The hydrological state of a hillslope prior to a sprinkling-induced
shallow landslide was monitored using electrical resistivity tomography
(ERT) along a 47-m-long transect, supplemented by local time-domain
reflectometry (TDR) and tensiometer measurements. The spatial and
temporal evolution of wetting patterns in the soil material indicated
attainment of a stationary fully saturated profile in a slope region
underlain by shallow sandstone bedrock. The significant decrease
in spatially averaged standard deviation of water saturation has
not been observed during an earlier failed attempt to trigger a landslide
by intense sprinkling. While for the ‘stable' experiment (no landslide
was triggered) water saturation and soil moisture variability were
still increasing with time, the ‘unstable' experiment reached a time-invariant
state of high pore water pressures and saturations, until it finally
failed. The results indicate that when large and interconnected regions
of hillslope are saturated (as confirmed by high volumetric water
content and low standard deviation of water saturation), additional
water cannot be redistributed to empty drier regions and may eventually
enhance local pore water pressure and seepage force, initiating large
shear deformation and failure. Accordingly, a transition to such
a critical steady state of high average water saturation, associated
with low and constant spatial standard deviation, may serve as additional
hydro-geophysical indicator for the imminence of a landslide release.