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GEOMORPHOLOGY. Experimental evidence for hillslope control of landscape scale

Authors:
Kristin E. Sweeney at University of Portland
Kristin E. Sweeney
Josh Roering at University of Oregon
Josh Roering
Christopher R Ellis at University of Minnesota Twin Cities
Christopher R Ellis
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Abstract and Figures

Landscape evolution theory suggests that climate sets the scale of landscape dissection by modulating the competition between diffusive processes that sculpt convex hillslopes and advective processes that carve concave valleys. However, the link between the relative dominance of hillslope and valley transport processes and landscape scale is difficult to demonstrate in natural landscapes due to the episodic nature of erosion. Here, we report results from laboratory experiments combining diffusive and advective processes in an eroding landscape. We demonstrate that rainsplash-driven disturbances in our experiments are a robust proxy for hillslope transport, such that increasing hillslope transport efficiency decreases drainage density. Our experimental results demonstrate how the coupling of climate-driven hillslope- and valley-forming processes, such as bioturbation and runoff, dictates the scale of eroding landscapes. Copyright © 2015, American Association for the Advancement of Science.
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GEOMORPHOLOGY
Experimental evidence for hillslope
control of landscape scale
K. E. Sweeney,
1
*J. J. Roering,
1
C. Ellis
2
Landscape evolution theory suggests that climate sets the scale of landscape
dissection by modulating the competition between diffusive processes that sculpt
convex hillslopes and advective processes that carve concave valleys. However, the link
between the relative dominance of hillslope and valley transport processes and
landscape scale is difficult to demonstrate in natural landscapes due to the episodic
nature of erosion. Here, we report results from laboratory experiments combining
diffusive and advective processes in an eroding landscape. We demonstrate that
rainsplash-driven disturbances in our experiments are a robust proxy for hillslope
transport, such that increasing hillslope transport efficiency decreases drainage
density. Our experimental results demonstrate how the coupling of climate-driven
hillslope- and valley-forming processes, such as bioturbation and runoff, dictates the
scale of eroding landscapes.
Convex hillslopes and concave valleys are
ubiquitous in eroding, soil-mantled land-
scapes (1–3) (Fig. 1, A and B). These dis-
tinct landforms are produced by equally
distinct sediment transport processes: On
hillslopes, abiotic (4,5)andbiotic(6,7)distur-
banceagentsdispersesedimentdownslope,where-
as in valleys, sediment is transported by debris
flows (8)orflowingwater(9). The transition be-
tween hillslopes and valleys has long been con-
sidered a fundamental landscape scale (3,10,11),
but there is debate over what controls its style
and position. Numerical results suggest that the
hillslope-valley transition may be dictated by the
minimum runoff necessary for sediment detach-
ment or landslide initiation (11–13)orbythecom-
petition between diffusive transport on hillslopes
and advective transport in channels (14,15).
These geomorphic models predict expansion
or contraction of the valley network from changes
in climatic variables such as precipitation and
vegetation (3,12,16). Hence, rigorous testing of
controls on the hillslope-valley transition is cen-
tral to our understanding of landscape response
to environmental perturbations. Due to the slow
and episodic nature of erosion, however, field
tests are limited to comparisons of steady-state
model predictions with natural topography [e.g.,
(2)]. Such approaches rely on the assumption that
topography reflects long-term average fluxes, ig-
noring the potentially important effects of initial
conditions (17)andtemporallagsbetweenland-
scape response and climatic and tectonic forcing
(18,19).
We conducted a series of laboratory experi-
ments to determine whether the competition
between hillslope transport and valley incision
sets the spatial scale of landscapes (15). The
theoretical underpinnings of the process con-
trol on the hillslope-valley transition derive
from a statement of mass conservation, where
the rate of elevation change (dz/dt) is equal to
uplift rate (U), minus erosion due to disturbance-
driven hillslope diffusion and channel advection
by surface runoff
@z
@t¼UþDr2z−KðPAÞmSnð1Þ
where Dis hillslope diffusivity, Kis the stream
power constant, Ais drainage area, Sis slope, Pis
precipitation rate (assuming steady, uniform rain-
fall), and mand narepositiveconstants(9). In
this framework, the strength of hillslope transport
relative to channel processes can be quantified
by the landscape Péclet number (Pe), assuming
n=1(15)
Pe ¼KL2mþ1
Dð2Þ
where Lis a horizontal length scale and the
hillslope-valley transition occurs at the critical
length scale L
c
where Pe =1 (15). In plots of slope
versusdrainagearea,L
c
corresponds to a local
maximum separating convex hillslope and con-
cave valley topography (20)(Fig.1,DandE).If
this framework holds for field-scale and experi-
mental landscapes, increasing the vigor of hillslope
transport relative to valley incision (decreasing
Pe) should result in longer hillslopes (higher L
c
)
SCIENCE sciencemag.org 3JULY2015•VOL 349 ISSUE 6243 51
1
Department of Geological Sciences, University of Oregon,
1272 University of Oregon, Eugene, OR 97403-1272, USA.
2
St. Anthony Falls Laboratory and National Center for Earth-
Surface Dynamics, College of Science and Engineering,
University of Minnesota, 2 Third Avenue SE, Minneapolis, MN
55414-2125, USA.
*Corresponding author. E-mail: kristin.e.sweeney@gmail.com
Fig. 1. Characteristic morphology of eroding landscapes. Photographs of eroding landscapes.
(A) Painted Hills unit of John Day Fossil Beds National Monument, Oregon. (B) Gabilan Mesa, Cal-
ifornia. (C) Laboratory landscape from this study with no hillslope diffusion, and associated plots
of local slope versus drainage area, calculated with steepest descent algorithm (Dto F). Pictures
in (A) and (C) taken by the author (K.E.S.); (B) from Google Earth. Topographic data to generate
slope-area plots from (D) field surveys; (E) Lidar data from National Center for Airborne Laser
Mapping; and (F) this study. Gray vertical bars in (D) to (F) demarcate the inferred hillslope-valley
transition (34).
RESEARCH |REPORTS
on July 2, 2015www.sciencemag.orgDownloaded from on July 2, 2015www.sciencemag.orgDownloaded from on July 2, 2015www.sciencemag.orgDownloaded from
and a contraction of the valley network (i.e., a
decrease in drainage density).
Experimental landscapes bridge the gap in
complexity between numerical models and nat-
ural landscapes (21) by enabling us to control the
confounding influences of tectonics, climate, and
lithology and observe surface evolution through
time. As previously noted [e.g., (21)], a complete
dynamical scaling of erosional landscapes in the
laboratory is typically intractable due to shallow
water depths, large grain sizes relative to the
size of the experiment, and other considerations.
Nonetheless, past landscape experiments have
successfully demonstrated the topographic man-
ifestation of changing uplift rate (22,23), precipi-
tation rate (24), and precipitation patterns (25).
In these experiments, however, erosion was ex-
clusively driven by surface runoff (e.g., Fig. 1, C
and F), intentionally excluding the representa-
tion of diffusive hillslope processes (22)andhence
precluding tests for the role of hillslope transport
in setting landscape scale.
Following Eq. 1, we created an experimental
system that distilled landscape evolution into
three essential ingredients: base-level fall (uplift),
surface runoff (channel advection), and sediment
disturbance via rainsplash (hillslope diffusion)
(Fig. 2). Our experiments in the eXperimental
Landscape Model (XLM) at the St. Anthony Falls
Laboratory systematically varied the strength
of disturbance-driven transport relative to sur-
face runoff (changing Pe) for steady, uniform
uplift. The XLM consists of a 0.5 by 0.5 by 0.3 m
3
flume with two parallel sliding walls, each con-
nected to a voltage-controlled dc motor to sim-
ulaterelativeuplift(Fig.2A).Theexperimental
substrate was crystalline silica (median grain
size = 30 mm) mixed with 33% water to increase
cohesion and reduce infiltration (Fig. 2C). We
began each experiment by filling the XLM with
sediment and allowing it to settle for ~24 hours
to homogenize moisture content. Topographic
data at 0.5 mm vertical accuracy were collected
at regular time intervals on a 0.5 by 0.5 mm
grid using a laser scanner.
Sediment transport in our experiments was
driven by two distinct rainfall systems: the mister,
a rotating ring fitted with 42 misting nozzles, and
the drip box, a polyvinyl chloride constant head
tank fitted with 625 blunt needles of 0.3 mm in-
terior diameter arranged in a 2 by 2 cm grid
(Fig. 2B). The fine drops produced by the mister
lack sufficient energy to disturb sediment on im-
pact and instead result only in surface runoff. By
contrast, the 2.8 mm diameter drops from the
drip box are sufficiently energetic to create rain-
splash and craters on the experiment surface that
result in sediment transport. We used four fans
mounted on the corners of the experiment to
generate turbulence and randomize drop loca-
tion during drip box rainfall. Importantly, sed-
iment transport due to drip box rainfall consists
of both hillslope diffusion from the cumulative
effect of drop impacts and nonnegligible advec-
tive transport due to the subsequent runoff of the
drops. Thus, we expect that changing the relative
contribution of rainsplash results in a change in
both hillslope and channel transport efficiency
(Dand K, respectively).
We ran five experiments, varying the time of
drip box rainfall (i.e., predominantly diffusive
transport) from 0 to 100% of total experiment
runtime (Fig. 3 and table S1) (26)andholding
base-level fall and water flux from the mister
and the drip box constant. During the experi-
ment, we alternated between drip box rainfall and
mister rainfall over 10-min periods (table S1); the
fans used for drip box rainfall prevented simulta-
neous operation. We continued each experiment
until we reached flux steady state such that the
spatially averaged erosion rate was equal to the
rate of base-level fall (figs S1 and S2). Each ex-
periment ran for 10 to 15 hours, encompassing
60 to 90 intervals of drip box and/or mister
rainfall.
The steady-state topography of our experiments
(Fig. 3, A to F) demonstrates how increasing the
relative dominance of rainsplash disturbance af-
fects landscape morphology. Qualitatively, land-
scapes formed by a higher percentage of drip box
rainfall (Fig. 3E) appear smoother, with wider,
more broadly spaced valleys and extensive un-
channelized areas. In contrast, landscapes with
more surface runoff transport (Fig. 3A), equivalent
to past experimental landscapes (24,27), are
52 3JULY2015•VOL 349 ISSUE 6243 sciencemag.org SCIENCE
Fig. 2. Experimental setup. (A) Schematic profile of experimental apparatus (XLM). Line arrows show
direction of sediment movement. (B) Photograph of misting ring and drip box looking from below.
(C) Photograph of sediment surface during 100% drip run.
Fig. 3. Steady-state topography and hillslope
morphology. (Ato E) Hillshades of experimental
topography for (A) 0% drip, (B) 18% drip, (C) 33%
drip, (D) 66% drip, and (E) 100% drip overlain with
channel networks (blue) and locations of hillslope
profiles (red). Topography is 475.5 mm wide in plan-
view. (F) Elevation profiles of hillslopes marked by
red lines in (A) to (E). Vertical and horizontal length
scales are equal.
RESEARCH |REPORTS
densely dissected. As the relative percentage of
rainsplash increases, hillslope transects increase
in both length and topographic curvature (Fig.
3F), confirming that our experimental procedure
can be used to adjust hillslope transport efficiency.
Hillslope relief in our experiments is approxi-
mately 10 to 20% of total landscape relief, a sim-
ilar value to natural landscapes (28).
To test the expected relationship between
Péclet number and landscape scale (15)(Eq.2),
we used steady-state relationships between land-
scape morphology and sediment transport laws
to independently calculate Dand K. This ap-
proach is often not possible in natural landscapes
and thus extends our theoretical understanding
beyond the slope-area plots shown in Fig. 1, D to
F. Specifically, we used the approach of (28)to
calculate D, which uses average hillslope length
and gradient, thereby avoiding the stochastic
imprint of individual raindrop impacts that can
obscure local metrics of hillslope form, such as
curvature. The following relationship relates
mean hillslope gradient (S) to hillslope length
(L
h
), critical slope (S
c
), erosion rate (E, equal to
Uat steady state), and hillslope diffusivity (D)
S
Sc
¼1
E*ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1þðE*Þ2
q−
ln 1
21þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1þðE*Þ2
q
−1ð3Þ
where E*=E(2L
h
/KS)(28). To calculate Kand
mforthestreampowermodel,weusedthepre-
diction of Eq. 1 that local steady-state channel slope
is a power-law function of drainage area (29):
S¼U
KPm

1
n
A
−m
=
nð4Þ
To quantify hillslope length and gradient, we
mapped the channel network by explicitly iden-
tifying channel forms (28–31) (fig. S3), then traced
hillslopes beginning at hilltop pixels by follow-
ing paths of steepest descent to the nearest chan-
nel (28) (fig. S3). We take S
c
to be sufficiently large
(table S1) that Eq. 3 approximates linear diffu-
sion. As the proportion of rainsplash transport
increases, Dcalculated from Eq. 3 also increases
(table S1), confirming that the morphologic trend
of individual hillslope transects (Fig. 3F) reflects
increasing hillslope transport efficiency.
To calculate the advective process parameters
(Eq. 4), we extracted slope and steepest-descent
drainage area data along networks defined by a
minimum drainage area of 250 mm
2
(larger than
the drainage area of channel initiation) and fit
power-law relationships to plots of slope versus
drainage area (29). Following (15,22), we assume
that n= 1 and use the intercept and slope of the
power-law fits to calculate mand Kfor each ex-
periment. Whereas mis relatively invariant for all
our experiments, Ktends to increase with the
fra ction of drip box transport, indicating that post-
impact rainfall runoff contributes to advective as
well as diffusive transport in our experiments.
Given that both Dand Kchange in our ex-
periments, we calculated Pe values (Eq. 2) for
each of our experiments to quantify how diffu-
sive and advective processes contribute to the
observed transition from smooth and weakly
channelized landscapes (100% drip box, Fig. 3A)
to highly dissected terrain (mist only, 0% drip
box, Fig. 3E). We calculated Pe for each exper-
iment (Eq. 2) by assuming that n= 1 and taking
the length scale Lto be the horizontal distance
fromthemaindividetotheoutlet(256mm).
Our results show that landscape Pe value increases
with the fraction of drip box transport, demon-
strating that an increase in hillslope transport
efficiency, D, is the dominant result of increasing
rainsplash frequency. Figure 4 reveals a positive
relationship between Pe and drainage density,
which is inversely related to hillslope length or
L
c
,suchthatincreasingPe in our experiments
results in higher drainage density (i.e., shorter
hillslopes). This finding is consistent with theo-
retical predictions for coupled hillslope-channel
process controls on the scale of landscape dis-
section (14,15).
In our experiments, hillslope transport im-
parts a first-order control on landscape scale,
emphasizing the need to establish functional
relationships between climate variables and
hillslope process rates and mechanisms for real
landscapes. Although climate change scenarios
typically focus on the influence of vegetation and
rainfall on overland flow and channel hydraulics
(3,12), climate controls on the vigor of hillslope
transport (e.g., 32,33) can drive changes in land-
scape form. Robust linkages between transport
processes and topography, as discussed here, are
an important component of interpreting plane-
tary surfaces as well as decoding paleolandscapes
and sedimentary deposits.
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ACKNO WLED GMEN TS
This work was supported by a National Science Foundation
grant (EAR 1252177) to J.J.R. and C.E. We thank D. Furbish and
A. Singh for fruitful discussions, S. Grieve and M. Hurst for
assistance with the calculation of hillslope metrics, and the
technical staff of St. Anthony Falls Laboratory for support during
experiments. All authors designed the experiments and wrote
the manuscript, C.E. built and troubleshot the experimental
apparatus, and K.E.S. conducted the experiments and analyzed
the data. Comments from two anonymous reviewers greatly
improved the manuscript. Topographic data are available from
the National Center for Earth Dynamics Data Repository at
https://repository.nced.umn.edu.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/349/6243/51/suppl/DC1
Materials and Methods
Figs. S1 to S3
Table S1
References (35)
3 March 2015; accepted 26 May 2015
10.1126/science.aab0017
SCIENCE sciencemag.org 3JULY2015•VOL 349 ISSUE 6243 53
Fig. 4. Effect of landscape Péclet num-
ber on landscape scale. Landscape
Péclet number for each experiment (circle,
0% drip; square, 18% drip; diamond,
33% drip; triangle, 66% drip; plus sign,
100% drip) versus drainage density of
GeoNet-derived drainage networks.
RESEARCH |REPORTS
DOI: 10.1126/science.aab0017
, 51 (2015);349 Science et al.K. E. Sweeney
Experimental evidence for hillslope control of landscape scale
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... Some experimental landscapes (28-31) were significantly less dynamic (i.e., tending toward static steady state) than other experiments (7,11,(32)(33)(34). Paola et al. (35) hypothesized that the relevant difference was experimental geometry. ...
... Paola et al. (35) hypothesized that the relevant difference was experimental geometry. The static steadystate experiments (28)(29)(30)(31) featured open boundaries along all landscape's edges that allowed material to exit, whereas the dynamic steady-state experiments (7,11,(32)(33)(34) constrained the locations where material could exit. In general, closed boundary conditions (7, 11) created large complex drainage basins that were dynamic, while open boundaries (28-31) promoted many relatively small subbasins that were generally static. ...
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Dendritic, i.e., tree-like, river networks are ubiquitous features on Earth's landscapes; however, how and why river networks organize themselves into this form are incompletely understood. A branching pattern has been argued to be an optimal state. Therefore , we should expect models of river evolution to drastically reorganize (suboptimal) purely nondendritic networks into (more optimal) dendritic networks. To date, current physically based models of river basin evolution are incapable of achieving this result without substantial allogenic forcing. Here, we present a model that does indeed accomplish massive drainage reorganization. The key feature in our model is basin-wide lateral incision of bedrock channels. The addition of this submodel allows for channels to laterally migrate, which generates river capture events and drainage migration. An important factor in the model that dictates the rate and frequency of drainage network reorganization is the ratio of two parameters, the lateral and vertical rock erodibility constants. In addition, our model is unique from others because its simulations approach a dynamic steady state. At a dynamic steady state, drainage networks persistently reorganize instead of approaching a stable configuration. Our model results suggest that lateral bedrock incision processes can drive major drainage reorganization and explain apparent long-lived transience in landscapes on Earth. drainage networks | landscape evolution | lateral migration | drainage reorganization W hat should a drainage network look like? Fig. 1A shows a single channel, winding its way through the catchment so as to have access to water and sediment from unchannelized zones in the same manner as the dendritic (tree-like) network of Fig. 1B. It appears straightforward that the dendritic pattern is a model for nature, and the single channel is not. Dendritic drainage networks are called such because of their similarity to branching trees, and their patterns are "characterized by irregular branching in all directions" (1) with "tributaries joining at acute angles" (2). Drainage networks can also take on other forms such as parallel, pinnate, rectangular, and trellis in nature (2). However, drainage networks in their most basic form without topographic, lithologic, and tectonic constraints should tend toward a dendritic form (2). In addition, drainage networks that take a branching, tree-like form have been argued to be "optimal channel networks" that minimize total energy dissipation (3, 4). Therefore, we would expect that models simulating river network formation, named landscape evolution models (LEMs), that use the nondendritic pattern of Fig. 1A as an initial condition to massively reorganize and approach the den-dritic steady state of Fig. 1B. To date, no numerical LEM has shown the ability to do this. Here, we present a LEM that can indeed accomplish such a reorganization. A corollary of this ability is the result that landscapes approach a dynamic, rather than static steady state. There is indeed debate as to whether landscapes tend toward an equilibrium that is frozen or highly dynamic (5). Hack (6) hypothesized that erosional landscapes attain a steady state where "all elements of the topography are downwasting at the same rate." This hypothesis has been tested in numerical models and small-scale experiments. Researchers found that numerical LEMs create static topographies (7, 8). In this state, erosion and uplift are in balance in all locations in the landscape, resulting in landscapes that are dissected by stable drainage networks in geometric equilibrium (9). The landscape has achieved geometric equilibrium in planform when a proxy for steady-state river elevation, named χ (10), has equal values across all drainage divides. In contrast, experimental landscapes (7, 11) develop drainage networks that persistently reorganize. Recent research on field landscapes suggests that drainage divides migrate until reaching geometric equilibrium (9), but other field-based research suggests that landscapes may never attain geometric equilibrium (12). The dynamism of the equilibrium state determines the persistence of initial conditions in experimental and model landscapes. It is important to understand initial condition effects (13) to better constrain uncertainty in LEM predictions. Kwang and Parker (7) demonstrate that numerical LEMs exhibit "extreme memory," where small topographic perturbations in initial conditions are amplified and preserved during a landscape's evolution (Fig. 2A). Extreme memory in the numerical models is closely related to the feasible optimality phenomenon found within the research on optimal channel networks (4). These researchers suggest that nature's search for the most "stable" river network configuration is "myopic" and unable to find configurations that completely ignore their initial condition. In contrast to numerical models, experimental landscapes (7, 11) reach a highly dynamic state where all Significance Most of Earth's landscapes are dissected by rivers, which are important for providing water, food, and energy resources. From remote imaging, we see that rivers organize themselves into dendritic (tree-like) networks. The shape and organization of river networks determine how and where water and sediment are routed on Earth. To date, models of river network formation, named landscape evolution models, are unable to explain how nondendritic networks evolve into strongly branching systems. Here, we capture this evolution utilizing a model that incorporates how rivers erode in the lateral direction.
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... The small droplets generate a dense mist that descends on the experimental surface, leading to surface erosion by water runoff. Material detachment due to droplet impacts ("rain-splash" effect) (e.g., Sweeney et al., 2015) was not detected, and hillslope-like processes were not observed as channelization reached very close to the water divides. ...
... Hack's parameters of higher order channels are emerging properties of the landscape. b In the experiment, channelization reached very close to the divide (Sweeney et al., 2015;Turowski et al., 2006), hence x c is negligible. ...
Plan‐Form Evolution of Drainage Basins in Response to Tectonic Changes: Insights From Experimental and Numerical Landscapes
Article
Full-text available
  • Mar 2023
Spatial gradients in rock uplift control the relief and slope distribution in uplifted terrains. Relief and slopes, in turn, promote channelization and fluvial incision. Consequently, the geometry of drainage basins is linked to the spatial pattern of uplift. When the uplift pattern changes, basin geometry is expected to change by migrating water divides. However, the relations between drainage pattern and changing uplift patterns remain elusive. The current study investigates the plan‐view evolution of drainage basins and the reorganization of drainage networks in response to changes in the spatial pattern of uplift, focusing on basin interactions that produce globally observed geometrical scaling relations. We combine landscape evolution experiments and simulations to explore a double‐stage scenario: the emergence of a fluvial network under block uplift conditions followed by tilting that forces drainage reorganization. We find that the globally observed basin spacing ratio and Hack's parameters emerge early in the basin formation and are maintained by differential basin growth. In response to the tilting, main divide migration induces basin size changes. However, basins' scaling relations are mostly preserved within a narrow range of values, assisted by incorporation and disconnection of basins to and from the migrating main divide. Lastly, owing to similarities in landscape dynamics and response rate to uplift pattern changes between experiments and simulations, we conclude that the stream power incision model can represent fluvial erosion processes operating in experimental settings.
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... The persistence of the low erosion frequency in the SWS over Quaternary timescales might explain the observed longer and undissected slopes with greater debris cover (Selby, 1982). Accordingly, persistent of more frequent erosion during the Quaternary in the NES could explain the greater degree of channelization on the NES slopes (e.g., Sweeney et al., 2015) and higher exposed vertical cliffs (Figures 4 and 5). ...
The Impact of Extreme Rainstorms on Escarpment Morphology in Arid Areas: Insights From the Central Negev Desert
Article
Full-text available
  • Oct 2023
The impact of climate on topography, which is a theme in landscape evolution studies, has been demonstrated, mostly, at mountain range scales and across climate zones. However, in drylands, spatiotemporal discontinuities of rainfall and the crucial role of extreme rainstorms raise questions and challenges in identifying climate properties that govern surface processes. Here, we combine methods to examine hyperarid escarpment sensitivity to storm‐scale forcing. Using a high‐resolution DEM and field measurements, we analyzed the topography of a 40‐km‐long escarpment in the Negev desert (Israel). We also used rainfall intensity data from a convection‐permitting numerical weather model for storm‐scale statistical analysis. We conducted hydrological simulations of synthetic rainstorms, revealing the frequency of sediment mobilization along the sub‐cliff slopes. Results show that cliff gradients along the hyperarid escarpment increase systematically from the wetter (90 mm yr⁻¹) southwestern to the drier (45 mm yr⁻¹) northeastern sides. Also, sub‐cliff slopes at the southwestern study site are longer and associated with milder gradients and coarser sediments. Storm‐scale statistical analysis reveals a trend of increasing extreme (>10 years return‐period) intensities toward the northeast site, opposite to the trend in mean annual rainfall. Hydrological simulations based on these statistics indicate a higher frequency of sediment mobilization in the northeast, which can explain the pronounced topographic differences between the sites. The variations in landscape and rainstorm properties across a relatively short distance highlight the sensitivity of arid landforms to extreme events.
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... Uncertainties for sampling depth (± 5 cm) and propagated error of model predictions for percent modern carbon (± 0.19%) are shown for each profile. erosion rate of ~ 4.9 mm/yr.50 , approximately 54 m of overburden could have been eroded from the site in the past 11,000 years. ...
Accumulation of radiocarbon in ancient landscapes: A small but significant input of unknown origin
Article
Full-text available
  • May 2023
The persistence of organic carbon (C) in soil is most often considered at timescales ranging from tens to thousands of years, but the study of organic C in paleosols (i.e., ancient, buried soils) suggests that paleosols may have the capacity to preserve organic compounds for tens of millions of years. However, a quantitative assessment of C sources and sinks from these ancient terrestrial landscapes is complicated by additions of geologically modern (~ 10 Ka) C, primarily due to the infiltration of dissolved organic carbon. In this study, we quantified total organic C and radiocarbon activity in samples collected from 28- to 33-million-year-old paleosols that are naturally exposed as unvegetated badlands near eastern Oregon’s “Painted Hills”. We also used thermal and evolved gas analysis to examine the thermodynamic stability of different pools of C in bulk samples. The study site is part of a ~ 400-m-thick sequence of Eocene–Oligocene (45–28 Ma) paleosols, and thus we expected to find radiocarbon-free samples preserved in deep layers of the lithified, brick-like exposed outcrops. Total organic C, measured in three individual profiles spanning depth transects from the outcrop surface to a 1-m depth, ranged from 0.01 to 0.2 wt% with no clear C-concentration or age-depth profile. Ten radiocarbon dates from the same profiles reveal radiocarbon ages of ~ 11,000–30,000 years BP that unexpectedly indicate additions of potentially modern organic C. A two-endmember mixing model for radiocarbon activity suggests that modern C may compose ~ 0.5–2.4% of the total organic C pool. Thermal and evolved gas analysis showed the presence of two distinct pools of organic C, but there was no direct evidence that C compounds were associated with clay minerals. These results challenge the assumption that ancient badland landscapes are inert and “frozen in time” and instead suggest they readily interact with the modern C cycle.
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... In particular, R S is the rainfall erosivity indicator (unitless) most associated with splash erosion. In this regard, Sweeney et al. [60] showed that rainsplashdriven disturbance is a robust proxy for hillslope transport, such that increasing hillslope transport efficiency decreases drainage density. R Q is the erosivity indicator (unitless) most closely associated with surface flow. ...
A framework for modelling emergent sediment loss in the Ombrone River Basin, central Italy
Article
Full-text available
  • Feb 2023
Water can represent a hazard causing soil erosion and it is essential to anticipate the potential environmental impacts of sustained rainwater energy to achieve sustainability. Here, we present the modelling of the erosive force of water for the production of soil sediment in a Mediterranean basin of central Italy (Ombrone River Basin, ORB). A point of departure is the historical recognition of the environmental factors causing sediments loss (SL) by water. A semi-empirical framework was then proposed for the upscaling of SL based on the Foster-Thornes approach (EUSEM: Environmental Upscaling Sediment Erosion Model) in order to give an insight into annual sediment losses (SL) over the period 1949–1977 (calibration) and over a longer time-frame (1942–2020: reconstruction). Two change-points were detected: 1967 and 1986. During this period, SL was affected by a sharp decrease from 625 Mg km ⁻² yr ⁻¹ , before the first change-point (when SL was only occasionally below the tolerable soil loss threshold of 150 Mg km ⁻² yr ⁻¹ ), to 233 Mg km ⁻² yr ⁻¹ , during the transition phase 1967–1985 (mostly above the warning treshold of 140 Mg km ⁻² yr ⁻¹ ). This decrease coincided with an enhancing of vegetation throughout the basin due to an ongoing afforestation process. After this period, a resurgence of climatic forcing led to a further, but more contained, increase in SL, from 1996 onwards. This case-study illustrates the application and results that can be obtained with the framework for the outcome of environmental change due to sediment losses in a Mediterranean fluvial basin. Limitations and perspectives of this approach are given as conclusion.
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... The majority of Earth's continental surface is occupied by hillslopes shaped by climate-surface processes interactions (e.g., Dixon et al., 2009;Sweeney et al., 2015;Perron, 2017;Richardson et al., 2019). Aspect-dependent slope asymmetry is considered a clear-cut example of landscape sensitivity to micro-climate and is a central observation concerning feedbacks between climate and erosion (Desta et al., 2004;Istanbulluoglu et al., 2008;Poulos et al., 2012;Pelletier et al., 2018). ...
Aspect-dependent bedrock weathering, cliff retreat, and cliff morphology in a hyperarid environment
Article
  • Nov 2022
Deciphering aspect-related hillslope asymmetry can enhance our understanding of the influence of climate on Earth’s surface morphology and the linkage between topographic morphology and erosion processes. Although hillslope asymmetry is documented worldwide, the role of microclimatic factors in the evolution of dryland cliffs has received little attention. Here, we address this gap by quantifying aspect-dependent bedrock weathering, slope-rill morphology, and sub-cliff clast transport rates in the hyperarid Negev desert, Israel, based on light detection and ranging (LiDAR)-derived topography, clast-size measurements, and cosmogenic 10Be concentrations. Cliff retreat rates were evaluated using extrapolated profiles from dated talus flatirons and 10Be measurements from the cliff face and sub-cliff sediments. We document systematic, aspect-dependent patterns of south-facing slopes being less steep and finer-grained relative to east- and north-facing aspects. In addition, cliff retreat and clast transport rates on slopes of the south-facing aspect are faster compared to the other aspects. Our data demonstrate that bedrock weathering of the cliff face and the corresponding grain size of cliff-derived clasts delivered to the slopes constitute a first-order control on cliff retreat and sediment transport rates. We demonstrate that the morphology of the cliff and the pattern of bedrock weathering co-vary with the solar radiation flux and hence that cliff evolution in hyperarid regions is modulated by aspect-dependent solar radiation. These results help to better understand interactions between climate and dryland surface processes.
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Responses of soil moisture at different topographic positions to rainfall events along a steep subtropical forested hillslope
Article
  • May 2024
Understanding the dynamic response of soil moisture to rainfall is crucial for describing hydrological processes at the hillslope scale. However, because of sparse monitoring coupled with the complexity of water movement and steep topography, the findings of rainfall‐related soil moisture dynamics have not always been consistent, indicating a need for systematic investigations of soil moisture dynamics and infiltration patterns following rainfall inputs at multiple topographic positions along a hillslope. This study aimed to examine the nature of these responses by characterizing and quantifying the response amplitude, rate and time for 37 large rainfall events at 25 combinations of topographic positions and soil depths along a steep forested hillslope. Our results showed that soil moisture responses under different rainfall patterns could be attributed to one or the other rainfall characteristics, such as rainfall intensity and amount. However, soil moisture dynamics at different hillslope positions after rainfall varied widely due to the controls of soil properties, topography, and non‐equilibrium flow. Preferential flow was more evident under dry initial soil conditions than under wet initial soil conditions. Findings of this study reveal that the dynamic response patterns of soil moisture to rainfall do not always follow topographic controls, which can improve our understanding of water cycling related to the infiltration process at the hillslope scale, and support water resources management in subtropical mountain ecosystems.
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Modeling Cosmogenic Nuclides in Transiently Evolving Topography and Chemically Weathering Soils
Article
Full-text available
  • Oct 2023
Terrestrial cosmogenic nuclides (TCN) are widely employed to infer denudation rates in mountainous landscapes. The calculation of an inferred denudation rate (Dinf) from TCN concentrations is typically performed under the assumptions that denudation rates were steady during TCN accumulation and that soil chemical weathering negligibly impacted soil mineral abundances. In many landscapes, however, denudation rates were not steady and soil composition was significantly impacted by chemical weathering, which complicates interpretation of TCN concentrations. We present a landscape evolution model that computes transient changes in topography, soil thickness, soil mineralogy, and soil TCN concentrations. We used this model to investigate TCN responses in transient landscapes by imposing idealized perturbations in tectonically (rock uplift rate) and climatically sensitive parameters (soil production efficiency, hillslope transport efficiency, and mineral dissolution rate) on initially steady‐state landscapes. These experiments revealed key insights about TCN responses in transient landscapes. (a) Accounting for soil chemical erosion is necessary to accurately calculate Dinf. (b) Responses of Dinf to tectonic perturbations differ from those to climatic perturbations, suggesting that spatial and temporal patterns in Dinf are signatures of perturbation type and magnitude. (c) If soil chemical erosion is accounted for, basin‐averaged Dinf inferred from TCN in stream sediment closely tracks actual basin‐averaged denudation rate, showing that Dinf is a reasonable proxy for actual denudation rate, even in many transient landscapes. (d) Response times of Dinf to perturbations increase with hillslope length, implying that response times should be sensitive to the climatic, biological, and lithologic processes that control hillslope length.
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Hillslope‐Channel Transitions and the Role of Water Tracks in a Changing Permafrost Landscape
Article
Full-text available
  • Sep 2023
The Arctic is experiencing rapid climate change, and the effect on hydrologic processes and resulting geomorphic changes to hillslopes and channels is unclear because we lack quantitative models and theory for rapid changes resulting from thawing permafrost. The presence of permafrost modulates water flow and the stability of soil‐mantled slopes, implying that there should be a signature of permafrost processes, including warming‐driven disturbance, in channel network extent. To inform understanding of hillslope‐channel dynamics under changing climates, we examined soil‐mantled hillslopes within a ∼300 km² area of the Seward Peninsula, western Alaska, where discontinuous permafrost is particularly susceptible to thaw and rapid landscape change. In this study, we pair high‐resolution topographic and satellite data to multi‐annual observations of InSAR‐derived surface displacement over a 5‐year period to quantify spatial variations in topographic change across an upland landscape. We find that neither the basin slope nor the presence of knickzones controls the magnitude of recent surface displacements within the study basin, as may be expected under conceptual models of temperate hillslope evolution. Rather, the highest displacement magnitudes tended to occur at the broad hillslope‐channel transition zone. In this study area, this zone is occupied by water tracks, which are zero‐order ecogeomorphic features that concentrate surface and subsurface flow paths. Our results suggest that water tracks, which appear to occupy hillslope positions between saturation and incision thresholds, are vulnerable to warming‐induced subsidence and incision. We hypothesize that gullying within water tracks will outpace infilling by hillslope processes, resulting in the growth of the channel network under future warming.
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Gopher bioturbation: Field evidence for non-linear hillslope diffusion
Article
Full-text available
  • Dec 2000
It has generally been assumed that diffusive sediment transport on soil-mantled hillslopes is linearly dependent on hillslope gradient. Fieldwork was done near Santa Barbara, California, to develop a sediment transport equation for bioturbation by the pocket gopher (Thomomys bottae) and to determine whether it supports linear diffusion. The route taken by the sediment is divided into two parts, a subsurface path followed by a surface path. The first is the transport of soil through the burrow to the burrow opening. The second is the discharge of sediment from the burrow opening onto the hillslope surface. The total volumetric sediment flux, as a function of hillslope gradient, is found to be: q(s) (cm(3) cm(-1) a(-1)) = 176(dz/dx)(3) - 189(dz/dx)(2) + 68(dz/dx) + 34(dz/dx)(0.4). This result does not support the use of linear diffusion for hillslopes where gopher bioturbation is the dominant mode of sediment transport. A one-dimensional hillslope evolution program was used to evolve hillslope profiles according to non-linear and linear diffusion and to compare them to a typical hillslope. The non-linear case more closely resembles the actual profile with a convex cap at the divide leading into a straight midslope section. Copyright (C) 2000 John Wiley & Sons, Ltd.
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Dynamics of the stream-power river incision model: Implications for height limits of mountain ranges, landscape response timescales, and research needs
Article
Full-text available
  • Aug 1999
The longitudinal profiles of bedrock channels are a major component of the relief structure of mountainous drainage basins and therefore limit the elevation of peaks and ridges. Further, bedrock channels communicate tectonic and climatic signals across the landscape, thus dictating, to first order, the dynamic response of mountainous landscapes to external forcings. We review and explore the stream-power erosion model in an effort to (1) elucidate its consequences in terms of large-scale topographic (fluvial) relief and its sensitivity to tectonic and climatic forcing, (2) derive a relationship for system response time to tectonic perturbations, (3) determine the sensitivity of model behavior to various model parameters, and (4) integrate the above to suggest useful guidelines for further study of bedrock channel systems and for future refinement of the streampower erosion law. Dimensional analysis reveals that the dynamic behavior of the stream-power erosion model is governed by a single nondimensional group that we term the uplift-erosion number, greatly reducing the number of variables that need to be considered in the sensitivity analysis. The degree of nonlinearity in the relationship between stream incision rate and channel gradient (slope exponent n) emerges as a fundamental unknown. The physics of the active erosion processes directly influence this nonlinearity, which is shown to dictate the relationship between the uplift-erosion number, the equilibrium stream channel gradient, and the total fluvial relief of mountain ranges. Similarly, the predicted response time to changes in rock uplift rate is shown to depend on climate, rock strength, and the magnitude of tectonic perturbation, with the slope exponent n controlling the degree of dependence on these various factors. For typical drainage basin geometries the response time is relatively insensitive to the size of the system. Work on the physics of bedrock erosion processes, their sensitivity to extreme floods, their transient responses to sudden changes in climate or uplift rate, and the scaling of local rock erosion studies to reach-scale modeling studies are most sorely needed.
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Shaping post-orogenic landscapes by climate and chemical weathering
Article
Full-text available
  • Oct 2013
  • GEOLOGY
The spacing of hills and valleys reflects the competition between disturbance-driven (or diffusive) transport on hillslopes and concentrative (or advective) transport in valleys, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, South Africa, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying mean annual precipitation (MAP = 470 mm, 550 mm, and 730 mm) impacts hill-valley spacing or landscape dissection. Catchment-averaged erosion rates, based on 10Be concentrations in river sands, are low (3–6 m/m.y.) and vary minimally across the three sites. Our lidar-derived slope-area analyses reveal that hillslopes in the dry site are gentle (3%) and short, such that the terrain is low relief and appears highly dissected. With increasing rainfall, hillslopes lengthen and increase in gradient (6%–8%), resulting in less-dissected, higher
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Controls on the spacing of first-order valleys
Article
Full-text available
  • Dec 2008
Many landscapes are composed of ridges and valleys that are uniformly spaced, even where valley locations are not controlled by bedrock structure. Models of long-term landscape evolution have reproduced this phenomenon, yet the process by which uniformly spaced valleys develop is not well understood, and there is no quantitative framework for predicting valley spacing. Here we use a numerical landscape evolution model to investigate the development of uniform valley spacing. We find that evenly spaced valleys arise from a competition between adjacent drainage basins for drainage area (a proxy for water flux) and that the spacing becomes more uniform as the landscape approaches a topographic equilibrium. Valley spacing is most sensitive to the relative rates of advective erosion processes (such as stream incision) and diffusion-like mass transport (such as soil creep) and less sensitive to the magnitude of a threshold that limits the spatial extent of stream incision. Analysis of a large number of numerical solutions reveals that valley spacing scales with a ratio of characteristic diffusion and advection timescales that is analogous to a Péclet number. We use this result to derive expressions for equilibrium valley spacing and drainage basin relief as a function of the rates of advective and diffusive processes and the spatial extent of the landscape. The observed scaling relationships also provide insight into the cause of transitions from rill-like drainage networks to branching networks, the spatial scale of first-order drainage basins, the contributing area at which hillslopes transition into valleys, and the narrow range of width-to-length ratios of first-order basins.
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A Detachment-Limited Model of Drainage-Basin Evolution
Article
Full-text available
A drainage basin simulation model introduced here incorporates creep and threshold slumping and both detachment- and transport-limited fluvial processes. Fluvial erosion of natural slopes and headwater channels is argued to be dominantly detachment-limited. Such slopes undergo nearly parallel retreat and replacement with alluvial surfaces under fixed base level, in contrast with gradual slope decline for transport-limited conditions. The arrangement of divides and valleys is sensitive to initial conditions, although average morphology is insensitive. Dissected, initially flat surfaces in which downstream concavity is slight exhibit nearly parallel drainage, compared to very wandering main valleys when concavity is great. Steady state is reached after a cumulative base level drop approximately 3 times the final relief. Simulated valley systems are similar to those predicted by a previous model of optimal drainage basins. A critical value of slope divergence normalized by average slope gradient is a useful criterion for defining the valley network.
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Erosion Thresholds and Land Surface Morphology
Article
Full-text available
  • Jan 1992
  • GEOLOGY
We propose a graphical technique to analyze the entirety of landforms in a catchment to define quantitatively the spatial variation in the dominance of different erosion processes. High-resolution digital elevation data of a 1.2 km2 hilly area where the channel network had been mapped in the field were used in the digital terrain model, TOPOG, to test threshold theories for erosion. The field observation that saturation overland flow is rare outside convergent zones provided a significant constraint on the hydrologic parameter in the model. This model was used in threshold theories to predict areas of slope instability and areas subject to erosion by saturation overland flow, both of which can contribute to channel initiation. Overall, the landscape can be divided, using erosion threshold lines, into areas prone to channel instability due to runoff and stable areas where diffusive transport predominates. -from Authors
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The relationship between frost heave and downslope soil movement: Field measurements in the Japanese Alps
Article
  • Apr 1998
  • PERMAFROST PERIGLAC
This paper presents three years of data on soil movement on two alpine slopes. Automatic instrumentation provided data on ground surface heaving and downslope subsurface debris displacements. Manual measurements of painted lines indicated downslope surface debris displacements. Observations highlight the role of diurnal frost heaving in soil movements. Frost heave of up to 3 cm takes place 30 to 70 times per year, reflecting both needle ice growth and/or near-surface ice lens formation. The frost heave activity is accompanied by downslope displacements of the uppermost 20 cm of soil. The surface debris moves downslope at a velocity of about 50 cm a-1 on the 30° slope and about 5 cm a-1 on the 14° slope. The potential frost creep predicted by the cumulative heave amount considerably underestimates the surface velocity on the 30° slope, and slightly overestimates it on the 14° slope. The surface velocity on both slopes is nearly proportional to the second power of the slope gradient. These conditions demonstrate that steeper slopes are dominated by the rolling of surface debris resulting from the downslope bending of ice needles. Velocity profiles are discontinuous below the surface debris, probably reflecting the differential movement by needle ice creep and frost creep.
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Landscape instability in a model drainage basin
Article
  • Dec 2000
  • GEOLOGY
Do drainage basins develop static river networks when subject to steady forcing? While current landscape evolution models differ in formulation and implementation, they have the common characteristic that when run for long times at constant forcing, they evolve to a static steady-state configuration in which erosion everywhere balances uplift rate. This results in temporally stationary ridge and valley networks. We have constructed a physical model of a drainage basin in which we can impose constant rainfall and uplift conditions. The model landscapes never become static, and they are not sensitive to initial surface conditions. Ridges migrate laterally, change length, and undergo topographic inversion (streams occupy former ridge locations). Lateral stream migration can also produce strath terraces. This occurs without any change in external forcing, so the terraces must be considered autocyclic. The experimental drainage basin also exhibits autocyclic (internally generated) oscillations in erosion rate over a variety of time scales, despite constant forcing. The experimental landforms are clearly not perfect analogs of natural erosional networks, but the results raise the possibility that natural systems may be more dynamic than the current models would suggest, and that features like strath terraces that are generally interpreted in terms of external forcing may arise autocyclically as well.
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