A physically based model concept for transport modelling in coupled hydrosystems

Hydrosystems are very complex systems with numerous processes occuring simultaneously at different spatial and temporal scales. In this paper we present the concept of a compartment approach for the analysis of coupled hydrosystems including heat and mass transport. In this concept, flow and transport processes are coupled via their compartment (or process domain) boundaries without giving up the computational necessities and optimisations for the numerical solution of each individual process. In this new approach, random walk particle tracking (RWPT) methods are integrated into the coupled hydrosystem analysis. We briefly introduce the RWPT method and the governing equations for water flow, heat, and mass transport in aquifers, soils and on surfaces. Flow processes are described by diffusion equations (Darcy equation for groundwater flow, Richards equation for flow in the unsaturated zone, and the diffusive wave approximation for overland flow) which are coupled by exchange fluxes. Transport processes are described by advection-diffusion equations and coupled with the exchange fluxes by advection. We present three application examples concerning flow, mass and heat transport in coupled hydrosystems based on Horton- and Dunne overland flow as well as on hyporheic flows.