Reevaluation of the Multistep Outflow Method for Determining Unsaturated Hydraulic Conductivity

Abstract

The multistep outflow (MSO) method is widely used for determining the unsaturated hydraulic conductivity, which is essential for accurate prediction of water flow in soils. However, the reliability of this method has yet to be adequately verified through comparisons with independently measured conductivities of soils having different textures and structures. We conducted MSO experiments with three soils of different textures to test the reliability of the conventional MSO. Three methods were used to determine the hydraulic conductivities, K( theta from identical experiments: an optimization method with readings of a tensiometer (OM), a direct method using one tensiometer (DM1), and a direct method using two tensiometers (DM2). Outflow data were also used in the three methods. Additionally, steady-state downward flow experiments using two tensiometers were performed for comparison. For two of the three soils, OM and DM1 clearly underestimated K( theta determined by SDFM. On the other hand, DM2 gave good agreement with the SDFM data. These results and the discrepancies between the fitted and observed tensiometer readings near the bottom imply the existence of a hydraulic resistance at the soil-porous plate interface. The hydraulic resistance might be caused by pore plugging with fine particles (colloids) transported to the ceramic plate after stepwise changes in pressure. We suggest that an additional tensiometer be installed near the bottom of the sample and be used for checking the optimized K( theta with the direct method (DM2).