With mounting pressure on growers to account for their precious water use, irrigation tools are relevant now more than ever.  Daniele Zaccaria, UC Irrigation Specialist, brought his impressive International team of student researchers and scientists to the foothills August 10 to discuss their study findings and grower friendly irrigation tools at a UCCE field day.  About twenty growers made the trip over to meet with us at Safari vineyards in Pilot Hill, where we have been gathering data for 4 years on the effect of aspect and slope on vine water use.

Tools for scheduling irrigation fall into three categories: we can monitor the weather, the soil or the plant.  Monitoring weather includes the classic estimation of crop evapotranspiration (ET) using reference ET data (ET₀), based on the amount of water an area of well-watered grass uses, and multiplying that number by a suitable crop coefficient (K_c) to determine how much water a crop has used during any given time period.  The idea being, if you can estimate what a crop has used, you will know how much water to replace with irrigation. The reference ET₀ depends on temperature and other local weather factors and is available online (for free, you only need to create a login) from 

CIMIS (CA. Irrigation Management Information System).  CIMIS stations measure the grass ET in different locations, there are currently 2 CIMIS stations in El Dorado County (Camino and Diamond Springs) and one in Plymouth. 

If we were growing corn or apples or almonds or practically anything else it would end there; you would have the ET₀ from CIMIS and a crop coefficient given based on research and you would multiply the two and then, voila!, you would have an estimate of how much water your crop has used.  But alas, winegrape production is more complicated.  This is because winegrapes, as most of you know, are not irrigated to "full ET".  We typically impose an irrigation deficit to winegrapes at key times of the year to increase the quality of the crop and intensify flavors in the berry skins.  So, for winegrapes, we multiply the reference ET₀ by a crop coefficient and then by yet another factor for "stress".  The stress factor is one that a grower determines (say, 70%).

The final equation is: ET_{grapes-stress}=ET₀ * K_c-grapes* K_stress

Furthermore, the crop coefficient is not one number, but a number multiplied by the amount of shading present.  This grape coefficient was developed based on research conducted in the Valley (where it is FLAT) by our very own UC Irrigation Specialist Larry Williams and Jim Ayars, in which they found a good correlation between the percent shading and the crop coefficient.  The formula they published in 2005 is K_{c grapes}= 0.017 * % shaded area.

But how do we get shaded area? And does this formula really work on sloped vineyards?  While many growers may eyeball

percent shading, Kristen Shapiro and Sloane Rice demonstrated the "Paso Panel" developed by my UCCE colleague and gadget whiz by his own right Mark Battany. Kristen and Sloane have been using Mark's Paso Panel to estimate percent shading in this sloped vineyard site, and have so far found that the Williams/Ayars model to predict K_c does not seem to work accurately on hillside vineyards.

There are several ways we can monitor the plant to determine vine water needs.  One of the most tried and true is to measure stem or leaf water potential using a pressure chamber.  Guilia Marino, a plant physiologist who hails from Sicily, demonstrated using a pressure chamber to measure vine stress.  To use a pressure chamber, a leaf is bagged (when measuring STEM water potential), allowed to equilibrate for at least 10 minutes, then the petiole is cut and the cut bagged leaf is placed in a chamber.  The chamber is sealed, pressure is applied, and a hand lens is used to note the pressure at which water is pressed out of the cut petiole.  Guilia likened using the pressure chamber to taking the blood pressure of the vine.  If the vine is under alot of stress, it will take more pressure (higher negative reading) to force water out of the xylem of the cut petiole.  I've done some work using the pressure chamber and I think it is an excellent tool for growers to use in the foothills, because it avoids the variability we frequently see in soils and focuses on what the plant is actually receiving.

Soil moisture sensors are, perhaps, the oldest technology we have and if placed properly, can give us good information about soil conditions in their location.  Pedro Lima of Brazil has been using Watermark sensors to measure soil moisture on the two slopes at Safari vineyards at several depths.  The sensors consist of porous media that require good contact with the soil, so proper installation is critical. Data from the sensors is uploaded to a station so that the sensors themselves remain undisturbed.  The sensor data has shown different tension, likely because the soils on the two slopes are different.  One other important note: beware of rodents chewing your sensor wires! Pedro noted that something (hmmm) was nibbling on the sensor wires which disrupted data collection.  I think these are great tools for young vineyards, when the roots haven't yet penetrated to greater depths.

But ET isn't only about crop transpiration.  Camilo Souto demonstrated his work measuring soil evaporation (the "E" in ET) using buried canisters and weighing them periodically to determine the component of soil evaporation.  Camilo noted that the top 4 inches of soil is where most evaporation occurs and he has estimated that evaporation of moisture from the soil accounts for 20-30% of seasonal ET!

The Zaccaria Team ended our field day by discussing how critically important it is for growers to begin using a tool, if they are not already, to better understand water use in their own vineyards.  Not only for improving winegrape quality, but as competition for water resources increases, the cost of water will surely continue to rise (consider the cost of running a pump) and with it the increase in regulations that require growers to demonstrate their good stewardship of water.  Our growing future depends on it.