Transpiration cools plants and thus, depends on climatic conditions - primarily air temperature, relative humidity and solar radiation. Transpiration increases when air temperature and solar radiation increase. Conversely, high humidity levels suppress transpiration. Wind enhances the transport of water vapor from plant leaves into the atmosphere; therefore, ET increases as wind intensifies. However, stomata may close for high winds, and transpiration may decrease.
Other factors affecting ET include plant species, growth stage and relative maturity; planting date and density; canopy characteristics and surface cover; soil water status and irrigation regime; and tillage practices and crop residue levels.
Effect of Residue - Crop residue can have a significant effect on evaporation of water from the soil surface. A University of Nebraska study found that plots with residue removed required 1.5 to 2.5 inches more of irrigation water to achieve the same yield as plots with residue on the surface. In addition, at the end of the growing season, plots with residue contained 1.5 inches more water in the top 4 feet of soil than plots with no residue cover, i.e., bare plots. Thus, residue on the soil surface could have saved 3 to 4 inches of irrigation water compared to bare soil. Water conservation from residue cover diminishes for very arid areas where the time between wetting events is long.
For example, a producer wants to estimate how much water his corn crop used last week. The corn is in the 10-leaf stage of growth, and an ETgage near the field shows reference crop water use of 2.4 inches last week. Estimating the water use of corn relies on the crop coefficient in Table 1. The result is:
* Portions of this article have been adapted from the Center Pivot Irrigation Handbook, 2012, by the Biological Systems Engineering Department and Extension at the University of Nebraska-Lincoln. Lincoln, Neb.
1 We gratefully acknowledge the contribution of the authors who are Professors and a Research Associate in Biological Systems Engineering at the University of Nebraska-Lincoln.
2 Pioneer Agronomy Research Manager, Johnston, Iowa.
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