  Movement of soil moisture associated with tree root-water uptake is
ecologically important, but technically challenging to measure. Here, the
self-potential (SP) method, a passive electrical geophysical method, is used to
characterize water flow in situ. Unlike tensiometers, which use a measurement
of state (i.e. matric pressure) at two locations to infer fluid flow, the SP
method directly measures signals generated by water movement. We collected SP
measurements in a two-dimensional array at the base of a Douglas-fir tree
(Pseudotsuga menziesii) in the H.J. Andrews Experimental Forest in western
Oregon over five months to provide insight on the propagation of transpiration
signals into the subsurface under variable soil moisture. During dry
conditions, SP data appear to show downward unsaturated flow, while nearby
tensiometer data appear to suggest upward flow during this period. After the
trees enter dormancy in the fall, precipitation-induced vertical flow dominates
in the SP and tensiometer data. Diel variations in SP data correspond to
periods of tree transpiration. Changes in volumetric water content occurring
from soil moisture movement during transpiration are not large enough to appear
in volumetric water content data. Fluid flow and electrokinetic coupling (i.e.
electrical potential distribution) were simulated using COMSOL Multiphysics to
explore the system controls on field data. The coupled model, which included a
root-water uptake term, reproduced components of both the long-term and diel
variations in SP measurements, thus indicating that SP has potential to provide
spatially and temporally dense measurements of transpiration-induced changes in
water flow. This manuscript presents the first SP measurements focusing on the
movement of soil moisture in response to tree transpiration.
