ISHS Acta Horticulturae 278: Symposium on Scheduling of Irrigation for Vegetable Crops under Field Condition
PLANT-ATMOSPHERE INTERACTIONS, EVAPOTRANSPIRATION, AND IRRIGATION SCHEDULING |
| Author: | T. C. Hsiao |
| Abstract:
This paper discusses the interactions between the crop and the atmosphere in determining water use and plant water status and their implications for possible improvements of irrigation management. Considered first is how evapotranspiration (ET) is determined by energy supply and the important of radiation absorbed by the crop field (net radiation). Warming or cooling of the field by the air mass overhead accounts for the difference between net radiation and ET when ET is expressed in energy units. Reference or potential ET, ETo, is a measure of the capacity of the atmosphere to supply energy and carry away water vapor. Crop coefficient (Kc), the ratio of crop ET to ETo, accounts for the effects of crop characteristics and soil surface wetness on ET, and can be considered as a description of the "wetness" and "roughness" of the field. Crop canopies with open stomata are effectively fully wet. Early in the season canopy covers the ground only partly so the field is often less than fully wet because of frequent drying of the exposed soil surface. The dependence of ET and Kc on canopy cover and irrigation or rainfall frequency is examined using a simulation model and illustrated with some experimental data. Much of the literature emphasizes the control of transpiration by stomata. That control may be overstated since it is usually observed in gas exchange chambers where the maintenance of constant leaf temperature eliminates energy supply as a limiting factor. Additionally, boundary layer conductance for crop canopies is smaller than that for individual leaves, lessening further the control of transpiration by epidermal (stomatal) conductance. The decoupling factor developed by Jarvis and McNaughton for the analysis of the extent of stomata control in different canopies is discussed. An example is given of a lack of control in a dense and short alfalfa canopy, until canopy conductance was reduced to a small fraction of the original value by water stress. The last section discusses soil-plant-atmosphere-continuum and the fact that plant water status is not a unique function of soil water status but is also dependent on transpiration rate and evaporative demand. This complicates the use of plant indicators for irrigation scheduling. Also elaborated on is how allowable depletion of soil water is altered by transpiration rate and evaporative demand. | |
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