Geoscience Reference
In-Depth Information
K
c
=
K
cb
+
K
e
1.4
1.2
K
e
1.0
0.8
0.6
0.4
K
cb
0.2
0.0
Initial
Crop development
Mid-season
Late season
Time
Figure 8.3
Crop coeficient curves showing the basal
K
cb
, soil evaporation
K
e
and the
corresponding single
K
c
=
K
cb
+
K
e
curves.
Dual Crop Coeficient
One of the main disadvantages of the single-crop coeficient is that two processes
(transpiration and soil evaporation) have to be covered by one crop coeficient. After
all, the relationship between actual soil evaporation and
E
ref
has nothing to do with
plant characteristics. Therefore Allen et al. (
1998
) proposed to use for more accurate
and detailed studies (e.g., studies on daily basis) dual-crop coeficients:
(
)
EK KE
=+
ref
(8.2)
cb
e
where
K
cb
is the basal crop factor and
K
e
the soil evaporation coeficient.
K
cb
is
deined as the ratio
E
/
E
ref
when the soil surface is dry.
K
e
describes the evaporation
component of
E
.
Figure 8.3
illustrates the methodology. The value of
K
cb
is smaller
than the value of
K
c
in
Figure 8.2
because the latter includes the average soil evap-
oration. If the soil is wet following rain or irrigation
K
e
may be large. However, the
sum of
K
cb
and
K
e
can never exceed a maximum value as determined by the total
energy amount available for evapotranspiration.
K
e
decreases sharply when the top
soil dries out. The corresponding smoothed
K
c
(i.e.,
K
cb
+
K
e
) curve is also shown in
Figure 8.3
and illustrates the effect of averaging
K
cb
and
K
e
over time. The estima-
tion of
K
e
requires a daily water balance computation for the water content in the
top soil. Allen et al. (
1998
) describe in detail the procedure to determine both
K
cb
and
K
e
. Compared to the single-crop coeficient approach, the dual-crop coeficient
approach is more suitable to analyse daily irrigation scheduling or other research
studies in which daily variations in soil surface wetness affect evapotranspiration
and soil water luxes.
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