Methodological Issues in Combining Pores Micromorphometry and Hydraulic Functions in Soils.
Angelo Basile1, Giacomo Mele1, Roberto De Mascellis1, and Fabio Terribile2. (1) CNR ISAFOM, Via Patacca 85, Ercolano (Na), Italy, (2) Disspa Univ di Napoli Federico II, via Universita 100, Portici, Italy
Soil hydrology and soil micromorphology analyse pores in soils from very different viewpoints addressing different related properties, such as water retention, saturated and unsaturated hydraulic conductivity, sorptivity, etc. in the first case and counting, shape, size distribution, connectivity etc. in the second one. It is an old challenge of the scientific community the attempt to explain and relate soil hydraulic behaviour to the architecture of the soil porous system. For example, the water retention curve, experimentally obtained, can be explained by several models of the porous system. Among others, the simple bundle of parallel capillaries, the 2D lattice-type network, the 3D spherical/throats interconnected pores or the new bi-modal pore space coupled with three steady-state and laminar flow regimes. Moreover, the most applied methods of the analytical and numerical formula of the unsaturated hydraulic conductivity lye on the knowledge of the pore size distribution, mainly derived by the soil water retention curve. In such framework, the measurement and estimation of the tortuosity factor still remain a critical point. On the other hand, the study of soil structure by means of image analysis techniques is a well-established field of research since the seventies. Image analysis of soil thin sections and soil blocks have been mainly applied to the quantitative description of the soil aggregate and pore geometry and evaluation of soil physical qualities such as the effect of the tillage practices in many different soils. If the combined hydrological and micromorphological approach have to be efficiently used a major care on methodologies is very much needed. Issues such as spatial resolution of the techniques, algorithms for pores detection (object based vs raster based), functional vs descriptive analysis of the pore system must be addressed. Such methodological issues are presented through the analysis of the following selected case studies: - pore size distribution obtained from 2D image analysis measurements vs unsaturated hydraulic conductivity curve k(h) obtained by tension infiltrometer in a Vertisol; - 3D mechanical tomography (serial sectioning) and X-ray Micro CT vs soil water retention curve and unsaturated hydraulic conductivity obtained by Wind's evaporation method in a sandy Luvisol; - 2D pore size distribution analysis vs sorptivity in soil crusts formed after irrigation on a Vertisol; These examples have shown the power of using a 2D and 3D micromorphological characterization for an insight improving of the standard methodologies of both water retention and hydraulic conductivity measurements.