On a warm, sunny afternoon in late April, University of California Cooperative Extension Viticulture-Soils Farm Advisor Mark Battany guided the GMC pick-up truck across hilly, drought-parched pasture near Paso Robles on the Central Coast.

The truck moved slowly – dodging a gopher hole – to reach a new state-of-the-art weather station which extended 35-feet skyward on land owned by the J. Lohr Winery.

Along the way, Battany discussed how this Central Coast region is a microcosm of sorts for much of the rest of the state – coming to tough terms with drought-caused limitations in the water supply, and simply put – finding ways to accomplish more with less water.

“The current drought has put a lot of pressure on our groundwater supply,” Battany said. “Growers have to pump more water just to maintain the vines for normal growth.”

Weather station

This weather station located on an undulating bench above the Estrella River is one of a dozen weather stations which the wine grape advisor will strategically install across the Paso Robles wine grape region by year’s end.

Findings from the stations hopefully will provide grape growers with a variety of new water-saving tools; vital information to maximize groundwater irrigation use in this drought-stricken coastal corner part of the parched Golden State.

This region relies almost exclusively on groundwater for agricultural irrigation. Rainfall here totals about 15 inches in an average year and less as the crow flies eastward.

“Since this groundwater basin is resupplied primarily by rainfall, access to improved weather data on regional rainfall will help us going forward to better understand the conditions which lead to recharge in the basin.”

Gaining more accurate rainfall totals are just one part of the weather stations’ many abilities.

Buried soil sensors

This first stop on Battany’s afternoon tour included soil moisture sensors buried up to four feet deep in the alluvial-type soil.

“These sensors will allow us to better track soil moisture levels throughout the season and try to determine when soil moisture storage reaches its peak in the springtime,” Battany said.

This is important, the 14-year UCCE veteran says, since knowing when the peak moisture soil content occurs can inform growers that it’s time to terminate vineyard cover crops to save the moisture for vine and grape development later in the season.  

The weather stations are largely funded through two California Department of Food and Agriculture’s specialty crops block grants. The latest grant for $160,000 covers three years.

Each weather station costs several thousand dollars each.

In addition to rain and soil moisture measurements, the station has a temperature sensor at five feet above the station’s ground level (vine height) and at the 35-foot top to read current air temperatures.

On a cold frost-risk night, the air temperature may increase with the height above the ground. This is a temperature inversion.

Wind machines save water

During the late winter and early spring, many grape growers here have traditionally used sprinklers during cold snaps to protect grape plants from possible frost. The weather station temperature readings could help growers transition to wind machines for frost protection and save water by identifying weather or not sufficient inversion conditions exist from frost risk periods. Wind machines can stir the air up to 35-feet in the air.

This would in turn move the higher height warmer air down to the vine level, achieving the same goal as sprinklers but without the water.

“Going forward, it will be harder to justify sprinkler use in some areas, especially if we can determine that the conditions exist for wind machines to provide useful protection in those areas,” Battany said.

The weather stations could also protect human health. A black globe sensor located on the station provides data for a heat stress index to help growers make the decision to move workers out of the vineyard when summer temperatures could become unsafe.

Summer temperatures can exceed 100 degrees on the hottest days.

Battany believes this weather data could be applied to similar grape-growing regions on the Central Coast and in other parts of California.

Grape top crop

In 2013, wine grapes were the top crop in farm gate value in San Luis Obispo County, covering 40,000-plus acres with a farm gate value of about $220 million, according to the county agriculture commissioner’s office. Strawberries ranked second at $210 million.

Red grapes are the most popular type grown in the Paso Robles area with Cabernet Sauvignon the dominant variety. Other popular varietals include Zinfandel and Merlot. Some table grapes are grown east of Paso Robles near Shandon.

Battany says water is a premium resource in this farming area and elsewhere. While water use per vine in the county is highly variable between vineyards, vines on average require about a foot of water annually, delivered mostly by above-ground drip.

Slamming into brick wall

While new grape plantings have rapidly increased over the last few decades, new plantings slammed into a brick wall two years ago. An urgency ordinance imposed in the Paso Robles groundwater basin effectively prohibited the expansion or establishment of new irrigated acreage.

One exception was a one-to-one water use offset allowing acreage to be replaced; for example, allowing several acres of wine grapes to replace one acre of alfalfa. Projects already ‘in the pipeline’ were also allowed to be planted, so there has been an increase in irrigated acreage while the ordinance has been in effect.

The current ordinance is set to expire in August. It’s unknown whether the moratorium will be extended though a similar program will likely continue.

Battany said, “The drought has hit us at a pretty bad time in terms of trying to resolve groundwater supply issues here.”

Rural-urban issue

Water has not only become a rural debate here between growers but an urban versus rural debate. A large number of residential water users live in rural San Luis Obispo County.

“There is a bit of a clash between agricultural water users – especially the newer vineyards coming into the area – and the residential water users in the rural areas. They are competing users for the same groundwater source.”

With these water issues and more, Battany says his weather station research is part of the water solution in the area.

“We are trying to maximize the efficiency of agricultural water use and gain the maximum benefit from it,” Battany shared. “We want to make sure growers can manage irrigation to achieve the types of production they have in place.”

He added, “The weather station research is about trying to improve the information available to growers to help them make the most informed decisions when managing irrigation. If we can be more efficient with our water use and maximize the crop value based on the water use, this benefits everybody in the long run.”

Some wells in the area have gone dry. New agricultural well depths are often 1,500-feet and deeper.

Grapes in the county are mostly mechanically harvested, with an increasing amount picked at night to maximize grape quality.

Surface renewal measurement

A second weather station Battany visited, also on the J. Lohr farm, was a surface renewal measurement station where research is studying a newer way to measure crop evapotranspiration or ET – basically the amount of water in the plant lost to the atmosphere and how much irrigation water should be applied through irrigation to make up for the loss.

For years, a pressure bomb measuring the water content of a single leaf at a time has been the common tool to determine plant water content.

Battany, along with Andrew McElrone of the USDA-ARS, Chris Parry at UC Davis, and Anji Perry of the J. Lohr Winery, are testing an ET sensor developed and licensed by UC Davis and commercially available through Tule Technologies. The company says the sensor can measure actual ET over a 1-10 acre area.

“Our goal is to better understand how this new tool can be useful and practical for a grower to better manage irrigation,” Battany said.

2015 is the second year of this trial. The Extension advisor hopes to have some good data by year’s end to extrapolate irrigation suggestions.

Irrigator’s dream

“For many irrigators, this has been their dream to have automated data available which tells them how well vineyards are performing water wise at any given time,” Battany said. “This technology has a lot of management potential in this area.”

In fact, the system could help a grower build a water-use record over a number of years and better understand a vineyard block’s water needs. If a given vineyard block has a thinner soil and tends to dry out sooner, the grower would be aware of this and better respond with irrigation.

The third and last weather station visited that afternoon dealt with reference evapotranspiration value compared to surface renewal measurements. This station included a device from the ETgauge Company, plus four evaporation cups added by Battany.

“We get the standard weather station ET but also a water evaporation based ET. We’re trying to see which system could be a better fit.”

Weather tool answers

In closing, Battany shared the importance of weather tool development for the agricultural industry.

“Developing better tools and information for growers to base their decisions is one of the best things we can do to help make sure that everyone has the information needed to make the best possible decisions on water management,” he said.