December 22, 2009 –
Getting the most out of every drop of water is a high priority for grape
growers in the southern Napa Valley, where summers are hot and dry and vines
have to be irrigated to make it through the growing season.
December 22, 2009 –
Getting the most out of every drop of water is a high priority for grape
growers in the southern Napa Valley, where summers are hot and dry and vines
have to be irrigated to make it through the growing season.
But Stanford University researchers
have found that a significant portion of the water applied to the vines zips
right by the plants, hardly even pausing.
“We found that about 10
per cent of the water that is applied is lost below the vine rooting zone and
does not have contact with the soil and vine roots,” said Eve Hinckley, who
worked on the project for her PhD thesis in the department of geological and
environmental sciences at Stanford. “This is a conservative estimate.”
The problem lies in deep
cracks that are a chronic feature in the clay-rich soils of the area. Due to
the physical and chemical properties of these soils, they naturally swell when
wet and shrink as they dry, producing cracks. Hinckley says that tendency is
exacerbated by the weekly cycle of irrigating during the growing season, when
vines are typically watered for four hours a week. Under a regular regimen of
swelling and shrinking, the cracks become more pronounced and water speeds
through them without interacting with the soil.
Hinckley presented her
results at the fall meeting of the American Geophysical Union held Dec. 16.
She gathered her data by
burying devices called lysimeters about 16 inches down in the soil – just below
the root zone of the vines. That is also the depth to which many of the deep
cracks penetrate in the vineyard where she did her study. The lysimeters
captured water flowing through the soil, giving her data on the volume,
chemical composition, and residence time of water in the soil.
The speedy passage of so
much water through the cracks in the soil affects more than just the job of
getting enough water to the vines. There are significant consequences on either
end of that rapid flow. Upstream, it means that more water has to be stockpiled
each winter than the vines are actually using.
All the water needed to
sustain the vines through the summer has to be captured for each vineyard by
the grower during the preceding winter. Most of that water is diverted from
rivers and streams that are temporarily swollen – in a good year – by the
winter rains. A lesser portion comes from rain falling directly into the
reservoirs and runoff from adjacent slopes.
“You will often see a
string of reservoirs coming off of a stream,” Hinckley said. “The lowest one
has the first water rights. When it’s full, the grower closes it off and then
the next grower up the slope is allowed to fill.” In a winter with low
rainfall, sometimes the higher reservoirs in the string never fill completely.
“Diversions are a pretty
big deal up in the (river) system,” Hinckley said. “And that is what has been a
concern to the public, because it is siphoning water from the supply that would
be going to groundwater recharge or to streams, where fish may be spawning.”
Chinook salmon and steelhead trout both spawn in the Napa River and its
tributaries.
Hinckley said growers
could take several approaches to reducing their water loss. Most vineyards have
irrigation drip lines about one foot to 18 inches above the ground surface.
Lowering those lines onto the ground – or even burying them – would reduce the
speed and force with which irrigation water hits the ground, slowing its pace
through the soil. But lowered or buried the lines are at risk of breakage
during tilling operations and buried lines can get plugged.
Another possibility is
slowing the rate at which water is delivered from the drip emitter, Hinckley
said. “They could irrigate earlier in the day when evaporation rates are lower,
and could irrigate for a bit longer, but still deliver less water to the vine
and there would be more time for the water to soak into the soil.”
Hinckley said some growers
have systems that send small pipes down into the root zone of each vine,
putting water directly where it is needed. “That is very labour intensive,” she
said, which makes it expensive to install. “But we are living in a world where
water is a precious resource, so many growers are taking those measures.”
Slower delivery could also
help mitigate the problem of animal burrows, mainly ground squirrels, which are
usually just below and parallel to the ground surface. Like the cracks, burrows
offer water an easier path than slowly percolating through the soil and thus
contribute to routing water away from the vines. Hinckley’s lysimeters did not
intercept water flowing through these burrows, which is one reason why she says
the 10 percent estimate of water loss is a minimum. She said she's been out in
the vineyards during big rainstorms and seen the burrow’s effect.
“It basically looks like
an artesian well,” she said. “Water is flowing up, just spurting out from the
subsurface.”
The winter storms also
revealed another cause for concern.
To determine the residency
time of irrigation water in the soil, Hinckley analyzed the sulphur in the
irrigation water she captured. Growers typically apply sulphur to their vines
weekly throughout the growing season to combat mildew. The form of sulphur they
use is chemically distinguishable from the sulphur found in the soil naturally,
so by determining the quantity and type of sulphur in the water, she could tell
whether the water had lingered in the soil long enough to react with it.
In addition to enabling
her to calculate that at least 10 per cent of the irrigation water was zipping
past the root zone without reacting, she discovered that during the winter
rainstorms, all the sulphur applied to the vines during the previous growing
season was getting washed below the rooting zone of the vines, and potentially
out of the vineyard. That could have significant consequences for areas
downstream, she said.
“The growers absolutely
bathe the landscape in sulphur,” Hinckley said. “They are broadcast spraying it
across the whole vineyard.”
“The next stage of work is
to look at what the implications of that sulphur input are for aquatic systems
down-gradient of the vineyards,” she said. “There, sulphur may interact with
other elements, such as heavy metals, which could have ecological
consequences.”
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