Dynamic modeling of tension-controlled irrigation of container-grown nursery plants

A mathematical model of a closed-loop feedback control system for controlling microirrigation of container-grown nursery plants was developed and tested. Moisture tension in the container medium was the control variable. The system's characteristic transfer function parameters included a time delay (infiltration time), on-off control (irrigation pump), pumping rate, mass of potting medium, moisture tension as a function of potting medium moisture, and a tensiometer response function. Calibration and validation of the model was done using experimental data collected for Red Sunset red maple (Acer rubrum 'Red Sunset') during late summer of 1997. A time-varying, vapor pressure deficit/evapotranspiration transfer function, ETVPD, was used to estimate the plant's rate of removal of water from the root medium (R-2 = 0.684). The transfer function underestimated total irrigation water by 25% and overestimated irrigation frequency by 37.5% (11 vs. 8 cycles) during the 14-day experimental validation period. Major differences between experimental validation and simulated results were attributed to the accuracy of the ETVPD model and the use of zero infiltration time (tau(DT)). Key factors identified as directly affecting stability and accuracy of tensiometer-controlled irrigation, in addition to the identified system's parameters, were size of the plant root system and placement of the tensiometer(s). Results showed that only 18% of the total soil medium mass was affecting the systems response function based on the experimental setup of one tensiometer in the potting medium.