A soil‐plant‐atmosphere model for transpiration and availability of soil water

In a soil‐plant‐atmosphere model the soil was divided into several layers with different root densities and soil water potentials, but the aboveground portion was represented by single values both of diffusive resistance and of plant water potential Ψ. The model used Cowan's theory for root uptake, Monteith's combination equation for vapor flux, constant internal resistance per unit of root length, and stomatal response to light, humidity, temperature, and Ψ. For forests, unstressed transpiration E u , defined as the transpiration that would occur if stomatal opening were not reduced by low Ψ, may be a more useful value than potential evapotranspiration. The model simulated field data from a hardwood forest in terms of both Ψ and stomatal behavior within a day and over a summer. Stomata had a threshold type of response to Ψ, with closure only occurring as Ψ approached a critical potential Ψ c . Internal resistance was high enough that when E u was high, Ψ became low enough to cause some stomatal closure and reduce transpiration even when soil was wet. As soil dried, potential gradient decreased because plant potential could not decline further than Ψ c . But internal resistance remained generally larger than rhizosphere resistance, so decrease in uptake with soil drying depended more on the decreasing soil‐plant gradient than on increasing rhizosphere resistance. Simple functions, either of total soil water content or of vertical root and soil water distribution, can be used to determine the reduction of transpiration below E u in hydrologic models.