Scale Dependent Spatial Covariance of Soil Water Flux Across Soil Horizon Interfaces.

Field studies have shown that vertical water flow through soil, even at the pedon scale, can have significant horizontal spatial variability.  Stochastic convective transport models have been developed to describe the influence of this flow variability on solute transport/dispersion.  Understanding and predicting the vertical continuity/persistence of preferred (high versus low) flow regions (with depth) remains a challenge.  In addition, all soils by definition have at least two soil horizons (layers) and all models require assumptions about the vertical continuity of flow and correlation of hydraulic properties across horizon boundaries.  Time domain reflectometry (TDR) methodologies were developed to measure the spatial patterns of transient and steady state, local soil water flux above and below an A/B horizon interface, and implemented in field water flow and transport experiments.  Results indicate the soil horizon interface is hydrologically significant.  Specifically, the hydrological influence of the horizon interface on the continuity of vertical soil water flux was: 1) different for transient infiltration versus steady state flow (under constant water application); 2) dependent on the average rate of soil water flow; 3) scale (spatial) dependent; and 4) influenced by a spatial covariance of the interface shape and soil hydraulic properties (as expressed by the steady state soil water content).  These results indicate that accurate parameterization of hydrologic models and pedotransfer functions must account for the nature and spatial variability of the shape of the soil horizon interface in addition to the individual average properties of soil horizons.