February 2014 Issue of Wines & Vines

Surface-renewal measurements of actual evapotranspiration

by Tom Shapland, Ph.D.
practical winery vineyard
Water conservation is critical to California viticulture as population and environmental pressures make fresh water resources less reliable. Irrigation management impacts grape quality, but reducing water applications to meet quality objectives can put the crop at risk.

A new technology from the University of California, Davis, provides viticulturists with a tool to quantify the amount of water that evaporates from a broad area of a vineyard. Surface-renewal technology has recently been improved to perform well in vineyards.

Grapegrowers use the measurements of actual evapotranspiration (ET) to help them decide how much water to replace in a vineyard when they irrigate. ET is a combination of water transpired through the vines and other vegetation along with water evaporating from the soil and other surfaces in the field.

Surface-renewal technology provides viticulturists with remote monitoring of vine-water status by tracking changes in actual vineyard ET between irrigation events. The new technology assists growers in maintaining yield and quality objectives while increasing water-use efficiency by more accurately assessing crop water use.

In order to measure how much water the vines are using, surface renewal quantifies how much heat and water vapor the wind carries away from a 2- to 5-acre vineyard block.1 A surface-renewal station looks like a basic weather station installed at one location within a vineyard block.

Each morning, the surface-renewal station sends data to the Internet via a cellular connection. The viticulturist receives an automatically generated email at customized intervals that reports:

1) How much water evaporated from a hypothetical well-watered grass field (i.e., Reference ET) for both the previous week and forecast for the following week. The California Irrigation Management Information System (CIMIS) provides growers with Reference ET values. By estimating the ET of a well-watered grass field, Reference ET describes the evaporative demand of the atmosphere. With the past and forecasted Reference ET, the grapegrower can anticipate how evaporative demand is expected to change during the next week.

2) How much water has evaporated from the vineyard during the past week. Actual vineyard ET is different from Reference ET, because it is the water loss from a vineyard itself and not the water loss from a hypothetical well-watered grass crop somewhere else. The actual vineyard ET is affected by the evaporative demand of the atmosphere, vine canopy size and availability of water in the vineyard soil.

3) The ratio of the actual vineyard ET to the Reference ET. Assuming the vine canopy area has not changed much since the previous irrigation, a decrease in actual vineyard ET may be due to either a decrease in evaporative demand or a decrease in water availability.

The ratio of the actual vineyard ET to the Reference ET accounts for changes in evaporative demand. A decrease in the ratio of the actual vineyard ET to the Reference ET indicates that the vines are responding to lower soil water availability. Grapegrowers may monitor the ratio of actual vineyard ET to the Reference ET to determine a level of vine water deficit and maintain a desired water status level.

4) The run-time for a pump required to irrigate a vineyard at a given ratio of actual vineyard ET to Reference ET. The information facilitates the viticulturist’s decision to decrease, maintain or increase the amount of water the vineyard receives relative to what it has been receiving.

Growers currently estimate the potential water use of a fully irrigated vineyard by using the Reference ET and crop coefficient (Kc) model. The area of transpiring leaves of a vineyard is typically smaller than the transpirational area of a reference grass crop. The Reference ET is multiplied by the crop coefficient (Kc), which corrects for leaf area differences between the reference crop and the vineyard, to estimate the evapotranspiration of the fully irrigated vineyard.

The weekly potential water use of the 25-acre Cabernet Sauvignon vineyard at Wente Vineyards in Livermore, Calif., was 34.3-43.8 gallons per vine per week in July (Table 1). The actual vineyard evapotranspiration was close to the modeled evapotranspiration, demonstrating that the vineyard had sufficient water in the soil to meet the potential demand.

Recent UC Davis breakthrough
The turbulent motions of the wind drive the movement of water from a vineyard into the greater atmosphere. As the wind moves across a vineyard, heat and water vapor are transferred from the vineyard into eddies of the wind. When an eddy of wind reaches a surface-renewal station in the vineyard, it reports the heat and water vapor transfer over the area that the eddy has traveled.

A UC Davis research group of Drs. Andrew McElrone, Kyaw Tha Paw U, Rick Snyder and Tom Shapland in the Departments of Viticulture & Enology and Atmospheric Science developed the breakthrough in surface renewal.2 In each field it was deployed, surface renewal previously had required calibration against expensive direct evapotranspiration measurement methods.

Once the surface-renewal system was calibrated, it matched the actual evapotranspiration measurements from a lysimeter or eddy covariance system. In the field of evaporation and irrigation science, lysimetry and eddy covariance are the “gold standards” for measuring water use, and they often are used to calibrate other technologies.

The research group developed a method to eliminate the calibration requirement. The method was validated by applying it to 20 years of surface renewal studies reported in scientific literature related to various crops including wine grape and table grape vineyards. It has also been validated with new data sets from 2012 and 2013 in California vineyards.

Commercial measurements of actual evapotranspiration have not been available previously because expensive instruments and equipment were required to characterize the turbulent wind motions. The expense of those methods, such as lysimetry and eddy covariance, limited actual evapotranspiration measurements to the research sphere and well out of reach of practical agriculture.

Surface-renewal equipment is relatively inexpensive, making actual evapotranspiration measurements commercially viable for grapegrowers. UC Davis owns the intellectual property, and Tule Technologies has the exclusive license to commercialize the technology. Tule Technologies charges an annual subscription fee for providing actual vineyard ET data to growers.

Technology validation
At the UC Kearney Agricultural Center vineyard in Parlier, Calif., surface renewal was validated against two research techniques: lysimetry and eddy covariance in 2012 and 2013. Figure 2 shows a sample of data from the experiment in 2013.

In two commercial Napa Valley vineyards (each 2.5 acres of Cabernet Sauvignon) in 2012, surface renewal was validated against eddy covariance. The two vineyard blocks were planted on different rootstocks, 420A and 110R. The actual vineyard evapotranspiration differed between the two blocks according to the influence of the rootstocks.

A commercial 15-acre vineyard of Chardonnay near Esparto, Calif., had three sub-blocks of 5 acres each where surface renewal was validated against eddy covariance in 2013. Different irrigation treatments were applied to each block. Surface renewal ET measurements matched eddy covariance ET measurements in each block and irrigation treatment.

A surface-renewal station was installed in a commercial 35-acre table grape vineyard site in Delano, Calif., in 2013. A station was installed in a 35-acre Cabernet Sauvignon vineyard in Paso Robles, Calif., in 2013.

Case study
Wente Vineyards has led adoption of the new technology. Throughout the 2013 season, the vineyard and winemaking team used four surface-renewal stations in four separate vineyards to monitor actual vineyard evapotranspiration and vine water status. Two of the vineyards are hillside sites. The total 52 acres includes 10 acres of Cabernet Sauvignon, 25 acres of Cabernet Sauvignon (Figure I), 7 acres of Cabernet Sauvignon and a 10-acre Merlot block.

How surface renewal works
Solar energy absorbed by a vineyard vaporizes water, is conducted into the ground or warms the air. This is called an energy balance and is considered on the scale of a field, not an individual vine.

The amount of energy that vaporizes water (latent heat of evaporation) is obtained algebraically by first measuring the light energy (net radiation), then subtracting from it the ground conduction energy (soil heat flux) and the air-warming energy (sensible heat flux).

The energy that vaporizes water can be converted to the mass of water evaporated from the vineyard (actual evapotranspiration) using a basic units conversion because the amount of energy required to vaporize a given volume of water is very close to constant.

The size of the area, or footprint, over which surface renewal measures evapotranspiration depends on the height of the instruments above the vine canopy and local environmental conditions. The footprint increases as the height of the temperature sensor increases.

For practical application, the maximum sensor height is limited by the clearance of a sprayer. For most vineyard and sprayer configurations, the maximum temperature sensor height (10 feet) allows surface renewal to measure approximately 5 acres. Conversely, for small vineyard blocks, the temperature sensor can be closer (7 feet) to the vine canopy so that surface renewal effectively measures a 2- to 3-acre area.

1. Paw U, K.T., J. Qiu, H.B. Su, T. Watanabe, Y. Brunet. 1995 “Surface renewal analysis: a new method to obtain scalar fluxes without velocity data.” Agricultural & Forest Meteorology. 74: 119–137.

2. Shapland, T.M., R.L. Snyder, K.T. Paw U, A.J. McElrone. 2014 “Thermocouple frequency response compensation leads to convergence of the surface renewal alpha calibration.” Agricultural & Forest Meteorology. In Press.

3. Williams, L.E., J.E. Ayars. 2005 “Grapevine water use and the crop coefficient are linear functions of the shaded area measured beneath the canopy.” Agricultural & Forest Meteorology. 132: 201–211.

4. Williams, L.E., 2001 “Irrigation of winegrapes in California.” Practical Winery & Vineyard. practicalwinery.com/novdec01p42.htm.

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