Evidence for spatial variability in hydraulic redistribution within an oak–pine forest from resistivity imaging

Abstract

Electrical resistivity imaging (ERI) was used to determine spatiotemporal resistivity changes in the vadose root zone of an oak–pine forest during an 8-day period of drying following a rainfall event. Electrical images from 0 to 2 m depth, incorporating the shallow root zone, were inferred to represent spatiotemporal variability of moisture between tree and non-tree areas resulting from hydraulic redistribution (HR). Electrical resistance measurements on a grid of 96 electrodes were collected, six times a day, from July 2nd to July 10th, 2009. Images were produced from a 3-D inversion of resistance measurements that were assigned weights based on error models that accounted for both measurement (reciprocal) and modeling errors. To account for temperature variations, a sinusoidal diurnal temperature correction based on the 1-D heat conduction equation and measurements at 5 cm depth was applied to the resistivity results. Laboratory measurements of resistivity as a function of moisture content for samples from the site obeyed Archie’s Law, suggesting that electrolytic conduction is the dominant current pathway in these soils. Spatiotemporal variability in moisture dynamics due to root zone processes was therefore inferred from spatial variation in changes of electrical conductivity over time. Electrical conductivity depth profiles were generated for tree versus non-tree locations using conductivities from corresponding finite element mesh voxel coordinates. These profiles reveal a stable temporal profile beneath trees relative to a pronounced decrease in electrical conductivity over time at non-tree locations. Relative changes in electrical conductivity between 6 am and 6 pm surveys revealed that both tree and non-tree locations are more conductive at 6 am, although changes are greater at tree locations. This finding is likely diagnostic of hydraulic lift (HL) overnight by tree roots. Regardless of location, the differences between 6 am and 6 pm surveys are consistent with HR, impacting moisture content across the entire survey area.