Geoscience Reference
In-Depth Information
Figure 23.3
Typical variation
in moisture stress function
f
(
q
s
)
with average volumetric soil
moisture content,
q
s
, in a
volume of soil considered
accessible to the atmosphere via
plants. The soil is saturated
when
q
is
q
s
sat
, at 'field capacity'
when
q
is
q
s
fc
, and at 'wilting
point' when
q
is
q
s
wilt
.
1
f(
)
q
s
0
θ
s
wilt
θ
s
d
θ
s
fc
θ
s
sat
- Open water evaporation, which is given by Equation (23.9) and associated
equations with calculation illustrated by example in Table 23.3.
- Reference crop evaporation, which is given by Equation (23.15) and asso-
ciated equations with calculation illustrated by example in Table 23.4.
Compromise estimates of evaporation
: when not all the required weather
variables are available to estimate Reference Crop Evaporation from Equation
(23.15), a compromise estimate using less/other variables is required, includ-
ing making a
radiation-based estimate
using Equation (23.21); a
temperature-
based estimate
using Equation (23.22); or a
pan-based
estimate using
Equation (23.23) with either Equation (23.28) or (23.29), calculations of all
also being illustrated by example in Table 23.4.
●
Matt-Shuttleworth approach
: estimates of crop evaporation are calculated
more realistically from standard weather variables measured at 2 m using the
Matt-Shuttleworth approach which involves using a version of the Penman-
Monteith equation indexed to a (50 m) blending height (Equation (23.37),
and associated equations) with crop-specific values of surface resistance
either calculated by Equation (23.34) for partial crop cover or taken from
Table 23.5 for full crop cover. These calculations are illustrated by example in
Table 23.5.
●
Evaporation in water-stressed conditions
: often evaporation from water-
stressed vegetation is calculated using Equation (23.39) with the value of the
water-stress factor given as a function of ambient soil water content in the
plant rooting zone modeled by a daily running water balance.
●
References
Allen, R. G., Pereira, L. S., Raes, D. and Smith, M. (1998) Crop evapotranspiration.
Irrigation
and Drainage Paper 56
. UN Food and Agriculture Organization, Rome, Italy.
Barton, A. B. and Mayer, W. S. (2008) An analysis of method and meteorological
measurement of evapotranspiration estimation. Part 2: Results using measured
evapotranspiration and weather data from Ayr (Qld), Kununurra (WA) and Griffith
