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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