LOW-TENSION WATER-FLOW IN STRUCTURED SOILS

Water transport through structured clayey soils may be prone to by-pas s flow, a mechanism that may lead to rapid transport of contaminants t o ground water. To quantify the significance of low-tension water flow in structured soils, apparent steady-state infiltration rates at wate r potentials from -0.24 to 0 m were measured using tension infiltromet ers on 18 soils of varying texture and structure. Each infiltration me asurement was conducted sequentially at -0.24, -0.12, -0.06, -0.03, -0 .02, -0.01, and 0 m supply potentials (Psi(supply)), all at the same s oil location, to separate different size pores effective in transmitti ng water. Results from 96 soil horizons showed that 76 +/- 18% (mean /- SD) of the water fluxes at Psi(supply) = 0 m (total water flux) wer e transmitted through macropores (active at Psi(supply) greater than o r equal to -0.03 m), although macropores usually constituted a small p ortion of a soil's total porosity. Mesopores (active at Psi(supply) gr eater than or equal to -0.24 m) contributed 19 +/- 13% of total water flux. Micropores dominated the soils' total porosities, but generally contributed < 10% of the total water flux. Macropores and mesopores sh owed greater influence on water flow in sands at Psi(supply) > -0.24 m . Values of soil macroscopic lambda(c) and microscopic lambda(m) capil lary length scales were determined from the change in infiltration rat es with Psi(supply). Values of lambda(c), a hydraulic conductivity-wei ghted mean capillary water potential, were greater for sands (63 mm) t han loams (50 mm), and greater for loams than days (22 mm). Values of lambda(m) the mean hydraulically effective pore size, were greater for clays (0.33 mm) than loams (0.15 mm), and greater for loams than sand s (0.12 mm). Most of the soils studied showed hydraulic characteristic s associated with by-pass flow.