DETAILS ABOUT THE MODEL
The level of plant water potential at any particular moment depends on both soil and weather conditions. In fully irrigated plants (i.e., plants under non-limiting soil water conditions) there is a linear relationship between midday SWP (and in the case of grapevine, LWP) and midday air vapor pressure deficit (VPD) in prune (McCutchan and Shackel, 1992), almond (Shackel, 2011), grapevines (Williams and Baeza, 2007), and walnut (Shackel, unpublished data). The Model section of this website calculates a reference water potential estimate for prune, almond, grapevines, and walnut using this relationship and hourly VPD data from CIMIS weather stations located throughout the state. Air VPD for the hours (s) of water potential measurement (optimally at midday, 1:00 – 3:00), is obtained from a nearby CIMIS (http://www.cimis.water.ca.gov) or other weather station, and the non-limiting soil water value corresponding to this VPD is calculated as a reference or “baseline” water potential value. If your measured water potential is reasonably close to this baseline value (within 1 bar or 0.1 MPa), then the soil is close to a non-limiting water condition, and no further improvement in water potential can be expected from irrigation.
The need for irrigation depends on the plant species and the desired outcome, and for some crops, research-based recommendations or “target” levels of SWP are available. For instance, a progression from mild (-0.6 MPa SWP) to moderate (-1.5 MPa SWP) stress over the season has been found to reduce drying costs and improve economic yield in prunes (Shackel et al, 2000) and moderate levels of stress (-1.4 to -1.8 MPa SWP) during hull split have been recommended for the control of hull rot in almonds (Teviotdale et al., 2011). Approximate values for mild, moderate, and severe stress, and the tree symptoms associated with these stress levels, are summarized for prune, almond and walnut in the Data Interpretation section of this website.