Predicting soil interpedal macroporosity and hydraulic conductivity dynamics: A model for integrating laser-scanned profile imagery with soil moisture sensor data

The size and spatial distribution of soil pores control the infiltration, percolation, and retention of water within a pedon. These distributions are often represented within hydrologic flux equations as static hydraulic properties such as saturated hydraulic conductivity and water retention parameters. However, the assumption that these hydraulic properties are static does not adequately represent the potentially rapid response of highly-structured soil to moisture variability-induced shrink-swell processes. We use a recently-developed, high-resolution (180 um) laser imaging technique to capture structural macropore data and derive a function that relates interpedal, planar macropore width to matrix water content. Subsequently, we develop an expression for transient hydraulic conductivity that accounts for dynamic macropore geometries and propose a method for partitioning total soil water content obtained from in situ sensor data into matrix and macropore water content. The model was applied to a soil profile in northeastern Kansas where intact soil monoliths had been imaged to quantify soil macorpore properties and continuous soil water content data were collected at multiple depths. Model-predicted macropore width showed significant sensitivity to matrix water content. Rainfall events that followed periods of low soil moisture were predicted to allow water to fill macropores - created by the shrinkage of soil structural units - which significantly and rapidly increased unsaturated hydraulic conductivity. This model offers a means by which to monitor and characterize the dynamic hydraulic properties of soils susceptible to shrink-swell processes that impact hydrologic partitioning and preferential flow.