Multi-step outflow and evaporation experiments – Gaining large undisturbed samples and comparison of the two methods

Abstract

The aim of the study was the comparison of the soil hydraulic properties determined based on two different experimental methods of four large and macroporous soil samples. An undisturbed cylindrical soil column (h = 60 cm and d = 30 cm) was obtained at a natural slope and quartered horizontally in the laboratory. The four sub-samples were saturated and subsequently drained using the extended multi-step outflow method (XMSO). Retention curves and unsaturated hydraulic conductivity curves were derived via inverse modeling based on the measured drainage and corresponding pore water pressure. After resaturation the evaporation method (EVA) was used on the same four samples. The samples were exposed to evaporation and the effective water contents and the average pore water pressures were recorded over time to determine soil hydraulic functions.While both methods yielded similar results in the medium moisture range, the advantage of the XMSO-method is its higher information content with respect to the hydraulic conductivity close to water saturation whereas the advantage of the EVA-method is its fast execution. Moreover, the EVA-method is easier to handle and has significantly lower computational requirements than the XMSO-method.Furthermore, since the four soil samples represent a soil profile in the field, a depth dependent characterizing of the soil profile was possible and showed that the soil properties near saturation are greatly dominated by the ratio of macropores in the sample and that a decrease in the macropore ratio with soil depth was clearly reflected in the hydraulic functions.