Agriculture Reference
In-Depth Information
Evapotranspiration from this standard reference surface is denoted as ET
0
(mm/day of water). ET
0
is a function of the net daily radiation at the crop
surface, the daily soil heat flux density, the mean daily air temperature and
wind speed at 2 m height and the mean daily water vapour pressure in the air.
Thus ET
0
incorporates the effects of weather conditions on evapotranspiration.
Allen
et al.
(1998) recommend the Penman-Monteith equation, since it allows
ET
0
to be unambiguously determined to provide consistent values in all regions
and climates using readily measured meteorological data.
Characteristics that distinguish a particular crop species from the
reference crop are integrated into the 'crop coefficient', K
C
. The evaporation
from a disease-free, well-fertilized crop grown in large fields without water
stress, ET
C
, is predicted by multiplying ET
0
by K
C
:
ET
C
= ET
0
K
C
(Eqn 6.8)
The crop coefficient can be divided into two components: K
CB
, 'the basal
crop coefficient' for evaporation from the crop leaves and K
E
for evaporation
from bare soil between plants; therefore:
K
C
= K
CB
+ K
E
(Eqn 6.9)
The crop coefficient integrates the effects of four main characteristics that
distinguish a crop from the reference grass: (i) crop height, which influences
resistance to water vapour transfer from crop to atmosphere; (ii) the reflectance
(albedo) of the crop-soil surface, which influences the net radiation absorbed;
(iii) the canopy resistance of the crop to vapour transfer, which varies with leaf
area and the degree of stomatal control; and (iv) the evaporation from soil. The
K
C
of well-grown onion and garlic crops at their maximum leaf cover is fairly
close to unity (see Table 6.3), and hence allium crop evapotranspiration, in
common with that of other short vegetables, is similar to the grass reference
evapotranspiration, ET
0
.
Similar values probably apply to leeks and Japanese bunching onions,
given their similarity of root and shoot morphology to the crops in Table 6.3.
The K
C
and K
CB
values in Table 6.3 are those expected under a standard
climatic condition defined as sub-humid with a mean RH of 70%, a minimum
RH of 45% and moderate wind speeds averaging 2 m/s. K
C
depends to a degree
on climate, increasing as wind speed increases and humidity decreases and
decreasing at low wind speeds and high humidity, to give a range for onions of
0.94-1.22 around the value of 1.05 for the standard climate (see Fig. 6.12).
When a crop is small and hardly shading the ground, K
C
depends largely on
soil evaporation, and this decreases drastically as the soil surface goes from wet
to dry. The K
C
for wet soil may exceed 1.0, but a dry soil surface can have a K
C
as
low as 0.1. As a crop develops, K
C
will change as the proportion of leaf cover to
bare soil increases. For evapotranspiration calculation purposes, values of K
C
are divided into four phases: (i) 'initial', from sowing to 10% ground cover; (ii)
'crop development', from 10% ground cover to full leaf cover; (iii) 'mid-season',
