December 2021 - 17

Southern Oregon winegrapes may need less water
By Kym Pokorny
Oregon State University
Oregon winegrape growers in
southern Oregon may be able to cut their
water usage by almost half, according to
an Oregon State University (OSU) study.
Alexander Levin, assistant professor
and viticulturist at the Southern Oregon
Research and Extension Center, found
that winegrape water usage estimated
by the U.S. Bureau of Reclamation's
AgriMet network for irrigation was 44%
higher than necessary. AgriNet is a free
weather station widely used by growers
to determine how much water they need
to irrigate their crops.
AgriMet estimates winegrapes
need 20.2 inches of water per year in
southern Oregon. In his study, Levin
estimated annual winegrape needs at
11.4 inches annually, depending on
time of year - a vast difference for
water-starved southern Oregon, which
is already bracing for another drought.
" Federal recommendations are built
into the weather station and network, "
Levin said. " The network attempts
to simplify irrigation scheduling for
farmers. We use it regularly for our
research. It provides the basic values
for them to determine their irrigation
needs. "
In southern Oregon - stretching
from south of Eugene to the California
state line - 9,242 acres were planted
with 25,000 tons of winegrapes in
2019, an amount valued at $50 million,
according to the Oregon Vineyard
and Winery Report by the University
of Oregon. When the value of wine
is considered, the worth of southern
Oregon winegrapes sits right around
$238 million, Levin said.
Levin conducted his research at
two vineyards in Jackson County,
Oregon, from 2017-2019. Using
solar panels, he estimated the vines'
evapotranspiration - the amount of
water transferred from land and plants
to the atmosphere. First, he placed the
solar panel next to the grapevines in
full sun. The same solar panel was then
placed under the vines so that part of
the panel was shaded. The panel gave a
reading of the power produced by the
sun compared to shade. This results in a
value called a crop coefficient. Growers
plug it into a mathematical equation
to make irrigation adjustments for
different crops.
Well watered and maintained turfgrass
is used as a reference crop because it
covers the ground and needs more water
than most crops. With grapes, there's
less evapotranspiration and less need for
water than for turf.
As the growing season progresses,
vineyard water requirements change
with grapevine canopy growth and
warmer, drier conditions, Levin said.
Many growers in the Pacific Northwest
already use personal or publicly available
weather stations like AgriMet. However,
the crop coefficients used for irrigation
scheduling were developed in California,
where crop growth, cultural practices
FGN | DECEMBER 2021 | 17
and water requirements differ.
" The California-based coefficients
don't quite match the pattern of plant
growth and development we have
here in Oregon, " Levin said. " When
the crop coefficient maxes out, you'd
expect that to be when the canopies are
done growing. They should match. The
coefficients developed in California
work well in California - better than
we expected. But we noticed they
weren't matching what we saw on the
ground in Oregon. "
Current values for grapevine water
requirements either lack precision
or were developed in other regions
characterized with different growing
conditions. For growers to establish
sustainable irrigation practices, they
need improved crop coefficients that
take into consideration management
practices and climate, Levin said.
" We accepted both hypotheses that
new regional estimates of coefficients are
more accurate and have the potential to
result in increased irrigation efficiency
and significant water savings, " Levin
said. " Ultimately, the improved accuracy
of the new estimated coefficient values
should allow growers to optimize
irrigation scheduling for improved fruit
and wine quality. " FGN
Alec Levin researches winegrapes and
irrigation in southern Oregon.
Photo: Stephen Ward
December 2021

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