Soil-moisture limits on plant uptake: An upscaled relationship for water-limited ecosystems

Transpiration in water-limited ecosystems is controlled by the availability of soil moisture. Rain events in these natural environments may only partially wet the root zone, leading to a heterogeneous distribution of available water. This spatial variability coupled with the non-linearity of the function relating local uptake to local saturation produces a non-unique relationship when these quantities are scaled up. This work proposes a simple multi-valued relationship between plant transpiration and average root-zone saturation predicated on the distinct spatial patterns of wetting and drying. Predictions of daily transpiration from an upscaled model that uses this relationship match closely those from a vertically resolved model that employs an Ohm's law analogy for plant uptake.