Curt Pierce, Irrigation and Water Resources Advisor, Glenn, Tehama, Colusa, and Shasta Counties

If you are using a pressure chamber or FloraPulse sensors to manage your prune irrigation, you may be interested in our new online tool for stem water potential (SWP) measurements. Tables have been developed and published by UC to determine baseline SWP values, which depends not only on crop, but temperature and relative humidity (RH) at the location where SWP samples are being taken. Once you cross-reference your temperature and RH on the appropriate table, you can determine the baseline value for that sample and compare the SWP from the pressure chamber to it to determine if it is time to start irrigating.

Now, we have created an online calculator that you can use instead of the tables. Simply 1) select your crop, 2) enter the temperature and RH, and 3) enter the SWP reading you get from your sample. The tool will output both the baseline value, and your sample’s deviation from it. In the example shown in the screenshot (Figure 1), we had a pressure chamber reading of -7 bars, and a baseline of -5.7 bars. The deviation from baseline is -1.3 bars, so the tool displays that your pressure chamber reading is 1.3 “bars below baseline.”

Figure 1. The new online SWP calculator at https://www.sacvalleyorchards.com/swp-baseline-calculator-tool/ provides both baseline and bars below baseline values for SWP samples provided with temperature and relative humidity data

While we no longer recommend using SWP to determine start of irrigation in prune, knowing when your trees have begun deviating from baseline SWP values can still be helpful. For more detailed information on recommendations for start of irrigation in prune, please refer to the article Determining when to start irrigation in prune.