This thermal image of vines shows a wide temperature difference among leaves. Source: Martin Mendez
—At a recent meeting of the Napa Valley Vineyard Technical Group devoted to irrigation, Martin Mendez, a senior research scientist at E. & J. Gallo Winery
, said, “Management of water depends on what type of wine you want to make.”
An example: Mendez noted that Napa Valley generally starts with saturated soil in the spring and then drops to a threshold where growers start irrigating. The actual irrigation start date and in-season management should reflect the targets set for yield and grape and wine quality.
Too much water, of course, encourages vegetative growth. Therefore, the correct amount of water depends on the yield and quality desired based on the cultivar and style of wine.
“Deficit irrigation is widely used in later stages of ripening, but cutting water off before harvest isn’t the best solution,” Mendez said. It can stress the plants unnecessarily and reduces yield but doesn’t always improve flavors.
On the other hand, water may be wasted if applied where or when it’s not needed.
Unfortunately, growers may not have complete flexibility to irrigate. Availability of water for vine roots and protecting vines from frost by overhead spraying is increasingly looming as the largest issue facing grapegrowers in California. Grapevines will grow and produce grapes in many areas with the natural rainfall, but much falls at the wrong time (winter), varies by year and doesn’t necessarily provide optimum yield and quality.
Measuring water use traditionally
Mendez, who has a Ph.D. focusing on irrigation management and how it influences grape yield and grape and wine quality, discussed new techniques for remotely measuring vineyard water use, and ways to correlate that to actual plant needs.
He started with a reminder of all the standard tools available to growers. Traditionally, these included visual clues like cessation of tip growth, soil moisture sensors and plant-based measurements like leaf water potentials (measured using a pressure chamber), sap-flow sensors or leaf porometers that measure leaf stomatal conductance.
These tools often have been used to estimate plant evapotranspiration (ET) using reference coefficients, a process that invokes controversy as it’s not related to the specific site and therefore does not always reflect the intrinsic characteristics of each vineyard.
Moreover, it’s difficult to find representative leaves or vines, as vineyards are largely variable in terms of soil texture, rooting depth and other characteristics that affect vine growth, yield and grape and wine quality.
Remote sensing becomes practical
More recently, remote sensing using thermography and energy balances have become practical for assessing large areas instead of individual leaves or vines.
They operate on a simple principle: Transpiring plants have lower leaf temperature than those that aren’t giving off water. In fact, Mendez said that there is a linear correlation between stomatal conductance and temperature; you can calibrate measurements with a leaf covered with water vs. one covered with Vaseline, which inhibits transpiration, although a more practical approach is to look at the difference between air and leaf temperature on a larger scale.
An expensive thermo graphic camera flown over the vineyard, or even tied to a balloon or installed in a satellite, can measure temperature and locate warm and cool areas (i.e, those with less or more water being transpired).
Another approach to remote sensing are vegetation indexes, perhaps the most commonly used being NDVI (Normalized Difference Vegetation Index), which can be used to estimate plant leaf area based on the different reflectance of light from plants at different wavelengths.
One limitation of remote sensing is the impact of bare ground or cover crops that can skew readings of the measured indices, though a solar array on the ground can provide an indication of solar activity for compensation.
It’s also important to differentiate between actual vine evapotranspiration (ETa) from potential (ETp), which is more commonly referenced. One approach is SEBAL (Surface Energy Balance Algorithm for Land), which combines spectral radiance readings from satellite-based sensors with actual meteorological data to calculate the energy balance at the earth’s surface. It produces water consumption, or actual evapotranspiration and biomass production of agricultural crops and native vegetation. Another energy balance model is METRIC (Mapping Evapo-Transpiration at high Resolution with Internalized Calibration), developed by Dr. Rick Allen of the University of Idaho.
In sum, Mendez noted that methods of evaluating proper irrigation levels are evolving from visual clues and vine and soil-based measurements to large-scale monitoring via remoter sensing. This can give a better indication of vineyard health and account for differences between and within vineyards, while avoiding the uncertainty caused by extrapolating a vineyard’s health from perhaps unrepresentative samples.