Impact of plant water uptake strategy on soil moisture and evapotranspiration dynamics during drydown

Experiments have shown that plants can compensate for water stress in the upper, more densely rooted, soil layers by increasing the water uptake from deeper layers. By adapting root water uptake to water availability, plants are able to extend the period of unstressed transpiration. This strategy conflicts with the approach in many land surface schemes, where plant water uptake is treated as a static process. Here we derive expressions for the typical drydown trajectories of evapotranspiration and soil moisture for both strategies. We show that the maximum difference in evapotranspiration between the two strategies during drydown can exceed 50%. This in turn leads to a difference in root zone soil moisture of up to 25%. The results stress the importance of incorporating realistic root water uptake concepts in land surface schemes.