2019 ASHS Annual Conference

Deficit irrigation has been shown to offer a number of benefits for tree fruit production and orchard upkeep. To most effectively implement this strategy, accurate real-time assessment of tree hydration is necessary to prevent crop losses while maximizing benefits. Sap flow sensors have been used extensively to estimate transpiration, but accurate calibration can be challenging, requiring large amounts of time and expertise. The use of these sensors for determination of sap velocity, however can be easily implemented. Sap velocity has been shown to be well correlated with evaporative demand. Short reference Penman-Monteith is widely reported and utilized with crop coefficients for irrigation scheduling but is not sensitive enough for precision irrigation scheduling alone. We propose an index of tree hydration for use in deficit irrigation scheduling using a ratio of sap velocity measurements to modeled evaporative demand. Initial comparisons of sap velocity measurements to short reference Penman-Monteith modeled evapotranspiration under sunny conditions were highly correlated (r² = 0.95) on an hourly basis and well correlated when integrated into a daily time interval (r² = 0.71). Under these conditions the proposed index deviated from average by roughly 4% in a well-watered tree. However, modeled evapotranspiration declined faster in relation to measured sap velocity during days with high cloud cover, causing the index to spike. Estimates of evapotranspiration based on short reference Penman-Monteith are highly sensitive to net incoming radiation, while transpiration in tall discontinuous canopies has been reported to be strongly coupled with bulk air parameters. Further studies are needed to determine whether this limitation in the model will affect the viability of the proposed index for deficit irrigation scheduling.