Accounting for humidity in canopy-temperature-controlled irrigation scheduling

Abstract

High moisture content in the air surrounding crop canopies can reduce transpiration and increase canopy temperature ( T c ) independently of soil moisture. Humid conditions can affect the accuracy of irrigation signals produced by a canopy-temperature-based irrigation scheduling procedure that uses a time threshold (TT), which is the daily summation of time above the temperature threshold ( T o) defined as the midpoint of the crop's optimum temperature range. Because historical crop canopy temperature data were unavailable, an energy balance model was used to simulate time threshold values for different climates. A limiting relative humidity (LRH) algorithm was added to the model to estimate whether canopy temperatures that exceed the ( T o) were affected by high humidity. The LRH was computed from T a and δ T, denoted as ( T o) - T wb ∗ , where T wb ∗ is the highest wet bulb temperature that does not increase T c. Time periods of restricted transpiration were identified by calculating ambient relative humidity (RH) and comparing it to the LRH value. If RH > LRH, canopy temperature was assumed to be increased by a reduction in transpiration. In a humid climate the LRH criterion reduced the simulated average TT value by 27%, 51%, and 69%, respectively, for δ T values between 3°C and 5°C. This same LRH reduced the TT values by 16%, 32% and 36%, respectively, in a semiarid climate. The LRH criterion had no effect on the average TT value in the and climate. Estimated TT values had the lowest variability among years for a AT value of 4°C in the humid and semiarid climates. A generalized curve described the TT versus ΔT relationship across a wide spectrum of climates. The LRH procedure produced consistent adjustments to TT; however, further refinements may be needed to improve the accuracy of estimating daily TT when weather conditions are highly variable.