EFFECTIVE POROSITY AND FLOW-RATE WITH INFILTRATION AT LOW TENSIONS INTO A WELL-STRUCTURED SUBSOIL

A combination of tension infiltrometers and dye tracers was used to in vestigate the extent and nature of water movement at low tensions thro ugh the well-structured subsoil of Ships clay (very-fine, mixed, therm ic, Chromic Hapluderts). Dyed-water was placed in the reservoirs of te nsion infiltrometers and allowed to infiltrate into the soil under sev en tension sequences to separate different size pores effective in tra nsmitting water. Flow patterns were then revealed by exposing dye-stai ned soil after each infiltration sequence. The results showed that, wh en water was supplied at tensions <24 cm, the effective porosity (frac tion of pores that were stained) in this subsoil was primarily macro- and mesoporosity, which constituted about 5% of the total soil porosit y. Under flow, at 0-cm tension, macropores greater than or equal to 0. 5 mm and mesopores from 0.06 to 0.5 mm (radius for cylindrical pores o r width for planar pores) contributed about 89% and 10% of the total w ater flux, respectively. Micropores <0.06 mm contributed the remaining 1% of the total water flux, but constituted about 95% of the total so il porosity. Dye stain patterns showed that water pow at tensions <24 cm in this structured subsoil was primarily controlled by slickenside fissures, root channels, and vertical fissures. The nonuniform flow pa ttern caused water to penetrate to depths as much as II times deeper t han expected from a Green-Ampt model that considered the total soil po rosity to be active in transmitting water By replacing the air-filled porosity in the Green-Ampt model with the active macro- plus mesoporos ity, calculated wetting front depths were close to the observed maximu m dye depths.