A midday stem water potential reading says less on its own than it seems to. Here is what the number measures, why we report it as a distance below a fully watered vine, and what that distance did in Napa this month.
What stem water potential measures
A vine moves water from the soil to its leaves by pulling on it. As leaves lose water to the air, they pull on a continuous column of water running down the trunk to the roots. Stem water potential is how hard that column is being pulled.
The pull is a tension, so the number is negative. It is reported in bars, and one bar is roughly the pressure of the air at sea level (ten bars make one megapascal, the unit most research papers use). A reading close to zero means water is moving into the vine easily. The further below zero, the harder the vine is working to get it.
Growers have measured it for decades with a pressure chamber on a bagged leaf at midday. Our sensors measure the same quantity continuously, from a probe in the trunk.
Why a single reading is hard to interpret
The weather moves a reading as much as the soil does. On a hot, dry afternoon the air pulls water out of the leaves faster, so even a vine with all the water it could want reads further below zero than it would on a mild day.
So one reading mixes the weather, which nobody controls, with how much water the vine can get from the soil, which is the part an irrigation decision acts on.
The baseline is what a fully watered vine would read
Researchers separated the two by measuring fully irrigated plants across many kinds of weather and describing how their midday reading tracks the dryness of the air. For grapevines the reference is Williams and Baeza (2007). For prune, and by extension almond and other stone fruit, it is McCutchan and Shackel (1992).
That relationship is the baseline. Give it a block’s own temperature and humidity for the day, and it returns what a fully watered plant would have read at midday under that same weather.
In the Napa Valley vineyards we monitor, the baseline on September 3, a mild day with an average high of 68°F, came out at about −2.6 bars. On September 10, with highs near 98°F, it was about −5.7 bars. A fully watered vine would have read about 3.1 bars further below zero on the hot day, purely because of the heat.
Deviation from baseline is the number we report
The number on our dashboard and in our posts is the vine’s measured midday reading minus that baseline. We call it the deviation from baseline, and it is sometimes written as Δbar.
Zero means the vine reads exactly what a fully watered vine would that day, so it is keeping up with the weather. A negative number is how many bars further below zero it sits. Say a vine reads −9 bars at midday on a day when the baseline is −3 bars. It is 6 bars below baseline, and those 6 bars are the part the weather does not explain.
With the weather taken out, a deviation can be compared from one day to the next and from one block to another. A block 6 bars below baseline in a cool week and a block 6 bars below baseline in a heat wave are equally far behind. Two raw readings of −9 bars on those two days would describe very different vines.
A worked example from this month’s Napa heat
Early September brought Napa Valley a heat wave in the middle of harvest. At the weather stations matched to each vineyard, the average daily high went from 68°F on September 3 to 100°F on September 9, then eased back into the 80s.
| Date | Average high | Fully watered vine (baseline) | Our vines, deviation from baseline |
|---|---|---|---|
| September 3 | 68°F | −2.6 bars | −4.5 bars |
| September 7 | 89°F | −4.5 bars | −6.6 bars |
| September 9 | 100°F | −5.7 bars | −7.0 bars |
| September 10 | 98°F | −5.7 bars | −7.6 bars |
| September 12 | 82°F | −3.9 bars | −5.6 bars |
| September 14 | 86°F | −4.2 bars | −6.4 bars |
Read the right-hand column on its own. As the heat built, the vines fell further behind a fully watered vine every day, from 4.5 bars below baseline on September 3 to 7.6 bars below on September 10. When the heat broke they came back part of the way, to 5.6 bars below by September 12. By September 14 they were 6.4 bars below again.
Now look at the raw midday reading. Between September 3 and September 10 it fell about 6.2 bars, which looks alarming. But a fully watered vine would have fallen about 3.1 bars on the heat alone, so the other 3.1 bars is how much further the vines fell behind. A raw reading has no way to tell those two halves apart. The deviation separates them.
Some of that distance is a decision. Some wine grape growers are comfortable pushing vines further from baseline as the fruit ripens, and others keep them closer. The deviation does not say whether a block is where its grower wants it. It says where the block is, in a unit that means the same thing on a mild day and on a hot one.
Two numbers that look alike
Many guides and research papers give irrigation figures as absolute stem water potential, the raw midday reading. A deviation is a different measurement. On September 10 in Napa, a vine reading −12 bars at midday would have been about 6 bars below baseline. A deviation of −12 bars would describe a vine twice as far behind.
So before comparing a number to a guideline, check which of the two it uses. UC Cooperative Extension advisors Allan Fulton and Luke Milliron, for example, write their almond guidance in bars below baseline in Advanced SWP Interpretation in Almond.
Reading the number on your own blocks
Watch the direction over several days more than any one day, which can move with passing weather and ordinary scatter between sensors. Compare a block with itself before comparing it with its neighbor, since soil, rooting depth and irrigation history give each block its own normal distance from baseline.
And when a reading moves, look at the baseline first. If the baseline moved about as much as the reading did, the weather did it. If the reading moved further, the vine is falling behind or catching up.
FloraPulse plots each block’s midday reading against its own baseline every day, from a probe in the trunk. See what vineyards are reading this week.
About the data. Figures cover Napa Valley wine grape blocks where the same sensors reported on every day from September 3 to 14, 2026, averaged block by block. Weather comes from the station matched to each vineyard. Data is anonymized and aggregated by region.