Measurement of Transient Soil Water Flux Across a Soil Horizon Interface

Abstract

Understanding the physics of water flow and transport in layered soils requires experimental observations of the magnitude and variability of these processes as they occur in the field. In this study, a time domain reflectometry (TDR) method to measure the spatial pattern of transient, local soil water flux above and below a soil horizon interface under quasi‐steady surface water application was developed and implemented in laboratory and field experiments. The method uses vertical TDR probes spanning two or more soil horizons or layers. Time series of soil water storage measured by the TDR probes are used to quantify transient, local soil water flux during infiltration under quasi‐steady surface water application. Results from the laboratory and field experiments showed that transient soil water flux estimates with the presented methodology were, on average, 106% of the applied water flux (i.e., mass recovery = 106%). Furthermore, in field experiments, excellent agreement between two independent measurements of local soil water flux through the A horizon showed that this methodology is robust and sensitive to spatial variations in soil water flux within soil horizons and changes in local soil water flux as the wetting front crosses the interface between two horizons. In the field experiments, transient local soil water flux through the A and B horizons (measured along a 6.75‐m‐long transect) were positively correlated to each other, but the strength of the correlation decreased with increasing surface water application rates. The flux‐dependent covariance between the measured patterns of transient A and B horizon soil water flux indicates that the soil horizon interface is a hydrologically significant component of the soil profile. In‐depth spatial analysis of the measured soil water flux patterns is the subject of a future study.