Geoscience Reference
In-Depth Information
E
RC
to temperature but here the Hargreaves equation (Hargreaves, 1975) is
recommended on the grounds that its performance is at least as good as alternatives
and it is simple to use. However, the estimate given by this equation is only rele-
vant for monthly average estimates of reference crop evaporation at humid sites.
The Hargreaves equation has the form:
E
≈
0.0023 (
d
)
0.5
S T
d
(
+
17.8)
(23.22)
RC
T
o
where
S
o
d
is the solar radiation incident at the top of the atmosphere in mm of
evaporated water per day,
d
T
is the difference between mean monthly maximum
temperature (
T
max
) and mean monthly minimum temperature (
T
min
) in
°
C, and
T
is the temperature in
C. Arguably the predictive ability of this empirical
equation is based on the fact that it bears some relationship to Equation (23.21).
The temperature variation of the term (
T
°
),
the equation has an explicit link to maximum solar radiation via
S
o
d
, and through
dT
, it also includes some implicit measure of the extent to which the radiation
at the top of the atmosphere reaches the surface to warm the atmosphere near
the ground.
+
17.8) approximates that of
Δ
/(
Δ+γ
Evaporation pan-based estimation of
E
RC
The measurement of weather variables requires the use of fairly expensive sensors.
For this reason evaporation pans (see Chapter 7) were often preferred in many
agricultural applications, and many pans remain in operation today (see
Chapter 7). The required estimate of reference crop evaporation is assumed to be
directly related to the measured rate of evaporation from the evaporation pan
using an equation similar to Equation (23.12), thus:
ET
=
K ET
(23.23)
RC
p
pan
The 'constant' in this equation is called a 'pan factor. In the past the value of the
pan factor has been defined empirically by comparing reference crop evaporation
rate,
lE
rc
, with measured pan evaporation rate,
lE
pan
, at one location and in one
climate, and then applying this ratio elsewhere. On this basis approximate values
of pan factor were tabulated in different weather conditions (e.g. Doorenbos and
Pruitt, 1977; Shuttleworth, 1993), but such tabulation was made without proper
theoretical understanding of the origins of such variations.
In recent years there has been research into the physics that controls evapora-
tion from the Class A evaporation pan. Rotstayn
et al
. (2006) developed the
'Penpan' equation which is based on the work of Thom
et al
. (1981) and Linacre
(1994), and which is a physically-based description of pan evaporation in terms
of ambient climate variables. The Penpan equation is an implementation of the
Penman-Monteith equation in which the effective aerodynamic resistance for a
Class A evaporation pan is prescribed to be:
