Using Baseline SWP for Precise Interpretation

Allan Fulton, UCCE Water Resources Advisor, Tehama, Shasta, Glenn and Colusa Counties; and Luke Milliron, UCCE Orchards Advisor, Butte, Tehama and Glenn Counties.

Experienced Pressure Chambers Users

As a person gains experience with using the pressure chamber, they become more familiar with some complexities of measuring and interpreting orchard water status. While the direct measurement of plant water tension is appreciated, it does become apparent that readings do require careful interpretation. This is because SWP is affected by the weather and environmental conditions at the time measurements are made. Readings can vary from one day to the next as the weather changes, even if irrigation management and soil moisture are relatively stable. Cloudy, partly cloudy, smoky and unusually hot or cool days are examples of the types of variable conditions that can affect SWP measurements and interpretation. This has led to the development of the baseline concept for irrigation managers who want to more precisely interpret SWP readings for irrigation scheduling.

Baseline concept to distinguish weather from irrigation effects

The fully irrigated baseline SWP for any given day and time is defined as the SWP that is expected if soil moisture is abundant and not limiting transpiration under the prevailing temperature and humidity conditions. The fully irrigated baseline should be thought of as an important reference for interpreting your trees’ SWP readings, and NOT a target SWP reading to achieve and maintain in your orchard.

Baseline may or may not vary with crop species. For example, we use the same baseline conditions for almond and prune (table 1, end of article), but the baseline for walnut is very different (table 2, end of article). Baseline values are derived from mathematical models, and have been validated in field experiments in almond, prune, and walnut.

Estimating baseline conditions requires access to public or private weather databases that provide hourly temperature and relative humidity data, or an inexpensive, simple handheld instrument (fig. 1, e.g. thermo-hygrometer pen) that can measure temperature and relative humidity in the orchard at the time of taking SWP measurements. In our experience, weather information from databases outside of the orchard are likely to give higher temperatures and lower relative humidity than measurements taken inside an orchard with a simple handheld instrument. This will result in lower baseline (more negative or stressed) SWP estimates when using weather information from databases, instead of from the orchard.  Fortunately, the differences are usually small enough to not affect water management decisions.

In addition to tables 1 and 2 (end of this article), an online calculator of baseline SWP (utilizing CIMIS weather data) is also available through the Fruit and Nut Research and Information Center.

Example of simple thermo-hygrometer pen hung in lower tree canopy. There are many brands available online. In this example the walnut orchard temperature is 95.9°F and 36.0 percent relative humidity. Using table 2, this equates to a baseline SWP of approximately -5.2 bars.
Figure 1. Example of simple thermo-hygrometer pen hung in lower tree canopy. There are many brands available online. In this example the walnut orchard temperature is 95.9°F and 36.0 percent relative humidity. Using table 2, this equates to a baseline SWP of approximately -5.2 bars.

Example comparing baseline SWP to orchard measurements

Comparing orchard SWP measurements to fully irrigated baseline SWP estimates allows you to express your orchard’s SWP in a normalized unit of bars below baseline. A good example would be for past days in late May 2018 and early June 2018 when afternoon temperatures were highly variable. On the afternoon of Wednesday, May 30, afternoon high temperatures were on the order of 75 °F and corresponding afternoon relative humidity was on the order of 55%. Under these conditions, baseline in walnuts is -3.7 bars (table 2). Meanwhile, the orchard SWP measurements taken in a mature orchard averaged -5.0 bars, or 1.3 bars below baseline.

When additional pressure chamber measurements were taken in the same walnut orchard three days later, on Saturday, June 2 when the mid-afternoon temperature reached a high of 96 °F with corresponding relatively humidity of 20 percent, the walnut baseline read -5.7 bars. Meanwhile, measurements of SWP in the orchard averaged -7.2 bars or 1.5 bars below baseline.

More precise interpretation of SWP measurements

When the predicted baseline SWP and the orchard measurements are used together to evaluate orchard water status for both May 30 and June 2, it is evident that the increase in orchard SWP measurements from an average of -5.0 to -7.2 bars in four days is mostly caused by the changes in weather and less by plant-water availability. The change in orchard water status after normalizing for weather using this baseline method was from 1.3 to 1.5 bars over the three-day period (only 0.2 bars of additional water stress).

If this baseline concept is not used to adjust for weather, it is possible to over-react to the average midday SWP levels measured on June 2 of -7.2 bars and irrigate when it is not yet necessary. Other potential benefits of comparing fully irrigated baseline to the actual SWP measurements include identifying orchards that are steadily near or above the fully irrigated baseline the entire season. These orchards may be at more risk of eventual tree loss from excess water and resulting diseases. Using baseline estimates may also enable earlier use of a pressure chamber and SWP in the spring when weather is more variable.

Stem Water Potential Tables

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