Pore rigidity in structured soils : only a theoretical boundary condition for hydraulic properties?(INVITED PAPER,<SPECIAL ISSUE>SOIL AND LAND DEGRADATION)

The quantification of water fluxes and the identification of site-specific hydraulic properties are widely required for various research, economic or environmental purposes. Pore rigidity, as one of the major boundary conditions, is always assumed to exist. Under in situ conditions the validity of this assumption is questionable and strongly depends on climate, land use, soil type and management. Changes of water content or water potential will change porosity for structured or homogeneous soils irrespective of geological origin and clay mineralogy. Shrinkage curves show volume loss with decreasing matric potential due to drying and this was more pronounced the less aggregated and the wetter the soils. Water retention curves as well as the hydraulic conductivity vs. matric potential curves differed all the more from the origin with increasing drying; the less rigid were the pores. The smaller the structural shrinkage range and the more pronounced the proportional shrinkage, the greater the difference between the measured and the corrected curves concerning the water retention and the hydraulic conductivity/matric potential. The shrinkage-induced volume change can result in up to 12% volume loss which is interpreted as water loss if pore rigidity is assumed. Underestimating the occurrence of shrinkage may result in incorrect data for modelling hydraulic functions. These incorrect hydraulic functions also introduce a high uncertainty in the forecast of changes in hydraulic site properties due to climatic change. Non-rigidity of soils should therefore be more intensely investigated in future research.