Corn aerodynamic and canopy surface resistances and their role in sprinkler irrigation efficiency



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Texas Tech University


The development of improved methods for estimating evapotranspiration (ET) rates is important to efficient utilization of declining water supplies. This study was conducted to evaluate commonly used procedures for determining crop aerodynamic and canopy resistances used to calculate ET. It also assessed how changes in these resistances and in the crop microclimate during and after a sprinkler irrigation event could reduce transpiration and thus partially offset net irrigation application losses.

Corn {Zea mays L.) was grown at Bushland, TX, in 1990 in a 10-ha field in which two 3x3x2.3-m weighing lysimeters containing Pullman clay loam are located. Relevant micrometeorological measurements were made at each lysimeter. Transpiration (T) was measured on individual plants in the lysimeters using heat balance sap flow gauges, which allowed a separate calculation of plant surface resistance. Aerodynamic and canopy surface resistances were measured using energy balance techniques.

Aerodynamic resistance calculation methods with very limited to no stability corrections produced the best linear fit when comparing calculated sensible heat flux with sensible heat flux determined as a residual of the energy balance equation.

Differences between surface resistances to vapor loss based on both soil and plant vapor loss (ET) and on plant vapor loss alone (T), especially early in the morning and late in the evening, indicated that soil surface resistance should not be ignored when estimating canopy surface resistance of a well-irrigated crop. Both plant (r ) and canopy surface (r^) resistances had seasonal trends, with daily minimum values increasing from 30-45 s/m for r and 40-55 s/m for r at mid-vegetative growth stage to 70-80 s/m for r^, and 100-110 s/m for r at full dent growth stage.

Sharply reduced transpiration rate and changes in the crop energy balance during sprinkler irrigation limited net irrigation application losses due to evaporation of canopy-intercepted water to <10% for most situations. Irrigation water applied only on the soil surface did not alter the microclimate or suppress transpiration. During a sprinkler irrigation event, vapor pressure deficit declined as much as 2 kPa, air temperature 5 °C, and canopy temperature 8.1 °C, with pre-irrigation levels conditions returning within two hours.