Geoscience Reference
In-Depth Information
Precipitation
input,
Evaporation
output,
P
E
“Runin”,
V
ri
Surface area,
A
Sample
volume
Figure 7.10
Water balance
of a sample volume used to
measure the net water loss to
the atmosphere as
evaporation by measuring the
other inputs and outputs to
the volume.
Storage,
V
s
Leakage,
V
L
“Runoff”,
V
ro
can then be calculated from this. The approach is to define a sample of the
evaporating surface with known area for which the water balance can be closed,
i.e., to define a sample for which all the water entering or leaving can be measured
or adequately estimated. The water balance equation for such a sample volume is
illustrated in Fig. 7.10, and the evaporation from the sample is calculated from:
(
−+Δ+
vV VV
ri
ro
s
L
EP
=−
(7.9)
A
where
E
is the (required) evaporation loss from and
P
is measured precipitation
input to the sample volume, both in mm depth of water;
A
is the surface area of the
sample, in m
2
;
V
ri
and
V
ro
are the 'runin' to and 'runoff ' from the sample volume,
respectively, measured in liters;
V
s
is the measured or estimated change in water
stored in the sample volume, in liters; and
V
L
is the unmeasured 'loss' from the
system, in liters.
V
L
is therefore the error in the evaporation arising from poor
water balance closure.
Δ
Evaporation pans
The first measurements of evaporation were of the evaporation from the surface of
samples of water held in a container exposed to the atmosphere. Measurement
from well-specified containers, usually called evaporation pans, is still much used
to provide an approximate index of the atmospheric conditions that influence the
evaporation rate from well-watered crops and soils. Many designs for evaporation
pans have been documented, one that has been widely adopted as a standard is the
US Weather Bureau '
Class A
' pan (Fig. 7.11). This is a cylindrical 'raised pan'
mounted above ground with an area of 1.21 m
2
and a depth of 0.255 m which is
made of 22-gauge galvanized iron or monel metal, and which is mounted horizon-
tally 0.15 m above ground on a wooden platform with soil build up to be within
