Subsurface Transport Modelling

Chemicals applied to soils are either transported by runoff water flowing along inclined soil surfaces (surface transport) or transported within the soil by soil water flow (subsurface transport), downwards during infiltration or upwards during evaporation. Both surface and subsurface transport have to be considered to simulate terrestrial ecosystem dynamics.

Simulation models describing subsurface transport usually include sub-models to calculate soil water flow, soil heat transfer and soil solute transport. In variably saturated soils also the transport in the gaseous phase is considered. This mode of transport is often dominated by diffusion which is driven by concentration gradients, causing for example the exchange of oxygen, carbon dioxide and nitrous oxide between soil air and atmosphere.

Transported chemicals may originate from surface deposits such as applied fertilizers (mainly in agroecosystems) or from atmospheric depositions (mainly in forest ecosystems) and may be transported to the groundwater. The focus of our research is on the modelling of nitrate transport and nitrous oxide transport in agricultural and silvicultural soils to assess possible risks of groundwater and air pollution.

Nitrate Transport Modelling

Simulation of subsurface transport requires an adequate description of water flow, since many chemicals are mainly transported with soil water in which they are dissolved. Soil water flow dynamics is described based on the driving forces that result from weather conditions (e.g. precipitation, sunshine, wind speed, air humidity, air temperature) and land use practice (e.g. irrigation, cultivated crop, tillage).

During vegetation periods transpiration and possibly hydraulic redistribution in root systems may play a dominating role in soil water flow dynamics. In these cases water flow of the total soil-plant continuum needs to be considered.

Modelling Water Flow in the Soil-Plant Continuum

The estimation of root water uptake and water flow in plants is crucial to quantify transpiration and hence the water exchange between land surface and atmosphere. In particular the soil water extraction by plant roots which provides the water supply of plants is a highly dynamic and non-linear process interacting with soil transport processes that are mainly determined by the natural soil variability at different scales.

To better consider this root-soil interaction we extended and further developed a finite element tree hydro-dynamics model based on the one-dimensional (1D) porous media equation. This is achieved by including in addition to the explicit three-dimensional (3D) architectural representation of the tree crown a corresponding 3D characterisation of the root system.
This 1D xylem water flow model was then coupled to a soil water flow model derived also from the 1D porous media equation. The model is able to correctly describe transpiration and soil water flow. Compared to a fully 3D model the 1D porous media approach provides a computationally efficient alternative, able to reproduce the main mechanisms of plant hydro-dynamics including root water uptake from soil and hydraulic redistribution.

At the present state of development the model is supposed to be already useful to simulate the impact of differences in plant architecture and soil properties on transpiration and water uptake under various climatic conditions, in particular considering differences of leaf area and root area distribution or of root and shoot branching. The model may also be applied to better describe differences in stand water budgets between mixed forest stands of different species composition.

Projects (finished)

  • Water relations of beech and interacting vegetation in beech-dominated forests (DFG FOR 788 Beech Research Group, sub-project TP04)
  • The interrelation of carbon and water balance in beech-dominated forests – from leaf level water use efficiency to stand and area scale assessments (DFG PAK 539, sub-project TP1)
  • Modelling carbon and nutrient turnover of different tree species within mixed stands (DFG GRK 1086, sub-project C3a)