Written by Matt Jones, Staff Research Associate, UCCE Merced County Determining the appropriate time to irrigate is among the most critical tasks facing growers and farm managers. Traditionally, growers have relied on orchard water budgets using ET and CIMIS stations, and monitoring soil moisture levels to develop irrigation schedules. For in-depth explanations of these methods, consult the UC ANR Almond Production Manual. However, these methods only indirectly measure water status of trees in an orchard. To directly and quantitatively measure tree water status requires the use of a pressure chamber. One of the quantities a pressure chamber can measure is stem water potential (SWP), or the amount of tension in the water column as it is pulled from the soil and through the plant. For a complete guide on pressure chamber use and plant-water relations, see UC ANR Publication# 8503. But what do these numbers mean, and how can they be used in irrigation management? Interpreting these numbers depends on temperature, relative humidity, and the degree and type of water stress you are trying to manage with an irrigation set. Knowing temperature and relative humidity will establish what normal or ‘baseline’ pressure chamber values (in bars) would be for an orchard that is fully irrigated. Baseline values can be  precisely determined by looking at table 14 in ANR Pub 8503. However, a rough estimate baseline (in almond) is to divide temperature by ten. For example, if it is 100 F, then your baseline value is -10 bars. The values measured in the field and how they deviate from baseline will determine the degree of tree water stress, and irrigation timing. If aiming for a fully irrigated, mature orchard, then irrigate when the measured SWP values are 4 bars lower (more negative) than the baseline. For example, if the baseline value