Theoretical and experimental multiscale investigation of dynamic nonequilibrium water flow in soils - DynSoilWater

• Applicants: Professor Dr. Efstathios Diamantopoulos; Dr. Sascha Iden; Professor Dr. Ilhan Özgen
• Subject Area: Soil Sciences
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 566343966

The Richards equation (RE) is the de facto standard model for describing water flow in soils. It is based on the assumption of a local equilibrium between soil water content and soil water potential. However, soil water flow often shows so-called "dynamic effects" or “dynamic non-equilibrium” (DNE). DNE leads to a decoupling of water content and water potential, to relaxation phenomena of these two state variables, and to errors in the determination of hydraulic properties of soils. At present, it is not possible to predict when DNE will occur, nor to what extent it will occur. While it is generally assumed that DNE only occurs in laboratory experiments with rapidly changing boundary conditions, our research shows that DNE is also important in slow laboratory experiments (e.g., evaporation, transpiration) and under field conditions. The objective of this project is to conduct multiscale experimental investigations of DNE under different boundary conditions, to compare different DNE models reflecting competing theories, and, if possible, to develop a model that effectively describes DNE at multiple spatial and temporal scales. To achieve these goals, we will conduct a series of experiments to investigate DNE in different textures (sand vs. loam), in structured and unstructured soil (repacked vs. undisturbed), at different spatial scales (cm, dm, and m), and under different boundary conditions. Our primary goal is to generalize DNE observations, quantify the extent of DNE in each experiment, and use modern visualization techniques to better understand the underlying processes. In addition, we will compare existing theories on DNE and the models derived from them and their predictions of system behavior. For this purpose, different DNE models will be implemented in numerical codes using modern, mass-conservative schemes. The models will be extended to account for capillary hysteresis and a recently proposed method to parameterize the hydraulic conductivity function under DNE. We will perform both forward and inverse simulations to identify which observations can be accurately explained by each theory, whether one theory can explain the DNE phenomena observed in all experiments, and to identify optimal DNE model parameters. By integrating the experimental data, theories and models, we aim to develop an improved theory to describe water flow in soils accounting for DNE and to quantify the errors introduced by applying the RE to field-scale variably-saturated water fluxes under atmospheric boundary conditions and varying groundwater tables.

• DFG Programme: Research Grants