Agriculture Reference
In-Depth Information
Runoff and Deep Drainage
The generation of surface runoff
R
s
and subsurface lateral flow
R
ss
is described in
section 6.3.1.
R
s
and
R
ss
are potential losses to the vines, but they contribute to
streamflow, and hence to any dams that are built to store water for irrigation. Sur-
face runoff during rain causes a rapid response in streams that drain the vineyard
area. This can be augmented by flow from
contributing areas
, which quickly reach
saturation during periods of significant rainfall. Such areas can often be identified
as seepage zones, fed by subsurface flow from higher areas, that occur at the base
of slopes and near streams.
Water that drains below the root zone is called
deep drainage D
. Such drainage
is a loss to the vines, but it may contribute to groundwater. Lateral flow of ground-
water is much slower than runoff and subsurface lateral flow. It provides the bulk
of the steady
base flow
of streams.
6.4.4
The Water Balance Equation
An area of vines that makes up a discrete hydrologic unit in the landscape is called
a
catchment
(or a
watershed
in North America). The combined effect of all the hy-
drologic processes in a catchment can be expressed through the
water balance equa-
tion
. This equation assumes a mass balance of water, so that
P
E
a
R
s
R
ss
D
S
(6.9)
All the terms in this equation are in mm.
E
a
is the actual evapotranspiration from
the soil-plant system, and
S
(delta
S
) is the change in soil water storage. Typi-
cally, in winter rainfall areas when the vines are dormant, the soil should be at or
close to
FC
. If the vineyard soil is well managed (adequate infiltration and good
structure),
R
s
and
R
ss
should be small to negligible. Hence, when
S
is small, equa-
tion 6.9 simplifies to
P
E
a
D
(6.10)
Because
E
a
is small during winter, drainage below the root zone during win-
ter can be significant. But from spring to early summer, the soil water content
starts to fall as the vine canopies develop and evapotranspiration increases. Dur-
ing summer,
D
is insignificant and equation 6.9 simplifies to
P
E
a
S
(6.11)
When
E
a
exceeds
P
for a period,
S
in equation 6.11 becomes negative and
a substantial
soil water deficit
(
SWD
) develops. This
SWD
represents a withdrawal
from the soil's
plant available water
(
PAW
) which depends on the
AWC
in mm/m
depth of soil (section 6.2.2) and the rooting depth of the vines in meters (section
7.1.3).
6.4.5
Managing the Soil Water Supply
6.5
Good vineyard management involves, as far as possible, managing the soil water
supply. Shoot growth indicates the vine's water and nutrient status. Well-watered
vines have actively growing tips and normal internode expansion. If water is too
