Soil Moisture Availability for Transpiration

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Abstract
Potential transpiration is a measure of the rate at which water can be transmitted from evaporation sites in plant leaves to the atmosphere. Potential soil‐moisture availability is defined as a measure of the capacity of a soil to transmit water to a root site. A differential equation is presented describing radial flow of soil moisture to a single vertical sink (root) in an infinite soil mass which is initially at a uniform moisture content. The relationship between moisture content and diffusivity for the soils studied may be represented by an exponential function. A numerical solution of the differential equation is used to determine the soil‐moisture flux. The results show that for specific soils the decrease in soil moisture with time occurs mainly in the immediate vicinity of the sink. Moisture flux increases with initial moisture content but is essentially time independent. In natural systems the flux would probably decrease with time because of multiple root interference. At large soil‐moisture contents, actual transpiration is limited by and equivalent to potential transpiration. At small soil‐moisture contents, actual transpiration is limited by and equivalent to potential soil‐moisture availability. (Key words: Evapotranspiration; plants; soil moisture)

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