Geoscience Reference
In-Depth Information
Water balance
of catchment
The unmeasured difference
between other measured
components of the
catchment water balance,
including incoming
precipitation, surface (and
preferably also
groundwater) outflow, and
the change in soil water
storage.
Assumes all the other
components of the catchment
water balance can be measured
as spatial averages with
sufficient accuracy for
evaporation to be reliably
calculated as perhaps a small
difference between them.
Gives an area-average measurement
for natural vegetation covers for a
hydrologically significant region
which relates to water resource
issues, but area-average
measurement of the other water
balance terms can be expensive and
difficult, especially groundwater flow
and soil water storage, consequently
only longer time-average estimates
are possible
Catchment scale
Varies with quality
of implementation
and size and nature
of catchment, but
errors as low as
~10-20% may be
achievable in
research
catchments with
persistent care
Lysimetry
Measures the change in
weight over time of an
isolated preferably
undisturbed sample of soil
and overlying vegetation
while simulataneously
measuring incoming
precipitation to and
outgoing drainage from the
sample
Assumes that the sample of soil
and overlying vegetation on
which measurements are made
are representative of the plot or
field for which evaporation
measurement is required in
terms of soil water content and
vegetation growth and vigor.
If the soil and vegetation sample is
truly representative (difficult to
achieve), the lysimeter is widely
accepted as being an unparalleled
standard against which to compare
and validate other evaporation
measurements/models of crop
evaporation, but modern high
precison lysimeters are very
expensive (~$50k) and require
expert supervision.
Sample scale
(assumed
representative
at plot or field
scale)
State of the art
lysimeters can
provide daily
measurements with
high accuracy
(few%), but errors
can easily become
substantial (few ×
10%) with
unrepresentative
sampling.
Soil moisture depletion
Measure the change in
water content of a
representative sample of
undisturbed soil and
vegetation while
simulataneously measuring
incoming precipitation and
run-on and runoff and
estimating deep drainage
for the sample plot
Assumes soil water measuring
device (resistance blocks,
tensiometers, neutron probes,
time-domain reflectometers,
capacitance sensors)
adequately determine change
in soil water, the effects of deep
roots and sensor placement are
small, and deep drainage can
be estimated adequately.
Measurement is reasonably
inexpensive and, in principle,
representative of the often crop
covered plot in which it is
implemented, but disturbance during
installation of soil water sensors and
deep roots extending below the
measurement depth can negatively
influence the measurement, and
deep drainage is hard to estimate.
Plot scale
(assumed
representative
at field scale)
Varies with quality
of implementation
but errors of
~5-15% likely
achievable with
TDR or neutron
probes; (soil
capacitance and
conductivity
sensors not yet
accurate enough.
