Agriculture Reference
In-Depth Information
Table
.
Crop coefficient (K
c
) values for apple orchards in sub-humid conditions
End season
a
Early season
Mid-season
Without ground cover
.
.
.
With ground cover
.
.
.
a
Prior to leaf fall. After leaf fall
K
c
is . for bare dry soil or dead ground
cover and
for actively growing ground cover (which will increase its
evapotranspiration when the trees lose their leaves).
Data from Allen
et al.
(
.
to
.
).
Evapotranspiration by orchards
To calculate the evapotranspiration for other crops (
ET
c
), including orchard
crops, the reference
ET
value is multiplied by a crop coefficient
K
c
.
ET
c
=
K
c
ET
(
.
)
K
c
depends on (a) crop height which affects roughness and aerodynamic re-
sistance; (b) crop-soil surface resistance, which is affected by leaf area, the
fraction of the ground covered by vegetation, leaf age and condition, stomatal
control and soil surface wetness; and (c) the albedo (reflectance) of the crop-
soil surface, which is affected by the fraction of ground covered by vegetation
and by the soil surface.
In general the ratio of
ET
c
to
ET
increases as windspeed increases and rela-
tivehumiditydecreases.
K
c
foratallcrop,e.g.
-
mhigh,increasesbyasmuch
as
% when changing from a calm humid climate to a windy arid climate.
Table
gives estimates of the
K
c
value for apple orchards in early, mid-,
and end of season conditions, assuming mature orchards with trees
.
m tall
and taking into account soil evaporation and ground-cover crop evapotran-
spiration (Allen
et al.
,
). Similar results for Washington State conditions
are shown in Figure
. The increase in
K
c
in the early part of the season
reflects the early leafing out of spur buds and rapid completion of shoot growth
(Chapter
.
). These values apply to mature orchards. At planting the fraction
of the ground covered by tree canopy is very low and the number of years
needed to attain final canopy dimensions depends on planting density and
vigour of growth. Orchard ground cover has been variously estimated as the
percentage of ground vertically beneath the tree canopy and by LAI. These are
both likely to introduce inaccuracy, the most relevant measurement being that
of the fraction of radiant energy intercepted, which is not a simple function of
LAI in orchard crops.
The estimates of potential water use calculated in this way provide a basis
for planning the relative irrigation requirements in different areas. Optimizing
irrigation water application, especially where this is by localized (trickle)
