Agriculture Reference
In-Depth Information
Box 6.8
(continued)
Figure B6.8.1
An example of a weather station in a vineyard. Photograph by the author.
is the slope of the graph of saturated vapor pressure
In these equations,
versus temperature,
is the psychrometric constant,
U
is the wind speed at a
e
d
) is the water vapor pressure deficit,
T
is the average
temperature (°C), and
height of 2 m, (
e
a
(lambda) is the latent heat of water vaporization.
Although the dimensionless constant
k
should be chosen for a particular region,
the value of 1.26 has been found to be widely applicable.
Depending on the surface albedo, evaporation from a wet soil surface is
comparable to
E
p
, which is approximately equal to 0.8
E
o
.
Soil Water Storage
Because the grapevine is a perennial plant that may live for more than 100 years,
it can develop deep roots, especially in deep permeable soils (section 3.1). But the
majority of vine roots (
70%) occur in the top meter of soil, which defines the
effective root depth
, especially for irrigated vines. The amount of water held in this
root zone at field capacity
FC
defines the
soil water storage S
. As explained in box
3.5, this is most conveniently expressed as a depth of water (in mm) per meter
depth of soil (
1000). For example, values of
6.4.3
at
FC
for a sandy soil of
0.25 and for a clay soil of 0.45 m
3
/m
3
(table 6.2) correspond to
S
values of 250-
and 450-mm/m depth, respectively. Methods for measuring soil water content are
discussed in box 6.9.
