Geoscience Reference
In-Depth Information
same between two winters. An example of this type
of lysimeter was the work of Law who investigated
the effect that trees had on the water balance at
Stocks Reservoir in Lancashire, UK (Law, 1956; see
Case Study on p. 42).
A well-planned and executed lysimeter study
probably provides the best information on evap-
oration that a hydrologist could find. However,
it must be remembered that it is not evaporation
that is being measured in a lysimeter - it is almost
everything else in the water balance equation, with
an assumption being made that whatever is left
must be caused by evaporation. One result of this
is that any errors in measurement of precipitation
and/or percolation will transfer and possibly magnify
into errors of evaporation measurement.
help in this decision the various advantages and
shortcomings of each technique are discussed.
Thornthwaite
Thornthwaite derived an empirical model (i.e.
derived from measurement not theoretical under-
standing) linking average air temperature to
potential evaporation. This is an inherently sensible
link in that we know air temperature is closely
linked to both available energy and the ability of air
to absorb water vapour.
The first part of the Thornthwaite estimation
technique (Thornthwaite, 1944, 1954) derives a
monthly heat index (
i
) for a region based on the
average temperature
t
(°C) for a month (equation
3.4).
1 514
.
=
t
ESTIMATION OF EVAPORATION
(3.4)
i
5
The difficulties in measuring evaporation using
either micro-meteorological instruments (problem-
atic when used over long time periods and at the
catchment scale) or water balance techniques
(accumulated errors and small scale) has led to much
effort being placed on estimating evaporation rather
than trying to actually measure it. Some of the
techniques outlined below are complicated and
this sometimes leads hydrologists to believe that
they are measuring, rather than estimating, evap-
oration. What they are actually doing is taking
climatological variables that are known to influence
evaporation and simulating evaporation rates from
these: an estimation technique. The majority of
research effort in this field has been to produce
models to estimate evaporation; however, more
recently, satellite remote sensing has provided
another method of estimating the evaporation flux.
The techniques described here represent a range
of sophistication and they are certainly not all
universally applicable. Almost all of these are
concerned with estimating the potential evaporation
over a land surface. As with most estimation
techniques the hydrologist is required to choose the
best techniques for the study situation. In order to
These terms are then summed to provide an annual
heat index
I
(equation 3.5).
12
=
∑
1
(3.5)
=
I
i
j
Thornthwaite then derived an equation to provide
evaporation estimates based on a series of observed
evaporation measurements (equation 3.6).
a
10
t
(3.6)
PE
=
16
b
I
The
a
and
b
terms in this equation can be derived in
the following ways. Term
b
is a correction factor to
account for unequal day length between months. Its
value can be found by looking up tables based on
the latitude of your study site. Term
a
is calibrated
as a cubic function from the
I
term such as is shown
in equation 3.7.
a
= 6.7
10
-7
I
3
- 7.7
10
-5
I
2
+
0.018
I
+ 0.49
(3.7)
