Measurement of Steady-State Soil Water Flux across a Soil Horizon Interface
Soil horizon interfaces have been shown to be focal points for localized, three-dimensional redistribution of water and solutes in field soils. Therefore, understanding of the physics of water flow and transport in layered soils requires experimental observations of the magnitude and variability of local soil water flux under a variety of well-defined boundary conditions. We developed a time domain reflectometry (TDR) method to measure the spatial pattern of steady-state, local soil water flux density above and below a soil horizon interface under quasi-steady surface water application and implemented it in laboratory and field experiments. Time series of TDR-measured bulk soil electrical conductivity and TDR-measured soil volumetric water content at each location are used to quantify the solute travel time under steady-state flow conditions, which is then used to quantify steady-state, local soil water flux density. Results from laboratory and field experiments showed that the proposed methodology yielded local soil water flux density estimates that were, on average, 104% of the applied surface water flux density (i.e., mass recovery = 104%), which is consistent with transient, local soil water flux density estimates from a previous study. For the field experiments, steady-state, local soil water flux density estimates above and below an A/B horizon interface (measured across the length of a 6.75-m transect) were negatively correlated to each other. The strength of the negative correlation, however, decreased with increasing surface water application rate, suggesting that the hydrologic response of the soil horizon interface is flux-dependent. This flux-dependent correlation between A and B horizon steady-state soil water flux density is probably attributable to the spatial covariance between A and B horizon soil hydraulic properties.