Large effects of small scales: modelling non equilibrium in soil hydraulics

Soil hydraulics is a dominant constituent of ecosystemal site conditions. How the water is redistributed and stored determines the fertility of soils, the fate of pollutants, and the capacity of carbon storage of these systems. Although studied excessively in labs and monitored in big facilities like lysimeters and field instrumentations, the modelling of the water redistribution lacks a dominant feature: non equilibrated fast flows, where water enters an unsaturated soil and gets conducted fast through a  network of larger pores. This has consequences at the large scale: water can either bypass the rooting zone and be lost to plant production, or it can be stored in lower soil horizons and be preserved from soil evaporation and be available for plant transpiration. The systemic soil model BODIUM uses a new approach for modelling soil water, which is capable of reproducing these effects. By implicitely considering redistribution of water locally at the pore scale, the macroscopic behaviour follows the observed non-equilibrium dynamics and better matches field measurements. The work shows the water redistribution and its influence on the vapor exchange at the land surface for the regional water balance, both in modelling and in lysimeter measurement.