Agriculture Reference
In-Depth Information
drier air if evaporation is to proceed. This replacement is a function of wind-
speed. The higher the relative humidity of the air at any given temperature,
the lower the evaporation. Evaporation is also increased the higher the ambi-
ent temperatures of the surface and the air. This is through the supply of heat
energy and also because as air temperature rises its capacity to absorb water
vapour increases. Evaporation is driven by gradients in atmospheric vapour
pressure deficit (VPD).
Evapotranspiration by a standard reference crop (
ET
)
Potential transpiration by vegetation completely shading the ground and ade-
quatelysuppliedwithwaterisgenerallylessthanpotentialevaporationbecause
stomata close at low light intensities, so the effect of nightfall is greater than that
due to the reduced energy receipts alone and because of the greater reflectivity
of vegetation than water. For a standard reference crop of grass
cm tall
the evapotranspiration (
ET
, the combined transpiration and evaporation) is
usually about
-
%of
E
.
ET
can be calculated from weather data and as-
sumed values of bulk surface resistance (
Sm
−
) and albedo (
.
) using the
FAO Penman-Monteith equation (Allen
et al.
,
).
ET
=
.
(
R
n
−
G
)
+
γ
u
(
e
a
−
e
d
)
T
+
(
.
)
+
γ
(
+
.
u
)
where
ET
measured is in mm d
−
;
R
n
is net radiation and
G
is soil heat flux
density (MJ m
−
d
−
) for use with
-h calculation time steps.
T
is mean daily
temperature (
◦
C),
is the slope of the saturation vapour pressure curve at
the mean daily air temperature (kPa
◦
C
−
),
γ
is the psychometric constant
(kPa
◦
C
−
),
u
is mean
m(ms
−
),
e
a
is mean daily
saturation vapour pressure at air temperature (kPa) and
e
d
is saturation vapour
pressure at dewpoint temperature (kPa) at
-h windspeed at
m height.
The modified Blaney-Criddle method can be used to estimate evapotran-
spiration from a standard reference crop when neither pan evaporation nor
radiation data are available (Doorenbos and Pruitt,
.
-
; James
et al.
,
;
Wilson,
). A monthly water use factor (
f
), expressed as mm per day, is cal-
culated from the mean of the daily maximum and minimum temperatures of
the month considered (
t
) and the mean daily percentage of the annual daytime
hours (
p
).
f
=
p
(
.
t
+
.
)
(
.
)
ET
values are then estimated from these
f
values at different levels of relative
humidity, windspeed and cloudiness using standard tables or graphs.
