Coordination
Zusammenfassung der Projektergebnisse
Water and energy fluxes close to the soil surface play an important role for the water and energy budget of terrestrial hydrosystems and determine distribution of water into surface and subsurface water. The fluxes are relevant for transport of water and nutrients to plants, for groundwater recharge, for ’loss’ of subsurface water by evaporation and for water quality. The work of MUSIS was focused on water and energy fluxes close to the soil surface. It was the basic hypothesis of MUSIS that these processes are strongly influenced by interfaces and their properties, namely i) the soil/atmosphere interface, ii) material interfaces on all relevant length scales and iii) the moving water air interface. The general approach of our work was to investigate the influence of different interfaces on the flow processes, to identify the important properties and phenomena and to derive modeling approaches that describe the movement of water in an averaged sense on a continuum scale including an upscaled description of the interface. The standard modeling approach for water movement on the continuum scale is the Richards equation, the mass balance for the water combined with the Buckingham-Darcy law for the flux. This standard model has well known deficits as it does not capture all phenomena of water flow in soils. It is known to have, with the classical coupling strategies to the atmosphere, limitations for the prediction of evaporation. It has also limitations for fast flow processes and for flow in heterogeneous media, in cases where the heterogeneity is not resolved, but captured by effective model parameters. Even under moderate flow conditions, the model does not capture, for example, slow redistribution of water on the pore scale or hysteretic behaviour for wetting or drying processes. The goal of MUSIS was to identify and quantify limiting conditions, to extend and improve the standard concept and to test alternatives. The team of MUSIS covered expertise of different methods. The general questions were approached with observations as well as with numerical models and theoretical methods. Different length scales were taken into account. In particular, processes were considered at the pore scale, where water-air interfaces could be observed or represented explicitly in a model. On the other hand, experiments were conducted and models were developed on a length scale of several centimeters (cluster scale), where the pore space is no longer resolved and the waterair interfaces are captured by volume percentage of water as state variable. Bridging this scale difference was a key question of MUSIS. The largest length scale considered in MUSIS is the plot scale, which covers several meters. With the restriction to these length scales, the work of MUSIS does not address length scales relevant for land surface modeling, directly. Nevertheless, the findings and modeling concepts derived in MUSIS are very relevant for the modeling approaches applied in land surface modeling. Different processes that were analyzed with this approach. An overarching topic in MUSIS was the evaporation flux from bare soil. The crossover from fluxes determined from atmospheric conditions to fluxes that are limited by breaking of the continuous liquid phase was related to textural properties of the soil, the soil roughness and the wind field properties. A second field was the infiltration of water into soils, where in particular the influence of structure on unstable fronts and the identification and handling of pressure distributions that are locally not at equilibrium were analyzed. Different modeling approaches for such situations were derived by different groups. As a third focus field, the drainage of soils was studied in particular for the situation that corner flow in dry parts become important. The onset of corner flow dominated fluxes was quantified and different upscaled modeling approaches were developed by the different groups. Another field was the transport of solutes in soils with changing flow directions. This topic was only addressed in the second funding period. The onset of situations where the flow dynamics influences the transport were quantified and upscaled modeling concepts for such situations were developed and tested.
