Evaluating macropore flow with temporal electrical resistivity imaging in riparian areas

Abstract

Riparian soils are uniquely susceptible to the formation of macropores, voids with preferential flow in comparison to surrounding strata, which are hypothesized to promote fast transport of water through soil layers. Electrical Resistivity Imaging (ERI) can locate spatial heterogeneities in soil wetting patterns caused by preferential flow through macropores, thus optimizing the design of riparian buffers. Temporal ERI (TERI) imaging was conducted in a fine and coarse field setting with artificial macropores to evaluate flow under unsaturated simulated rainfall conditions and saturated infiltrometer conditions.

Results from field data show that while macropores are detectable using TERI datasets, this results in an average field setting would detect the wetted zone in the vicinity of a macropore, not the macropore itself. The results were similar for both the primary fine grain soil site in Oklahoma as well as the coarse grain site in North Carolina. TERI data indicate that without artificial rainfall or macropores in low noise conditions, a single macropore would not be detected, a wetted zone would be the best detection. In a field evaluation of naturally occurring macropores, the TERI technique would detect the wetted zone around a macropore similar to an area of increased hydraulic conductivity in a heterogeneous soil matrix. The findings from the first set of experimentation indicate an appropriate resolution and electrode spacing for the second experiment in this thesis. The second experiment entails the tracer velocity mapping of alluvial soil. Preliminary results show TERI as a viable method for calculating the fluid velocity along a series of vertical profiles in the coarse-grained North Carolina field site.