Hysteresis Phenomenon and Threshold Water Potentials for Cranberry Irrigation.

Irrigation of cranberry can be monitored using tensiometers and appropriate setpoints has been identified at a value of matric potential of -7 kPa to initiate irrigation based on growth cabinet experiments under controlled conditions, and supported field data. However, it is theoretically expected that this setpoint may vary according to ET demand as well as the hysteretic soil behaviour for unsaturated hydraulic conductivity. This paper uses field data and a 2-D Hydrus model to investigate the sensitivity of cranberry irrigation setpoints to soil and environmental conditions. The modeling exercise confirmed the effect of the evapotranspiration demand on irrigation setpoints.  Ideal water potential seems to be in the range of -6 kPa at 1.6 mm/day of crop evapotranspiration to -4 kPa at 3.2  and 6 mm/day, if the soil was rewetted bottom up.  However, the threshold was much lower if the soil was on a drying curve (following a rainfall event for example), indicating an important effect of hysteresis on setpoint. Its estimation gave -12 kPa at 1.6 mm/day, -8 Kpa at 3.2 mm/day and -6 kPa at 6 mm/day. Experiments are ongoing  to check these setpoints at the field scale.