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Challenges in Soil Hydrological Modeling -- The Devil is in the Details ...

Presenting person: Prof. Dr. Wolfgang Durner, Leiter der Abteilung Bodenkunde und Bodenphysik, TU Braunschweig (Homepage)
Th. 2023-01-12

"Soil hydrological modeling" is the numerical simulation of the transport of water and associated heat, solutes, and gases in the near-surface (“vadose”) zone of the subsurface. Of particular interest, in addition to the state of the soil, are fluxes to groundwater and across the soil-plant-atmosphere continuum to the free atmosphere. The scale of interest ranges from centimeters to a few meters vertically, and extends horizontally from a few meters for point processes up to watersheds for regional studies.

Soil hydrology modeling is important for a variety of applications, notably in the context of increasing water scarcity and climate change around the world. Traditionally, simulations are done by the disciplines of soil physics or hydrology ("vadose zone hydrology”), which differ in their approach to scale. While soil physics applies bottom-up approaches based on pore-scale concepts to represent the behavior of water in porous media in a process-based manner and to infer the effective behavior at the scale of interest through integration, physical hydrology goes the other way, and tries to use statistics-based observations at the scale of interest to arrive at effective modeling formulations. Unfortunately, the closure of the scale gap from typical soil physics scales to the larger scales of interest has not been achieved to date. Provocatively put, hydrology begins at a larger scale than that of soil physics ends. Nevertheless, the effects of small-scale processes on large-scale behavior can be substantial, so that ignorance of certain small-scale processes will cause macroscopic errors. This can significantly impede scientific progress and distort fundamental considerations.

This will be illustrated by two examples dealing with "hydraulic functions of drying soils". One case study affects evaporation estimates. Here, we will show how the numerical simulation of evaporation is affected by the microscale process of "film flow", which is completely ignored in traditional hydrological models. The second example deals with high-voltage lines embedded in the ground that transport renewable energy from northern to southern Germany. The heating leads to drying of the surrounding soil in the vicinity of power cables. The simulated cable temperature depends on how the hydraulic properties are parameterized. In both cases, we find that the parameterization of hydraulic properties (admittedly a very special topic of soil hydrology) has a significant impact on large-scale processes.

*** invited by Efstathios Diamantopoulos, Soil Physics

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