EVAPORATION - Fundamentals of Hydrology - page 45

much larger lake, providing an extra energy source
and greater evaporation rate.
To overcome the edge effect, empirical (i.e.
derived from measurement) coefficients can be used
which link the evaporation pan estimates to larger
water body estimates. Doorenbos and Pruitt (1975)
give estimates for these coefficients that require
extra information on upwind fetch distance, wind
run and relative humidity at the pan (Goudie et al .,
1994). Grismer et al . (2002) provide empirical
relationships linking pan evaporation measurements
to potential evapotranspiration, i.e. from a vegetated
surface not open water evaporation.

would fill up with water. In the same manner as
an evaporation pan it is necessary to measure the
precipitation input immediately adjacent to the
lysimeter. Assuming that the only runoff ( Q ) is
through percolation, the water balance equation for
a lysimeter i shown in equation 3.3.
E = S - P - Q (3.3)
A lysimeter faces similar problems to a rain gauge
in that it is attempting to measure the evaporation
that would be lost from a surface if the lysimeter
were not there. The difference from a rain gauge
is that what is contained in the lysimeter should
closely match the surrounding plants and soil.
Although it is never possible to recreate the soil and
plants within a lysimeter perfectly, a close approxi-
mation can be made and this represents the best
efforts possible to measure evaporation. Although
lysimeters potentially suffer from the same edge
effect as evaporation pans, the ability to match the
surrounding vegetation means there is much less of
an edge effect.
A weighing lysimeter has a weighing device under-
neath that allows any change in storage to be
monitored. This can be an extremely sophisticated
device (e.g. Campbell and Murray, 1990; Yang et al .,
2000), where percolation is measured continuously
using the same mechanism for a tipping-bucket rain
gauge, weight changes are recorded continuously
using a hydraulic pressure gauge, and precipitation
is measured simultaneously. A variation on this is
to have a series of small weighing lysimeters (such
as small buckets) that can be removed and weighed
individually every day to provide a record of weight
loss. At the same time as weighing, the amount of
percolation needs to be recorded. This is a very
cheap way of estimating evaporation loss for a study
using low technology.
Without any instrument to weigh the lysimeter
(this is sometimes referred to as a percolation gauge )
it must be assumed that the change in soil moisture
over a period is zero and therefore evaporation equals
rainfall minus runoff. This may be a reasonable
assumption over a long time period such as a year
where the soil storage will be approximately the

Lysimeters

A lysimeter takes the same approach to measure-
ment as the evaporation pan, the fundamental
difference being that a lysimeter is filled with soil
and vegetation as opposed to water (see Figure 3.4).
This difference is important, as E t rather than E o
is being indirectly measured. A lysimeter can also
be made to blend in with the surrounding land
cover, lessening the edge effect described for an
evaporation pan.
There are many versions of lysimeters in use, but
all use some variation of the water balance equation
to estimate what the evaporation loss has been. One
major difference from an evaporation pan is that a
lysimeter allows percolation through the bottom,
although the amount is measured. Percolation is
necessary so that the lysimeter mimics as closely as
possible the soil surrounding it; without any it

Weighing lysimeter

Figure 3.4 A weighing lysimeter sitting flush with the
surface. The cylinder is filled with soil and vegetation
similar to the surroundings.