ACCOUNTING FOR HUMIDITY IN CANOPY-TEMPERATURE-CONTROLLED IRRIGATION SCHEDULING

High moisture content in the air surrounding crop canopies can reduce transpiration and increase canopy temperature (T-c) independently of s oil moisture. Humid conditions can affect the accuracy of irrigation s ignals produced by a canopy-temperature-based irrigation scheduling pr ocedure 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 cano py 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 w hether canopy temperatures that exceed the T-o were affected by high h umidity. The LRH was computed from T-a and Delta T, denoted as T-o-T-w b, where T-wb* is the highest wet bulb temperature that does not incr ease 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 re duced the simulated average TT value by 27%, 51%, and 69%, respectivel y, for Delta T values between 3 degrees C and 5 degrees C. This same L RH reduced the TT values by 16%, 32% and 36%, respectively, in a semia rid climate. The LRH criterion had no effect on the average TT value i n the arid climate. Estimated TT values had the lowest variability amo ng years for a Delta T value of 4 degrees C in the humid and semiarid climates. A generalized curve described the TT versus Delta T relation ship across a wide spectrum of climates. The LRH procedure produced co nsistent adjustments to TT; however, further refinements may be needed to improve the accuracy of estimating daily TT when weather condition s are highly variable. (C) 1997 Elsevier Science B.V.