A low‐dimensional model of bedrock weathering and lateral flow coevolution in hillslopes: 1. Hydraulic theory of reactive transport

This is the first of a two‐part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low‐dimensional model based on hydraulic groundwater theory (also known as Dupuit or Boussinesq theory). Here, we examine the effect of lateral flow on the downward fluxes of water and solutes through perched groundwater at steady state. We derive analytical expressions describing the decline in the downward flux rate with depth. Using these, we obtain analytical expressions for water age in a number of cases. The results show that when the permeability field is homogeneous, the spatial structure of water age depends qualitatively on a single dimensionless number, Hi. This number captures the relative contributions to the lateral hydraulic potential gradient of the relief of the lower‐most impermeable boundary (which may be below the weathering front within permeable or incipiently weathered bedrock) and the water table. A “scaled lateral symmetry” exists when Hi is low: age varies primarily in the vertical dimension, and variations in the horizontal dimension x almost disappear when the vertical dimension z is expressed as a fraction z/H(x) of the laterally flowing system thickness H(x). Taking advantage of this symmetry, we show how the lateral dimension of the advection–diffusion‐reaction equation can be collapsed, yielding a 1‐D vertical equation in which the advective flux downward declines with depth. The equation holds even when the permeability field is not homogeneous, as long as the variations in permeability have the same scaled lateral symmetry structure. This new 1‐D approximation is used in the accompanying paper to extend chemical weathering models derived for 1‐D columns to hillslope domains.     

Relating the depth of the water table to the depth of weathering

Weathering of bedrock creates and occludes permeability, affecting subsurface water flow. Often, weathering intensifies above the water table. On the contrary, weathering can also commence below the water table. To explore relationships between weathering and the water table, a simplified weathering model for an eroding hillslope was formulated that takes into account both saturated and unsaturated subsurface water flow (but does not fully account for changes in dissolved gas chemistry). The phreatic line was calculated using solutions to mathematical treatments for both zones. In the model, the infiltration rate at the hill surface sets both the original and the eventual steady-state position of the water table with respect to the weathering reaction front. Depending on parameters, the weathering front can locate either above or below the water table at steady state. Erosion also affects the water table position by changing porosity and permeability even when other hydrological conditions (e.g. hydraulic conductivity of parent material, infiltration rate at the surface) do not change. The total porosity in a hill (water storage capacity) was found to increase with infiltration rate (all else held constant). This effect was diminished by increasing the erosion rate. We also show examples of how the infiltration rate affects the position of the water table and how infiltration rate affects weathering advance. Published 2020. This article is a U.S. Government work and is in the public domain in the USA     

A numerical investigation of bedrock groundwater recharge and exfiltration on soil mantled hillslopes

Here we use Richards Equation models of variably saturated soil and bedrock groundwater flow to investigate first-order patterns of the coupling between soil and bedrock flow systems. We utilize a Monte Carlo sensitivity analysis to identify important hillslope parameters controlling bedrock recharge and then model the transient response of bedrock and soil flow to seasonal precipitation. Our results suggest that hillslopes can be divided into three conceptual zones of groundwater interaction, (a) the zone of lateral unsaturated soil moisture accumulation (upper portion of hillslope), (b) the zone of soil saturation and bedrock recharge (middle of hillslope) and (c) the zone of saturated-soil lateral flow and bedrock groundwater exfiltration (bottom of hillslope). Zones of groundwater interaction expand upslope during periods of precipitation and drain downslope during dry periods. The amount of water partitioned to the bedrock groundwater system a can be predicted by the ratio of bedrock to soil saturated hydraulic conductivity across a variety of hillslope configurations. Our modelled processes are qualitatively consistent with observations of shallow subsurface saturation and groundwater fluctuation on hillslopes studied in our two experimental watersheds and support a conceptual model of tightly coupled shallow and deep subsurface circulation where groundwater recharge and discharge continuously stores and releases water from longer residence time storage.     

Morphodynamics of a bedrock‐alluvial meander bend that incises as it migrates outward: approximate solution of permanent form

In meandering rivers cut into bedrock, erosion across a channel cross‐section can be strongly asymmetric. At a meander apex, deep undercutting of the outer bank can result in the formation of a hanging cliff (which may drive hillslope failure), whereas the inner bank adjoins a slip‐off slope that connects to the hillslope itself. Here we propose a physically‐based model for predicting channel planform migration and incision, point bar and slip‐off slope formation, bedrock abrasion, the spatial distribution of alluvial cover, and adaptation of channel width in a mixed bedrock‐alluvial channel. We simplify the analysis by considering a numerical model of steady, uniform bend flow satisfying cyclic boundary conditions. Thus in our analysis, ‘sediment supply’, i.e. the total volume of alluvium in the system, is conserved. In our numerical simulations, the migration rate of the outer bank is a specified parameter. Our simulations demonstrate the existence of an approximate state of dynamic equilibrium corresponding to a near‐solution of permanent form in which a bend of constant curvature, width, cross‐sectional shape and alluvial cover distribution migrates diagonally downward at constant speed, leaving a bedrock equivalent of a point bar on the inside of the bend. Channel width is set internally by the processes of migration and incision. We find that equilibrium width increases with increasing sediment supply, but is insensitive to outer bank migration rate. The slope of the bedrock point bar varies inversely with both outer bank migration rate and sediment supply. Although the migration rate of the outer bank is externally imposed here, we discuss a model modification that would allow lateral side‐wall abrasion to be treated in a manner similar to the process of bedrock incision. Copyright © 2016 John Wiley & Sons, Ltd.     

Climate driven coevolution of weathering profiles and hillslope topography generates dramatic differences in critical zone architecture

Considerable debate revolves around the relative importance of rock type, tectonics, and climate in creating the architecture of the critical zone. We demonstrate the importance of climate and in particular the rate of water recharge to the subsurface, using numerical models that incorporate hydrologic flowpaths, chemical weathering, and geomorphic rules for soil production and transport. We track alterations in both solid phase (plagioclase to clay) and water chemistry along hydrologic flowpaths that include lateral flow beneath the water table. To isolate the role of recharge, we simulate dry and wet cases and prescribe identical landscape evolution rules. The weathering patterns that develop differ dramatically beneath the resulting parabolic interfluves. In the dry case, incomplete weathering is shallow and surface parallel, whereas in the wet case, intense weathering occurs to depths approximating the base of the bounding channels, well below the water table. Exploration of intermediate cases reveals that the weathering state of the subsurface is strongly governed by the ratio of the rate of advance of the weathering front itself controlled by the water input rate, and the rate of erosion of the landscape. The system transitions between these end‐member behaviours rather abruptly at a weathering front speed ‐ erosion rate ratio of approximately 1. Although there are undoubtedly direct roles for tectonics and rock type in critical zone architecture, and yet more likely feedbacks between these and climate, we show here that differences in hillslope‐scale weathering patterns can be strongly controlled by climate.     

Rock damage and regolith transport by frost: an example of climate modulation of the geomorphology of the critical zone

In this article we craft process‐specific algorithms that capture climate control of hillslope evolution in order to elucidate the legacy of past climate on present critical zone architecture and topography. Models of hillslope evolution traditionally comprise rock detachment into the mobile layer, mobile regolith transport, and a channel incision or aggradation boundary condition. We extend this system into the deep critical zone by considering a weathering damage zone below the mobile regolith in which rock strength is diminished; the degree of damage conditions the rate of mobile regolith production. We first discuss generic damage profiles in which appropriate length and damage scales govern profile shapes, and examine their dependence upon exhumation rate. We then introduce climate control through the example of rock damage by frost‐generated crack growth. We augment existing frost cracking models by incorporating damage rate limitations for long transport distances for water to the freezing front. Finally we link the frost cracking damage model, a mobile regolith production rule in which rock entrainment is conditioned by the damage state of the rock, and a frost creep transport model, to examine the evolution of an interfluve under oscillating climate. Aspect‐related differences in mean annual surface temperatures result in differences in bedrock damage rate and mobile regolith transport efficiency, which in turn lead to asymmetries in critical zone architecture and hillslope form (divide migration). In a quasi‐steady state hillslope, the lowering rate is uniform, and the damage profile is better developed on north‐facing slopes where the frost damage process is most intense. Because the residence times of mobile regolith and weathered bedrock in such landscapes are on the order of 10 to 100 ka, climate cycles over similar timescales result in modulation of transport and damage efficiencies. These lead to temporal variation in mobile regolith thickness, and to corresponding changes in sediment delivery to bounding streams. Copyright © 2012 John Wiley & Sons, Ltd.     

Exploring geochemical controls on weathering and erosion of convex hillslopes: beyond the empirical regolith production function

Landscape curvature evolves in response to physical, chemical, and biological influences that cannot yet be quantified in models. Nonetheless, the simplest models predict the existence of equilibrium hillslope profiles. Here, we develop a model describing steady‐state regolith production caused by mineral dissolution on hillslopes which have attained an equilibrium parabolic profile. When the hillslope lowers at a constant rate, the rate of chemical weathering is highest at the ridgetop where curvature is highest and the ridge develops the thickest regolith. This result derives from inclusion of all the terms in the mathematical definition of curvature. Including these terms shows that the curvature of a parabolic hillslope profile varies with distance from the ridge. The hillslope model (meter‐scale) is similar to models of weathering rind formation (centimeter‐scale) where curvature‐driven solute transport causes development of the thickest rinds at highly curved clast corners. At the clast scale, models fit observations. Here, we similarly explore model predictions of the effect of curvature at the hillslope scale. The hillslope model shows that when erosion rates are small and vertical porefluid infiltration is moderate, the hill weathers at both ridge and valley in the erosive transport‐limited regime. For this regime, the reacting mineral is weathered away before it reaches the land surface: in other words, the model predicts completely developed element‐depth profiles at both ridge and valley. In contrast, when the erosion rate increases or porefluid velocity decreases, denudation occurs in the weathering‐limited regime. In this regime, the reacting mineral does not weather away before it reaches the land surface and simulations predict incompletely developed profiles at both ridge and valley. These predictions are broadly consistent with observations of completely developed element‐depth profiles along hillslopes denuding under erosive transport‐limitation but incompletely developed profiles along hillslopes denuding under weathering limitation in some field settings. Copyright © 2013 John Wiley & Sons, Ltd.     

Factors controlling seasonal groundwater and solute flux from snow‐dominated basins

Critical zone influences on hydrologic partitioning, subsurface flow paths and reactions along these flow paths dictate the timing and magnitude of groundwater and solute flux to streams. To isolate first‐order controls on seasonal streamflow generation within highly heterogeneous, snow‐dominated basins of the Colorado River, we employ a multivariate statistical approach of end‐member mixing analysis using a suite of daily chemical and isotopic observations. Mixing models are developed across 11 nested basins (0.4 to 85 km²) spanning a gradient of climatological, physical, and geological characteristics. Hydrograph separation using rain, snow, and groundwater as end‐members indicates that seasonal contributions of groundwater to streams is significant. Mean annual groundwater flux ranges from 12% to 33% whereas maximum groundwater contributions of 17% to 50% occur during baseflow. The direct relationship between snow water equivalent and groundwater flux to streams is scale dependent with a trend toward self‐similarity when basins exceed 5.5 km². We find groundwater recharge increases in basins of high relief and within the upper subalpine where maximum snow accumulation is coincident with reduced conifer cover and lower canopy densities. The mixing model developed for the furthest downstream site did not transfer to upstream basins. The resulting error in predicted stream concentrations points toward weathering reactions as a function of source rock and seasonal shifts in flow path. Additionally, the potential for microbial sulfate reduction in floodplain sediments along a low‐gradient, meandering portion of the river is sufficient to modify hillslope contributions and alter mixing ratios in the analysis. Soil flushing in response to snowmelt is not included as an end‐member but is identified as an important mechanism for release of solutes from these mountainous watersheds. End‐member mixing analysis used in combination with high‐frequency observations reveals important aspects of catchment hydrodynamics across scale.     

Contributions of bedrock groundwater to the upscaling of storm‐runoff generation processes in weathered granitic headwater catchments

We examined the contributions of bedrock groundwater to the upscaling of storm‐runoff generation processes in weathered granitic headwater catchments by conducting detailed hydrochemical observations in five catchments that ranged from zero to second order. End‐member mixing analysis (EMMA) was performed to identify the geographical sources of stream water. Throughfall, hillslope groundwater, shallow bedrock groundwater, and deep bedrock groundwater were identified as end members. The contribution of each end member to storm runoff differed among the catchments because of the differing quantities of riparian groundwater, which was recharged by the bedrock groundwater prior to rainfall events. Among the five catchments, the contribution of throughfall was highest during both baseflow and storm flow in a zero‐order catchment with little contribution from the bedrock groundwater to the riparian reservoir. In zero‐order catchments with some contribution from bedrock groundwater, stream water was dominated by shallow bedrock groundwater during baseflow, but it was significantly influenced by hillslope groundwater during storms. In the first‐order catchment, stream water was dominated by shallow bedrock groundwater during storms as well as baseflow periods. In the second‐order catchment, deeper bedrock groundwater than that found in the zero‐order and first‐order catchments contributed to stream water in all periods, except during large storm events. These results suggest that bedrock groundwater influences the upscaling of storm‐runoff generation processes by affecting the linkages of geomorphic units such as hillslopes, riparian zones, and stream channels. Our results highlight the need for a three‐dimensional approach that considers bedrock groundwater flow when studying the upscaling of storm‐runoff generation processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Groundwater dynamics in a till hillslope: flow directions,gradients and delay

Knowledge of groundwater dynamics is important for the understanding of hydrological controls on chemical processes along the water flow pathways. To increase our knowledge of groundwater dynamics in areas with shallow groundwater, the groundwater dynamics along a hillslope were studied in a boreal catchment in Southern Sweden. The forested hillslope had a 1‐ to 2‐m deep layer of sandy till above bedrock. The groundwater flow direction and slope were calculated under the assumption that the flow followed the slope of the groundwater table, which was computed for different triangles, each defined by three groundwater wells. The flow direction showed considerable variations over time, with a maximum variation of 75°. During periods of high groundwater levels the flow was almost perpendicular to the stream, but as the groundwater level fell, the flow direction became gradually more parallel to the stream, directed in the downstream direction. These findings are of importance for the interpretation of results from hillslope transects, where the flow direction usually is assumed to be invariable and always in the direction of the hillslope. The variations in the groundwater flow direction may also cause an apparent dispersion for groundwater‐based transport. In contrast to findings in several other studies, the groundwater level was most responsive to rainfall and snowmelt in the upper part of the hillslope, while the lower parts of the slope reached their highest groundwater level up to 40 h after the upper parts. This can be explained by the topography with a wetter hollow area in the upper part. Copyright © 2011 John Wiley & Sons, Ltd.     

Epigenetic gorges in fluvial landscapes

Epigenetic gorges form when channels that have been laterally displaced during episodes of river blockage or aggradation incise down into bedrock spurs or side‐walls of the former valley rather than excavating unconsolidated fills and reinhabiting the buried paleovalley. Valley‐filling events that promote epigenetic gorges can be localized, such as a landslide dam or an alluvial/debris flow fan deposit at a tributary junction, or widespread, such as fluvial aggradation in response to climate change or fluctuating base‐level. The formation of epigenetic gorges depends upon the competition between the resistance to transport, strength and roughness of valley‐filling sediments and a river's ability to sculpt and incise bedrock. The former affects the location and lateral mobility of a channel incising into valley‐filling deposits; the latter determines rates of bedrock incision should the path of the incising channel intersect with bedrock that is not the paleovalley bottom. Epigenetic gorge incision, by definition, post‐dates the incision that originally cut the valley. Strath terraces and sculpted bedrock walls that form in relation to epigenetic gorges should not be used to directly infer river incision induced by tectonic activity or climate variability. Rather, they are indicative of the variability of short‐term bedrock river incision and autogenic dynamics of actively incising fluvial landscapes. The rate of bedrock incision associated with an epigenetic gorge can be very high (>1 cm/yr), typically orders of magnitude higher than both short‐ and long‐term landscape denudation rates. In the context of bedrock river incision and landscape evolution, epigenetic gorges force rivers to incise more bedrock, slowing long‐term incision and delaying the adjustment of rivers to regional tectonic and climatic forcing. Copyright © 2008 John Wiley & Sons, Ltd.     

Testing etching hypothesis for the shaping of granite dome structures beneath lateritic weathering landsurfaces using ERT method

An Erratum has been published for this article in Earth Surface Processes and Landforms 28(13) 2003, 1491. Granite domes, boulders and knobs buried within saprolite have been detected beneath lateritic weathering landsurfaces using 2D electrical resistivity tomography (ERT). This technique provides a valuable means of mapping the bedrock topography and the regolith structures underneath landsurfaces, as it is intrinsically very sensitive to the electrical properties of superimposed pedological, hydrological and geological layers, allowing the determination of their relative geometry and spatial relationships. For instance, 2D inverse electrical resistivity models including topographic data permit the de?nition of lithostratigraphic cross‐sections. It shows that resistive layers, such as the more or less hardened ferruginous horizons and/or the bedrock, are generally well differentiated from poorly resistive layers, such as saprolite, including water‐saturated lenses, as has been corroborated by past and actual borehole observations. The analysis of the 2D geometrical relations between the weathering front, i.e. the bedrock topography, and the erosion surface, i.e. the landsurface topography, documents the weathering and erosion processes governing the development of the landforms and the underlying structures, thus allowing the etching hypothesis to be tested. The in?ltration waters are diverted by bedrock protrusions, which behave as structural thresholds compartmentalizing the saprolite domain, and also the regolith water table, into distinct perched saturated subdomains. The diverted waters are thus accumulated in bedrock troughs, which behave like underground channels where the saprolite production rate may be enhanced, provided that the water drainage is ef?cient. If the landsurface topography controls the runoff dynamics, the actual bedrock topography as depicted by ERT imaging in?uences the hydrodynamics beneath the landsurface. In some way, this may control the actual weathering rate and the shaping of bedrock protrusions as granite domes and knobs within thick saprolite, before their eventual future exposure. Copyright © 2003 John Wiley & Sons, Ltd.     

Quantifying river incision into low‐relief surfaces using local and catchment‐wide 10Be denudation rates

Relief generation in non‐glaciated regions is largely controlled by river incision into bedrock but datable fluvial terraces that allow quantifying incision rates are not always present. Here we suggest a new method to determine river incision rates in regions where low‐relief surfaces are dissected by streams. The approach consists of three steps and requires the ¹⁰Be concentrations of a stream sediment sample and a regolith sample from the low‐relief surface. In the first step, the spatial distribution of ¹⁰Be surface concentrations in the given catchment is modelled by assuming that denudation rates are controlled by the local hillslope angles. The slope–denudation rate relation for this catchment is then quantified by adjusting the relation between slope angle and denudation rate until the average ¹⁰Be concentration in the model is equal to the one measured in the stream sediment sample. In the second step, curved swath profiles are used to measure hillslope angles adjacent to the main river channel. Third, the mean slope angle derived from these swath profiles and the slope–denudation relation are used to quantify the river incision rate (assuming that the incision rate equals the denudation rate on adjacent hillslopes). We apply our approach to two study areas in southern Tibet and central Europe (Black Forest). In both regions, local ¹⁰Be denudation rates on flat parts of the incised low‐relief surface are lower than catchment‐wide denudation rates. As the latter integrate across the entire landscape, river incision rates must exceed these spatially averaged denudation rates. Our approach yields river incision rates between ~15 and ~30 m/Ma for the Tibetan study area and incision rates of ~70 to ~100 m/Ma in the Black Forest. Taking the lowering of the low‐relief surfaces into account suggests that relief in the two study areas increases at rates of 10–20 and 40–70 m/Ma, respectively. Copyright © 2018 John Wiley & Sons, Ltd.     

Defining hydrochemical evolution of streamflow through flowpath dynamics in Kawakami headwater catchment,Central Japan

The hydrochemical behaviour of catchments is often investigated by inferring stream chemistry through identification of source areas involved in hydrograph separation analysis, yet its dynamic evolution of hydrologic pathways has received little attention. Intensive hydrometric and hydrochemical measurements were performed during two different storms on March 29, 2001 and August 21–22, 2001 to define hydrochemical evolution under the dynamic of flow pathways in a 5·2 ha first‐order drainage of the Kawakami experimental basin (KEB), Central Japan, a forested headwater catchment with various soil depths (1·8 to 5 m) overlying late Neogene of volcanic bedrocks. The hydraulic potential distribution and flow lines data showed that the change in flow direction, which was controlled by rainfall amount and antecedent wetness of the soil profile, agreed well with the hydrochemical change across the slope segment during the storm. Hydrograph separation predicted by end‐member mixing analysis (EMMA) using Ca²⁺ and SiO₂ showed that near surface riparian, hillslope soil water and deep riparian groundwater were important in stream flow generation. The evidence of decrease in solutes concentration at a depth of 1 m in the hillslope and 0·6 m in the near surface riparian during peak storm suggested a flushing of high solutes concentration. Most of the solutes accumulated in the deep riparian groundwater zone, which was due to prominent downward flow and agreed well with the residence time. The distinct flow pathways and chemistry between the near surface riparian and deep riparian groundwater zones and the linkage hillslope aquifer and near surface riparian reservoir, which controls rapid flow and solutes flushing during the storm event, are in conflict with the typical assumption that the whole riparian zone resets flow pathways and chemical signature of hillslope soil water, as has been reported in a previous study. Copyright © 2005 John Wiley & Sons, Ltd.     

Microvariability in the early stages of cobble weathering by microenvironment on a glacier foreland,Storbreen, Jotunheimen,Norway

The initial stages of cobble weathering, measured as increasing percentage porosity, were calculated for sets of cobbles taken from the foreland of Storbreen, a cirque glacier in the Jotunheimen, Norway. Cobbles were taken from in front of the 1998 glacier snout, from the 1928, 1870, 1810 and 1750 moraine crests and from the ～10 000‐year‐old land surface beyond the Neoglacial foreland limit. Three microenvironments were examined within each site: (1) lichen‐free surfaces from exposed cobbles, (2) lichen‐covered surfaces from exposed cobbles and (3) buried cobbles taken from a soil depth of ～40 cm. Percentage porosity within plagioclase minerals was calculated using backscatter electron images of prepared thin sections. Porosity was calculated from five depth profiles into each cobble. Depth profiles were subdivided into a sequence of 50 µm × 50 µm rectangles running to at least a nominal depth of 500 µm within each cobble. Three cobbles from each dated land surface were sampled, except for the 1750 surface, which had five cobbles. Statistical analysis was by analysis of variance of rank‐order transformed data. Findings indicate that cobbles close to the glacier snout are largely unweathered; also, weathering is generally weak in the 1928, 1870 and 1810 positions, but statistically significantly higher in the 1750‐ and 10 000‐year‐old positions. Weathering of buried cobbles always exceeded weathering of exposed cobbles and may possibly reach a value beyond which it cannot progress while retaining surface cohesion. The degree of weathering on lichen‐free and lichen‐covered cobble surfaces is not initially distinguishable, but diverges sharply after ～250 years, when lichen‐covered surfaces experience significantly higher totals. Overall, the weathering trend in cobbles matches that found in soils at the same sites. Copyright © 2007 John Wiley & Sons, Ltd.     

Field evidence for the influence of weathering on rock erodibility and channel form in bedrock rivers

Erosion processes in bedrock‐floored rivers shape channel cross‐sectional geometry and the broader landscape. However, the influence of weathering on channel slope and geometry is not well understood. Weathering can produce variation in rock erodibility within channel cross‐sections. Recent numerical modeling results suggest that weathering may preferentially weaken rock on channel banks relative to the thalweg, strongly influencing channel form. Here, we present the first quantitative field study of differential weathering across channel cross‐sections. We hypothesize that average cross‐section erosion rate controls the magnitude of this contrast in weathering between the banks and the thalweg. Erosion rate, in turn, is moderated by the extent to which weathering processes increase bedrock erodibility. We test these hypotheses on tributaries to the Potomac River, Virginia, with inferred erosion rates from ~0.1 m/kyr to >0.8 m/kyr, with higher rates in knickpoints spawned by the migratory Great Falls knickzone. We selected nine channel cross‐sections on three tributaries spanning the full range of erosion rates, and at multiple flow heights we measured (1) rock compressive strength using a Schmidt hammer, (2) rock surface roughness using a contour gage combined with automated photograph analysis, and (3) crack density (crack length/area) at three cross‐sections on one channel. All cross‐sections showed significant (p < 0.01 for strength, p < 0.05 for roughness) increases in weathering by at least one metric with height above the thalweg. These results, assuming that the weathered state of rock is a proxy for erodibility, indicate that rock erodibility varies inversely with bedrock inundation frequency. Differences in weathering between the thalweg and the channel margins tend to decrease as inferred erosion rates increase, leading to variations in channel form related to the interplay of weathering and erosion rate. This observation is consistent with numerical modeling that predicts a strong influence of weathering‐related erodibility on channel morphology. Copyright © 2017 John Wiley & Sons, Ltd.     

The sensitivity of hillslope bedrock erosion to precipitation

Decoupling the impacts of climate and tectonics on hillslope erosion rates is a challenging problem. Hillslope erosion rates are well known to respond to changes in hillslope boundary conditions (e.g. channel incision rates) through their dependence on soil thickness, and precipitation is an important control on soil formation. Surprisingly though, compilations of hillslope denudation rates suggest little precipitation sensitivity. To isolate the effects of precipitation and boundary condition, we measured rates of soil production from bedrock and described soils on hillslopes along a semi‐arid to hyperarid precipitation gradient in northern Chile. In each climate zone, hillslopes with contrasting boundary conditions (actively incising channels versus non‐eroding landforms) were studied. Channel incision rates, which ultimately drive hillslope erosion, varied with precipitation rather than tectonic setting throughout the study area. These precipitation‐dependent incision rates are mirrored on the hillslopes, where erosion shifts from relatively fast and biologically‐driven to extremely slow and salt‐driven as precipitation decreases. Contrary to studies in humid regions, bedrock erosion rates increase with precipitation following a power law, from ～1 m Ma^?1 in the hyperarid region to ～40 m Ma^?1 in the semi‐arid region. The effect of boundary condition on soil thickness was observed in all climate zones (thicker soils on hillslopes with stable boundaries compared to hillslopes bounded by active channels), but the difference in bedrock erosion rates between the hillslopes within a climate region (slower erosion rates on hillslopes with stable boundaries) decreased as precipitation decreased. The biotic‐abiotic threshold also marks the precipitation rate below which bedrock erosion rates are no longer a function of soil thickness. Our work shows that hillslope processes become sensitive to precipitation as life disappears and the ability of the landscape to respond to tectonics decreases. Copyright © 2010 John Wiley & Sons, Ltd.     

Experiments on the morphological controls of velocity inversions in bedrock canyons

A better understanding of bedrock incision mechanisms and processes is essential to the study of long‐term landscape evolution. Yet, little is known about flow dynamics in bedrock rivers, limiting our ability to make realistic predictions of local bedrock incision rates. A recent investigation of flow through bedrock canyons of the Fraser River revealed that plunging flows, defined by the downward‐directed movement of near surface flow toward the channel bed, occur in channels that have low width‐to‐depth ratios. Plunging flows occur into deep scour pools, which are often coincident with lateral constrictions and channel spanning submerged ridges (sills). A phenomenological investigation was undertaken to reproduce the flow fields observed in the Fraser canyons and to explore morphological controls on the occurrence and relative strength of plunging flow in bedrock canyons. Our observations show that the plunging flow structure can be produced along a scour pool entrance slope by accelerating the flow at the canyon entrance either over submerged sills or through lateral constrictions. Plunging flow appears to be a function of convective deceleration into a scour pool which can be enhanced by sill height, the amount of the channel width that is constricted, pool entrance slope, discharge, and a reduction in channel width‐to‐depth ratio. Plunging flow greatly enhances the potential for incision to occur along the channel bed and is an extreme departure from the assumptions of steady, uniform flow in bedrock incision models, highlighting the need for improved formulations that account for fluid flow. Copyright © 2017 John Wiley & Sons, Ltd.     

Soil moisture states,lateral flow,and streamflow generation in a semi‐arid,snowmelt‐driven catchment

Hydraulic connectivity on hillslopes and the existence of preferred soil moisture states in a catchment have important controls on runoff generation. In this study we investigate the relationships between soil moisture patterns, lateral hillslope flow, and streamflow generation in a semi‐arid, snowmelt‐driven catchment. We identify five soil moisture conditions that occur during a year and present a conceptual model based on field studies and computer simulations of how streamflow is generated with respect to the soil moisture conditions. The five soil moisture conditions are (1) a summer dry period, (2) a transitional fall wetting period, (3) a winter wet, low‐flux period, (4) a spring wet, high‐flux period, and (5) a transitional late‐spring drying period. Transitions between the periods are driven by changes in the water balance between rain, snow, snowmelt and evapotranspiration. Low rates of water input to the soil during the winter allow dry soil regions to persist at the soil–bedrock interface, which act as barriers to lateral flow. Once the dry‐soil flow barriers are wetted, whole‐slope hydraulic connectivity is established, lateral flow can occur, and upland soils are in direct connection with the near‐stream soil moisture. This whole‐slope connectivity can alter near‐stream hydraulics and modify the delivery of water, pressure, and solutes to the stream. Copyright © 2005 John Wiley & Sons, Ltd.     

Spheroidal weathering of granite porphyry with well‐developed columnar joints by oxidation,iron precipitation,and rindlet exfoliation

Spheroidal weathering, one of the important rock weathering styles, has been attributed to chemical weathering by the water from joint surfaces, and mechanical aspects of the weathering have not been well addressed. We made an investigation on spheroidal weathering of Miocene granite porphyry with well‐developed columnar joints and found that this spheroidal weathering proceeds through chemical processes and accompanying mechanical processes. The investigation of the textures, physical properties, mineralogy, and chemistry of the porphyry revealed the presence of a brown band on the surface margins of corestones, representing the oxidation of pyrite and chlorite, and the precipitation of iron hydroxides, and the consequent generation of micro‐cracks within the band. During weathering, oxidation progresses inwards from joints that surround the rindlets, including both high‐angle columnar and low‐angle planar joints, and causes rounding of the unweathered interior portion of the rock. Microscopic observations of the brown band embedded with fluorescent resin show that pores are first filled with iron hydroxides, and that micro‐cracks then form parallel to the oxidation front in the outer portion of the brown band. Iron hydroxide precipitation increases the P‐wave velocity in the brown band, while micro‐crack formation decreases the tensile strength of the rock. Where the brown band has thickened to ~6 cm, the micro‐cracks are connected to one another to create continuous cracks, which separate the rindlets from the corestone. Micro‐crack formation parallel to the corestone surface may be attributed to compressive stresses generated by small amounts of volumetric expansion due to the precipitation of iron hydroxides in the brown band. Earth surface is under oxidizing environments so that precipitation of iron hydroxides commonly occurs; the spheroidal weathering in this paper is a typical example of the combination of chemical and mechanical processes under such environments. Copyright © 2016 John Wiley & Sons, Ltd.