001 ). Height of
deployment is strongly related to the upwind area that the measurement represents.
The roughness of the upwind area also greatly affects the sensor signal.
3.5.3 Estimation Methods
Numerous empirical methods have been developed to estimate evaporation, ranging
frommethods that require only measurement of air temperature to methods that have
a sound physical basis and require numerous parameters. Methods can be grouped
into those that quantify available energy to determine evaporation as the residual,
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