Irrigation rates and amounts are essentially equivalent to transpiration rates and amounts for most plants assuming there are no evaporative losses from the soil or potting mix. The primary purpose of this study was to evaluate the feasibility of accurately measuring and predicting transpiration rates of potted Acer rubrum (Red maple) trees in proportion to solar radiation, vapor pressure deficit (VPD), or a combination of both in the Combination Model.
The potted six-foot-tall trees were grown in a high-stress greenhouse environment. Transpiration was measured for 15 days with a highly accurate (plus/minus one gram) weighing scales. The Combination Model provided the best predictions of transpiration for the plants used in this study with a regression coefficient of 0.931. The next best predictors of transpiration were VPD with a coefficient of 0.8608 and solar radiation with a coefficient of 0.5904.
Transpiration rates averaged 470 gr. per tree per day for sunny days and 420 gr. per tree per day for partly cloudy days. The canopy resistances used in the Combination Equation were 50 seconds per meter for internal, 70 seconds per meter for external, and 200 seconds per meter for radiation resistance. The computer-controlled drip-irrigation system used in this research was accurately triggered by soil tensiometers. Set limits for soil moisture tension were 8 kiloPascals to start the irrigation and 1.6 kiloPascals to terminate it.