Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 23, 1997
Publication Date: N/A
Interpretive Summary: A canopy temperature based irrigation scheduling procedure uses daily accumulation of time above a temperature threshold (TT) to decide when irrigation is needed. The TT is computed by a simulation model but it does not predict when canopy temperatures are influenced by high humidity which slows plant cooling and increases the plant temperature. A method for identifying periods when plant cooling is restricted by humidity was developed using wet bulb temperatures to calculate when humidity limited plant cooling. Humidity reduced the time threshold by up to 67% in a humid climate, by 36% in a semi-arid climate, but had no effect in an arid environment. The wet bulb temperature procedure for identifying periods of humidity limited plant cooling produced reasonable adjustments to the time threshold which will improve irrigation scheduling accuracy across a broader spectrum of environments.
Technical Abstract: A canopy temperature based irrigation scheduling procedure uses daily accumulation of time above a temperature threshold (TT) to provide sensitivity to local climatic conditions. The TT are computed from a model based on an energy balance of the crop. However, there is no explicit relationship to determine whether canopy temperatures that exceed the temperature threshold are being influenced by high humidity conditions that interfere with transpirational cooling. A procedure for identifying periods of restrictive transpiration based on the degree of wet bulb depression below the temperature threshold was used. Wet bulb depressions were used to calculate limiting relative humidity (LRH) conditions. The effect of 1 deg C to 5 deg C wet bulb depressions on the accumulations of time on the TT was simulated using local environmental data for three locations representing humid, semi-arid, and arid conditions. In the humid climate the LRH restriction reduced the simulated average TT value by 23%, 49%, and 67%, respectively, for wet bulb depressions of 3 deg C, 4 deg C, and 5 deg C. The same LRH reduced the TT values by 16%, 32%, and 36% in a semi-arid climate. The LRH criterion had no effect on the average TT value in the arid environment. The general shape of the TT-wet bulb depression relationship begins with a horizontal segment, where wet bulb depression has no effect on TT, followed by a downward sloping segment as LRH eliminates more time periods from adding to the TT value. The LRH procedure for identifying periods of humidity that can affect canopy temperature produced reasonable adjustments to TT; however, further refinements may be needed to accommodate a broad spectrum of environmental situations.