Agriculture Reference
In-Depth Information
late-season stage K
CB
is about 0.1 lower than K
C
for the allium crops (see Table
6.3), which is typical of vegetable crops where the ground cover by leaves may
be 80% or less (Allen
et al.
, 1998, Table 18). K
CB
values can be adjusted for
climate using the formula:
K
CB
= K
CBT
+ [0.04(u
2
- 2) + 0.004(RH
min
- 45)](h/3)
0.3
(Eqn 6.10)
where
K
CBT
= value of K
CB
for the temperate, sub-humid climate given in Table
6.3
u
2
= mean value of daily wind speed 2 m above grass (m/s) for 1 < u
2
< 6m/s
RH
min
= mean value of daily minimum RH, provided 20% < RH
min
< 80%
h = mean plant height, m.
Prior to the mid-season stage, K
E
makes a substantial contribution to K
C
(see Fig. 6.12), depending on both the frequency of soil wetting and the
quantity of water wetting the soil, the evaporating power of the atmosphere
(ET
0
) and the texture of the soil. Finer-textured soils hold more water and carry
on releasing water for evaporation for longer following a wetting to field
capacity. As well as soil and weather factors, K
C
depends on the fraction of the
soil surface that is wetted by irrigation, and whether or not the wetted fraction
is covered by the leaves. Under drip irrigation (see below) the wet fraction of the
soil surface is frequently shaded by leaves, and K
C
can remain low even after
irrigating, whereas with furrow irrigation it is likely that the wet fraction of the
soil will be partly unshaded, and this will give rise to an increase in K
C
following
irrigation. Allen
et al.
(1998) describe how to compute daily values of K
C
for a
particular crop with its unique pattern of rainfall plus irrigation, soil texture
and ET
0
in order to schedule the irrigation of the crop.
THE EFFECTS OF WATER STRESS ON EVAPOTRANSPIRATION
As soil dries, the soil
water becomes less free to move and is more strongly bound. When the soil
reaches a certain dryness, water can no longer move to roots at sufficient rate
to satisfy the potential evapotranspiration (ET
C
), and the crop is said to be
water stressed. In these conditions stomata close and restrict gas exchange,
and the transpiration rate falls below its potential rate. The effects of water
stress on evapotranspiration can be described by adjusting the basal crop
coefficient by a water stress coefficient, K
S
(Eqn 6.11).
ET
Cadj
= (K
S
K
CB
+ K
E
)ET
0
(Eqn 6.11)
The total available water (TAW) to a crop depends on the rooting depth
and the volume of water held between field capacity and the wilting point per
unit volume of soil. This can vary from about 0.05 for a coarse sand to 0.20
(m
3
/m
3
) for a silt or clay. The fraction of available water that a crop can extract
without any restriction in transpiration rate (i.e. without stress) is termed the
readily available water (RAW) (Eqn 6.12).
RAW = p TAW
(Eqn 6.12)
