Geoscience Reference
In-Depth Information
measure over a small area). With these difficulties it
is not surprising that they tend to be used at the
very small scale, mostly to calibrate estimation
techniques (see pp. 46-52). They are accurate in the
assessment of an evaporation rate, hence their use
as a standard for the calibration of estimation
techniques. The real problem for hydrology is that
it is not a robust method that can be relied on for
long periods of time.
Evaporation pan
Figure 3.3 An evaporation pan. This sits above the
surface (to lessen rain splash) and has either an
instrument to record water depth or a continuous
weighing device, to measure changes in volume.
Indirect measurement (water balance
techniques)
An evaporation pan is filled with water, hence you
are measuring E o , the open water evaporation.
Although this is useful, there are severe problems
with using this value as an indicator of actual evap-
oration ( E t ) in a catchment. The first problem is that
E o will normally be considerably higher than E t
because the majority of evaporation in a catchment
will be occurring over a land surface where the
available water is contained within soil and may be
limited. This will lead to a large overestimation of
the actual evaporation. This factor is well known
and consequently evaporation pans are rarely used
in catchment water balance studies, although they
are useful for estimating water losses from lakes
and reservoirs.
There are also problems with evaporation pans
that make them problematic even for open water
evaporation estimates. A standard evaporation pan,
called a Class A evaporation pan, is 1,207 mm in
diameter and 254 mm deep. The size of the pan
makes them prone to the 'edge effect'. As warm air
blows across a body of water it absorbs any water
vapour evaporated from the surface. Numerous
studies have shown that the evaporation rate is
far higher near the edge of the water than towards
the centre where the air is able to absorb less water
vapour (this also applies to land surfaces). The
small size of an evaporation pan means that the
whole pan is effectively an 'edge' and will have a
higher evaporation rate than a much larger body of
water. A second, smaller, problem with evaporation
pans is that the sides, and the water inside, will
absorb radiation and warm up quicker than in a
Evaporation pans
The most common method for the measurement
of evaporation is using an evaporation pan (see
Figure 3.3). This is a large pan of water with a water
depth measuring instrument or weighing device
underneath that allows you to record how much
water is lost through evaporation over a time period.
This technique is actually a manipulation of the
water balance equation, hence the terminology used
here of a water balance technique. An evaporation
pan is constructed from impervious material and
the water level is maintained below the top so that
no seepage or leakage occurs. This eliminates runoff
( Q term) from the water balance. Therefore it can
be assumed that any change in storage is related to
either evaporative loss or precipitation gain. This
means that the water balance equation can be
rearranged as shown in equation 3.2.
S - P
E =
(3.2)
If there is a precipitation gauge immediately
adjacent to the evaporation pan then the P term can
be accounted for, leaving only the change in storage
( S ) to be measured as either a weight loss or a drop
in water depth. At a standard meteorological station
the evaporation is measured daily as the change in
water depth. For a finer temporal resolution (e.g.
hourly) there are load cell instruments available
which measure and record the weight at regular
intervals.
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