Water balance and flow patterns in a fragipan using in situ soil block

A soil block (1.5 m x 1.5 m x 1.5 m) was excavated at a footslope position on hillslope underlain by a fragipan horizon. The block vertical faces were sealed on three sides using bentonite, sand, and lumber, with horizontal pressure exerted from basement jacks. The downslope face of the block was instrumented to measure lateral water flow through time at two depths within the profile. A flood irrigation system maintained a 2-cm constant hydraulic head on the soil block surface. A mass balance approach determined vertical and horizontal movement of water through, within, add above the fragipan. Saturated hydraulic conductivity was measured using the Guelph permeameter and a double-ring infiltrometer. Spatial flow patterns in the block were investigated using flood irrigation with Brilliant Blue FCF dye. At steady state, 63% of the total input water moved laterally through soil horizons above the fragipan, 10% moved laterally through interconnected prism faces within a 50-cm-thick fragipan horizon, and 27% moved vertically through the 50-cm-thick fragipan horizon and below the lowest collector. Lateral flow rates were 1.29 L min^-1 above the fragipan and 0.22 L min^-1 within the fragipan. The relatively large amount of water that moved through the fragipan (37%) indicates that fragipans do conduct a significant amount of water and that much of it is through horizontal flow in the interconnected prism faces. This is especially important to hydrologic modeling applications, where fragipans are often incorrectly estimated to be nearly impermeable. However, most of the water moved laterally in soil horizons above the well expressed fragipan, indicating that the reduced hydraulic conductivity of the fragipan does promote significant lateral flow on hillslopes underlain by glacial till soils.