Theoretical Analysis of Soil and Plant Traits Influencing Daily Plant Water Flux on Drying Soils

Experimental evidence has been accumulating for a number of years that daily transpiration rates respond to the progressive drying of soil in a similar manner across a wide range of conditions when expressed as a function of available volumetric soil water content. There is little decrease in gas exchange until only about one-third of the available soil water remains in the soil, and then gas exchange decreases in a nearly linear manner until the available soil water is exhausted. There exists no theoretical derivation that explicitly provides a basis for explaining the consistency in this response observed over a wide range of conditions. A relatively simple derivation is presented by defining plant water flux on drying soil relative to a plant on well-watered soil and by examining the response in the relevant range of soil volumetric water content. This derivation resulted in a relatively simple expression that predicted daily transpiration rate response to drying soil that was consistent with experimental observations. Further, the results of this analysis showed the response was essentially independent of root length density, transpiration rate, and soil depth. Also, large decreases in soil hydraulic conductivity as the soil dries had only modest influences on the daily transpiration rate response. Expression of relative transpiration as a function of volumetric soil water content available to support transpiration also minimized the influence of soil texture on the overall response of water flux to drying soil. The derivation offers a theoretical basis to explain the stability in daily transpiration response to drying soil that has been observed over a wide range of conditions.