katelin alldritt, toby o’geen and randy dahlgren
TRANSCRIPT
Hydropedology and Hydrologic Connectivity of an Oak-Woodland Hillslope in the
Northern Sierra Foothills of California
Katelin Alldritt, Toby O’Geen and Randy Dahlgren
University of California, Davis Department of Land, Air and Water Resources SSSA Nov 4th, 2014 Poster #1426
Q1: Hydrostratigraphic Units
Research Questions
Hydropedology is the study of how soil morphology and stratigraphy influence hy-
drologic processes, which is particularly relevant at the hillslope scale, where soil
stratigraphy and spatial variability can exert first-order control on the hydrologic
flow paths. Hydrologic connectivity is a condition by which different stratigraphic
units across the hillslope become hydraulically linked via subsurface water flow
(Steiglitz et al 2003). Hydrologic connectivity occurs when isolated patches of sat-
uration become connected across the hillslope (Western et al 1996, Hopp and
McDonnell 2009, Ocampo et al 2006). Understanding soil stratigraphy and its in-
fluence on hydrologic connectivity and stream flow generation has implications
for water resource sustainability, water quality and other ecosystem services
(Devito et al 2005).
Q1: What are the significant hydropedologic properties of the hillslope and their spatial distribution?
Q2: How does soil stratigraphy and morphology influence
hydrologic connectivity?
Q3: What are possible causes for connection and disconnection of
hydrologic flow paths during and between rain storm events?
210 m transect within a 36 ha research catchment
Soil hydrology monitoring network and stream flow data collection
Transect excavated with backhoe to 150 cm depth or to bedrock and
mapped
16 soil profiles characterized in detail, including texture, structure, color,
and redoximorphic features. 11 of these sites coincided with the soil hy-
drology monitoring sites.
42 soil cores sampled for saturated hydraulic conductivity, retention curve
measurements and bulk density
Hillslope hydrology modeled with HYDRUS 2D (Simunek et al, 1998)
Figure 2. Soils found in the research
catchment are (left) those with a clay-
pan, and (right) those without a claypan
(O’Geen et al, 2010)
Introduction
Study Site and Methods
Q2: Hydrologic Connectivity Q3: Flow Path Connection and Disconnection
Six Hydrostratigraphic Units (HSUs) were identified
Biomantle = permeable, bioturbated, continuous and
homogenous
Permeable argillic =stable zone, near continuous
Claypan = low permeability, >40% clay content, abrupt
clay increase from above zone, discontinuous
Weathered bedrock type 1 = high bulk density, fractured
Weathered bedrock type 2 = low bulk density, massive
Hard bedrock = Metavolcanics
Figure 3. Cross-section of
the hillslope transect based
on HSUs. Vertical exaggera-
tion = 3X. The gap was due
to a rock outcrop blocking
excavation. Hillslope was
complex and comprised of a
discontinuous network of
claypan, undulating bedrock
topography and highly vari-
able weathered bedrock.
Table 1. Physical properties of the hydrostratigraphic units found in the hillslope
Figure 1. 210 m transect (black line)
within research catchment. Blue lines
are stream channels. Red dots are pre-
viously described soil pits
Conclusions References
Complex hillslope stratigraphy comprised of a discontinuous claypan, undulating bedrock topography and highly variable weathered
bedrock.
Primary hydrologic flow path during connectivity was rapid subsurface lateral flow in the biomantle.
Presence of a claypan decreased effective soil depth, increased antecedent wetness and created a perched water table.
Undulating bedrock created disconnected perched water tables along the hillslope.
Isolated zones of wetness only became connected when a storm event saturated the entire subsurface and moved the water table
into the biomantle.
Further investigation on the hydrologic role of weathered bedrock would improve understanding of hillslope hydrology
Figures 4 and 5. (Left) Duration of saturation for each tensiometer depth at all
five tensiometer sites during one stream flow event (black line). Numbers on
the side of each row correspond to the tensiometer sites (1-5). The colors rep-
resent the hydrostratigraphic unit (s). The grey bars highlight the stream flow
peaks and corresponding tensiometer data. Missing data (e.g. tensiometers 1
and 5) was due to sensor failure. Stream flow event induced by multiple precip-
itation event. (Right) Hillslope velocities (cm/hr) as modeled with HYDRUS 2D.
Sections correspond to sections in Figure 3. The continuous surface zone of rap-
id velocities (blue) corresponds to connected subsurface lateral flow in the bio-
mantle.
Hillslope connected Hillslope
disconnected
Figures 6 and 7. (Left) Close ups on upper, middle and lower sections of the
hillslope, which are hydrologically disconnected. (Right) Water table connec-
tion time series at a large hard bedrock berm site. The color scale for both fig-
ures is in matric potential (cm H2O). Figure 8. Discon-
nected perched wa-
ter tables due to
the spatially discon-
tinuous claypans.
The color scale is in
matric potential (cm
H2O)
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