Flow Patterns During Extended Saturated Flow in Two, Undisturbed Swelling Clay Soils with Different Macrostructures

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Abstract
Large, undisturbed soil columns with an average height of 30 cm, a diameter of 25 cm, and with two different types of macrostructure were sampled in moist clay soil. Type 1 had vertical wormchannels and “rough” peds; type 2 had no worm channels and “smooth” peds. Flow patterns were characterized by repeatedly measuring Ksat and chloride breakthrough curves. Effective pore size distributions were calculated from the latter with the Klinkenberg procedure.Initial chloride breakthrough was most rapid in soil columns of type 1 after displacement of < 1 vol. % of the initial water, whereas 3 vol. % was displaced in type 2. Rapid chloride breakthrough was associated with a high volume of immobile water (60 vol. % and 2 vol. % for types 1 and 2 respectively). These phenomena can be explained by the presence of relatively large, continuous pores (particularly in type 1), that occupy only a small volume, as was confirmed by micromorphometric analysis and calculations of effective pore diameters. Application of a gypsum crust on soil columns of type 1 erased the differences with type 2. Crusts induced pressure potentials of __5 mbar, thus excluding the largest pores only. Extended ponding and associated continued swelling of peds did not change flow phenomena in type 1; in type 2, Ksat, the volume of immobile water and the effective pore diameters gradually decreased. Equivalent pore diameters, calculated from the soil moisture characteristic, were quite different from the effective diameters and could not be used to independently predict the breakthrough curves. Direct measurement of the latter and empirical extrapolation to soils with identical texture, mineralogy, and macrostructure is recommended for field work because replicate columns, sampled in a large geographical area and with the two types of macrostructure, showed significantly different flow phenomena.