Low tension water flow in structured soils

Water transport through structured clayey soils may be prone to by-pass flow, a mechanism that may lead to rapid transport of contaminants to ground water. To quantify the significance of low-tension water flow in structured soils, apparent steady-state infiltration rates at water potentials from -0.24 to 0 m were measured using tension infiltrometers on 18 soils of varying texture and structure. Each infiltration measurement was conducted sequentially at -0.24, -0.12, -0.06, -0.03, -0.02, -0.01, and 0 m supply potentials (Ψ(supply)), all at the same soil location, to separate different size pores effective in transmitting water. Results from 96 soil horizons showed that 76 ± 18% (mean ± SD) of the water fluxes at Ψ(supply) = 0 m (total water flux) were transmitted through macropores (active at Ψ(supply) ≤ -0.03 m), although macropores usually constituted a small portion of a sows total porosity. Mesopores (active at Ψ(supply) > -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 showed greater influence on water flow in clays than in sands at Ψ(supply) > -0.24 m. Values of soil macroscopic λ(c) and microscopic λ(m) capillary length scales were determined from the change in infiltration rates with Ψ(supply). Values of λ(c), a hydraulic conductivity-weighted mean capillary water potential, were greater for sands (63 mm) than loams (50 mm), and greater for loams than clays (22 mm). Values of λ(m), the mean hydraulically effective pore size, were greater for clays (0.33 mm) than loams (0.1 5 mm), and greater for loams than sands (0.12 mm). Most of the soils studied showed hydraulic characteristics associated with by-pass flow.