Agriculture Reference
In-Depth Information
Fig. 6.14.
Relationships between onion bulb yields and crop evapotranspiration
during the bulb growth phase on a sandy loam soil at Farmington, New Mexico,
USA. (a) Total bulb yield as a function of crop evapotranspiration: (
), 1986 data;
(
), 1987 data; the line is fitted to both years' results. (b) The relationship between
relative yield decrease and relative evapotranspiration deficit; the yield response
factor to crop evapotranspiration, K
Y
, is the slope of this line (from Al-Jamal
et al
.,
2000. Courtesy of
Agricultural Water Management
).
Since yield reductions caused by water stress are proportional to the reduction
in transpiration caused by water stress (see above), Allen
et al.
(1998) used this
relation combined with Eqn 6.14 to derive a stress factor K
S
to describe how
salinity - and also combined salinity and soil water stress - will reduce
transpiration.
When EC
e
> EC
ethreshold
and D
r
< RAW:
K
S
= [1 - (b/100K
Y
)(EC
e
- EC
ethreshold
)]
(Eqn 6.15)
When EC
e
> EC
ethreshold
and D
r
> RAW:
K
S
= [1 - (b/100K
Y
)(EC
e
- EC
ethreshold
)][(TAW - D
r
)/
(TAW - RAW)]
(Eqn 6.16)
K
Y
is the yield response factor (see Fig. 6.14b).
These equations are only approximate estimates of the long-term impact of
salinity on ET
C
and are unlikely to hold where EC
e
and D
r
values exceed those
needed to reduce yields by 50% (Allen
et al.
, 1998). Figure 6.13 illustrates how
these equations predict that water stress and salinity together affect K
S
, and
therefore the transpiration rate, of onion on contrasting soil types.
Factors other than water stress and salinity can reduce the LAI to below that
of a well-grown crop (i.e. < 3 in mid-season), resulting in lower transpiration. For
example, pest, disease or hail damage or low plant density resulting from such
damage or from poor seedling establishment may all reduce LAI. Allen
et al.
(1998) give equations to estimate the consequences for evapotranspiration of
