Agriculture Reference
In-Depth Information
Box 2.3
(continued)
A comparable figure for water required is obtained for the Riverina region in
New South Wales, Australia, for a similar vine density. Note that for higher vine
densities, the water requirement increases proportionally. A similar calculation can be
done for the Lodi Viticultural Area in the Central Valley region, California (see
www.
lodiwine.com
),exceptthatwatervolumesaremeasuredinacre-inches.Peakdemand
occurs in July when a typical vineyard's average daily water need is about 0.33 acre-
inches. This requirement amounts to 0.33 inches of water spread over 1 acre of land
and is equivalent to 8.4 L/m
2
(conversion factors from metric to U.S. units and vice
versa are given in appendix 1). Thus the total quantity of water required daily per ha is
×
=
84 10 000
.
,
L/ha/day
84 000
,
L/ha/day
and one week's supply amounts to
×=
L/ha (rounded to 0.6 ML/ha)
However, regulated deficit irrigation (see “Controlling the Soil Water Deficit
through Irrigation,” chapter 4) is advocated in these summer-dry inland regions
to control vine vigor and improve grape quality. This is especially so in the Lodi
district where the peak water requirement, especially in the weeks between fruit set
and veraison, could be 30% to 65% of the calculated amount, depending on vine
density and canopy management. Thus for low-density, vertically trained vineyards
under regulated deficit irrigation at Lodi, the amount of water required per ha per
week could be as small as
84 000
,
7
588 000
,
×=
ML
or as large as 0.382 ML. Note that if a grass cover crop is growing actively in the
mid-rows, the amount of water required could be twice as much as that required by
the vines alone.
0 588
.
03
.
0 176
.
Water Quality
The most critical factor in water quality is the concentration of dissolved salts—
mainly, sodium chloride (NaCl), calcium chloride (CaCl
2
), and magnesium sulfate
(MgSO
4
). Over time, these salts accumulate in the soil because the water supplied
by irrigation is used by plants and evaporated. The salt concentration is most readily
measured by the
EC
of the water, usually expressed in units of dS/m. From analyses
of many water samples, the following approximate relationship has been developed:
=
1
(dS/m)
640
mg/L of total dissolved salts (
TDS
)
Table 2.3 explains the relationships between a number of the expressions used for
EC
and the concentration of salts in water.
