Use of contact material in tension infiltrometer measurements

Abstract

A critically important aspect of tension infiltrometer (TI) measurements is that good hydraulic contact be established and maintained between the infiltrometer and the soil. This is routinely achieved by placing a layer of contact material between the TI membrane and the soil surface. In this study, the effect of contact material on the pressure head and hydraulic head gradient under the TI was investigated using computer simulations and laboratory measurements. The discrepancy between the pressure head at the soil surface (ψ_ss) and the pressure head on the TI membrane (ψ_m) can be substantial and variable. The discrepancy depends on the thickness, sated hydraulic conductivity (K_cm) and water entry value (ψ_cm) of the contact material, and on the flow rate out of the TI. The hydraulic head gradients in the soil under the TI (G) are generally greater than unity, and depend on soil type and ψ_ss. These results indicate that the often used assumptions, ψ_ss = ψ_m and G = unity, are generally not correct. A relationship based on Darcy's law was developed to account for the discrepancy between ψ_ss and ψ_m. Use of this relationship in TI equations resulted in estimates of near-saturated hydraulic conductivity that fell within 10% of the actual values. A highly uniform glass sphere medium with K_cm = 1.1 (±0.1) × 10⁻⁴ ms⁻¹ and ψ_cm = −300 (± 30) mm was proposed as a good contact material for practical field use. Recommendations were also given for how the material could be used in field-based TI measurements.