Measuring Surface Heat Fluxes - Terrestrial Hydrometeorology - page 92

Table 7.1 Summary of attributes of different method for measuring evaporation along with opinions on their relative strengths and weaknesses, the scale at
which measurement is made, and likely errors in each method.

Micrometeorological
methods

Scale of
Measurement Brief Description Assumptions Strengths and weaknesses Error

Bowen Ratio - Energy
Budget

Calculate evaporation as the
latent heat from the surface
energy budget using the
ratio of sensible to latent
heat (Bowen ratio) derived
from the ratio between
atmospheric temperature
and humidity gradients
measured over a few meters
above the vegetation.

Assumes the turbulent diffusion
coefficient for sensible heat and
latent heat are the same in the
lower atmosphere in all
conditions of atmospheric
stability, and sample plot scale
measurements of energy budget
components (net radiation, soil
heat) are representive of
upwind conditions.

Fairly well established method which
is available as relatively inexpensive
proprietary systems that can be used
for both short crops and natural
vegetation but is problematic over
tall vegetation when atmospheric
gradients are low and commonly
cannot be used during hours around
dawn and dusk hours when the
Bowen ratio is minus one.

Field scale Errors associated

with assumptions
and
representativeness
and the errors in
required
supplementary
sensors imply
overall errors can
be ~5-15%.

Eddy correlation Calculate the evaporation

as 20 to 60 minute time
averages from the
correlation coefficient
between fluctuations in
vertical windspeed and
atmospheric humidity
measured at high frequency
(~10 Hz) and at the same
place, a few meters above
the vegetation.

Assumes transfer of water
vapor is all via turbulent
transfer at the sample point,
but does not occur in
turbulence with associated time
scales less than ~0.1 seconds
or greater than the selected
averaging time.

The preferred method for field scale
measurements in research
applications, gives routine
unsupervised data collection using
reasonably expensive proprietary
logger and co-located sensors, but
prone to systematic underestimation
of fluxes so best used to measure
Bowen ratio, with evaporation then
deduced from surface energy budget

Field scale Systematic

underestimation up
to 25% can occur
in the basic
measurement,
reduced to random
errors ~5-15% if
sensible heat also
measured and
energy balance
used

Water loss
measurements

Scale of
Measurement

Brief Description Assumptions Strengths and weaknesses Error

Evaporation pan Direct measurement of the

change in water level over
time for a sample of open
water in a “pan” with
well-specified dimensions
and siting.

Assumes that the relationship
between the measured
evaporation from pans (with
prescribed characteristics) and
the actual evaporation from the
adjacent area can be calibrated,
and that this calibration is
transferrable between different
locations and climates

Method is long established and
well-recognized, simple to
understand and implement, and
reasonably inexpensive; but because
it fundamentally relies on the validity
of an extrapolated calibration factor
previously defined elsewhere, it is
primarily used for crop evaporation
estimates rather than heterogeneous
natural vegetation covers

Plot scale
(assumed
representative
at field scale)

Varies with reliability
and relevance of
calibration factor,
but ~10-20% errors
are possible for
crops, with greater
errors likely for
natural vegetation
because calibration
may be unknown