Modelling root water uptake: from root segments up to the field plot scale

Uptake of water by plant roots is a major component of the soil water balance and an accurate simulation of this process is a prerequisite to simulate soil water fluxes and transport processes in the soil profile accurately. A reduction in water uptake when soils dry out leads to water stress of the vegetation which has an impact on growth. Therefore, accurately simulating root water uptake is also important for predicting vegetation/crop growth and associated with it carbon assimilation. To improve the mechanistic description of this process, we developed a simulation model that simulates the flow process in the three-dimensional root system architecture and coupled it to a description of flow in the soil. With this model, the impact of root system architecture and of the hydraulic properties of root segments and of the soil just around roots on the uptake capacity by the root system can be assessed and accounted for. Processes like root water uptake compensation, i.e., an increased uptake from wetter soil layers when part of the root zone dries out, and hydraulic redistribution, i.e., exudation of water by the root system and transfer of water from wetter to drier zones in the soil profile via the root system, are directly simulated by the model and do not require further parameterization. However, the description of three-dimensional flow comes with a high computational cost and makes it impossible to simulate processes at the field and landscape scales. Therefore, we developed upscaling approaches that describe uptake without representing flow to and in individual root segments. We found that these approaches show similarities but also some differences with empirical and macroscopic uptake models that are currently used in one-dimensional soil water balance models. By upscaling of the local mechanistic flow process models, the parameters of the macroscopic models can be linked directly to soil and root system properties. This opens ways to link root water uptake models to plant development models and to include two way feedbacks between drought and plant growth in these models.

*** invited by BayCEER-member Efstathios Diamantopoulos

• BayCEER Colloquium Prof. Jan Vanderborght