Hillslope Hollows Research Articles

Chernozem relics in the Hellweg Loess Belt (Westphalia, NW Germany) – Natural or man-made?

Chernozems and Chernozem-like soils are widespread e.g. in Central Germany but its formation is still controversially discussed. Clustered findings of Luvic Phaeozems and buried dark soil horizons in the Westphalian Hellweg Loess Belt (North Rhine-Westphalia) are traditionally interpreted as relics of early Holocene Chernozems. More recent research raised the question whether these soils are of (pre-) historic anthropogenic origin.Field observations in the east of Dortmund revealed the existence of buried black horizons in hillslope hollows. The underlying Stagnic Luvisol was penetrated by darkish clay illuviation veins forming a polygonal pattern. This feature was also found below the plough horizon in Luvisols beyond the hillslope hollows. A multi-analytical approach was used in order to clarify whether the black soil horizons are natural relics or man-made. In order to identify fire-derived black carbon (microcharcoal), benzenepolycarboxylic acids were used as molecular markers.Beside the position of the fossil black horizons on top of a well-developed Stagnic Luvisol, micromorphological thin section analysis confirmed the colluvial origin of the black horizons. Total organic carbon contents of the black horizons were only ∼1%, whereas high black carbon concentrations indicate large amounts of fire derived organic matter. High concentrations of black carbon were also found in the clay illuviation veins, testifying to a formerly more widespread occurrence of black carbon enriched soils.Radiocarbon ages of charcoal particles from the black horizons prove fire activities during the late Neolithic and Bronze Age (e.g. slash-and-burn). Finally, our results strongly indicate that the Chernozem-like soils in the study area are man-made and natural Chernozem formation can be excluded.

Water's Way at Sleepers River watershed – revisiting flow generation in a post‐glacial landscape, Vermont USA

AbstractThe Sleepers River Research Watershed (SRRW) in Vermont, USA, has been the site of active hydrologic research since 1959 and was the setting where Dunne and Black demonstrated the importance and controls of saturation‐excess overland flow (SOF) on streamflow generation. Here, we review the early studies from the SRRW and show how they guided our conceptual approach to hydrologic research at the SRRW during the most recent 25 years. In so doing, we chronicle a shift in the field from early studies that relied exclusively on hydrometric measurements to today's studies that include chemical and isotopic approaches to further elucidate streamflow generation mechanisms. Highlights of this evolution in hydrologic understanding include the following: (i) confirmation of the importance of SOF to streamflow generation, and at larger scales than first imagined; (ii) stored catchment water dominates stream response, except under unusual conditions such as deep frozen ground; (iii) hydrometric, chemical and isotopic approaches to hydrograph separation yield consistent and complementary results; (iv) nitrate and sulfate isotopic compositions specific to atmospheric inputs constrain new water contributions to streamflow; and (v) convergent areas, or ‘hillslope hollows’, contribute disproportionately to event hydrographs. We conclude by summarizing some remaining challenges that lead us to a vision for the future of research at the SRRW to address fundamental questions in the catchment sciences. Copyright © 2014 John Wiley & Sons, Ltd.

What we see now: Event-persistence and the predictability of hydro-eco-geomorphological systems

What we see now in the landscape is the result of a long history of events with varying degrees of persistence. We have only limited access to much of that history and we know that many current events have only a minimal impact on what we see. Even rather extreme events may have impacts that are not very long-lasting but can have the effect of changing the antecedent states for future events. That means that sampling of sequences of events might be important in understanding the evolution of the catchments. In some cases, however, extreme events can have an impact on the system that persists over hundreds or thousands of years. Any evolution of the landscape is then constrained by those past events, however much it might be also constrained by self-organisational principles. It might be difficult to verify those principles given the epistemic uncertainties about past histories and system properties that are generic to the studies that are possible within a research project or career. These arguments are investigated in a simple slab model of landslip failures in a hillslope hollow subject to stochastic forcing over long periods of time. The complementarity of an event-persistence approach to hydro-eco-geomorphological systems is captured in suggestions for future research questions.

Towards closing the watershed nitrogen budget: Spatial and temporal scaling of denitrification

AbstractEnhanced consideration of the hydrogeomorphic template of watersheds is critical to understanding watershed nitrogen budgets. We developed a framework to estimate the spatial distribution and temporal dynamics of soil moisture and soil oxygen in surficial soils to scale nitrogen transformations for a forested watershed (Pond Branch) in Maryland, USA. We sampled soil cores in upland, hillslope hollow, riparian hollow, and riparian hummock landscape positions in different seasons for biogeochemical fluxes including measurement of N2gas produced via denitrification. We extrapolated these rates in space and time with information derived from in situ soil oxygen and soil moisture probes to scale fluxes from plots to the catchment level. We addressed three questions: (1) How important are seasonal, daily, and storm event variations in soil oxygen for denitrification? (2) How is denitrification spatially distributed through the watershed? (3) How important is denitrification to the watershed nitrogen budget? We found that microtopography within the riparian zone is a significant influence on soil oxygen dynamics and therefore redox‐sensitive biogeochemical processes such as denitrification. Riparian zone hollows (lower topographic positions) represented 0.5%–1.0% of the catchment area, but accounted for >99% of total denitrification. Interestingly, topography was a much stronger controller of oxygen than rainfall, which had little influence on temporal variation in soil oxygen levels. Spatial and temporal extrapolations of measured rates suggest that a minimum of 16%–27% of atmospheric nitrogen deposition is lost to denitrification. These results suggest that the importance of denitrification in the nitrogen budget of forested watersheds depends fundamentally on the presence of landscape elements, such as riparian hollows that function as “hot spots” of activity.

Instrumental record of debris flow initiation during natural rainfall: Implications for modeling slope stability

The middle of a hillslope hollow in the Oregon Coast Range failed and mobilized as a debris flow during heavy rainfall in November 1996. Automated pressure transducers recorded high spatial variability of pore water pressure within the area that mobilized as a debris flow, which initiated where local upward flow from bedrock developed into overlying colluvium. Postfailure observations of the bedrock surface exposed in the debris flow scar reveal a strong spatial correspondence between elevated piezometric response and water discharging from bedrock fractures. Measurements of apparent root cohesion on the basal (Cb) and lateral (Cl) scarp demonstrate substantial local variability, with areally weighted values of Cb = 0.1 and Cl = 4.6 kPa. Using measured soil properties and basal root strength, the widely used infinite slope model, employed assuming slope parallel groundwater flow, provides a poor prediction of hydrologic conditions at failure. In contrast, a model including lateral root strength (but neglecting lateral frictional strength) gave a predicted critical value of relative soil saturation that fell within the range defined by the arithmetic and geometric mean values at the time of failure. The 3‐D slope stability model CLARA‐W, used with locally observed pore water pressure, predicted small areas with lower factors of safety within the overall slide mass at sites consistent with field observations of where the failure initiated. This highly variable and localized nature of small areas of high pore pressure that can trigger slope failure means, however, that substantial uncertainty appears inevitable for estimating hydrologic conditions within incipient debris flows under natural conditions.

Mercury transport in response to storm events from a northern forest landscape

AbstractConcentrations and fluxes of mercury (Hg) species in surface waters of forested watersheds are affected by hydrological events. The mechanisms of Hg transport during these events are poorly understood and yet may influence Hg bioavailability and exposure to aquatic biota. Three storm events with varying magnitude and intensity were investigated (June, September and November 2005) at a forested watershed in the Adirondack region of New York State, USA. Concentrations of Hg species increased during these events, both above and downstream of wetlands in the watershed. While Hg flux was higher from wetland drainage, the Hg flux from the upland site exhibited a greater relative increase to elevated runoff. Hg flux was controlled by discharge; however, Hg species concentrations were not well correlated with discharge, dissolved organic carbon (DOC), or total suspended solids (TSS) through the duration of events. A counter‐clockwise hysteresis response of DOC with increasing runoff contrasted with the clockwise response for total Hg, suggesting different contributions from source areas for these solutes. Correspondence with elevated total K and NO3− (α < 0·05) during the rising limb of the hydrograph suggests rapid delivery of throughfall Hg, potentially enhanced by hillslope hollows, to the stream channel. As the watershed saturated, stream Hg appears to be derived from the soil Hg pool. Results suggest that particulate Hg did not contribute substantially to total Hg flux during events (<25%). These results emphasize the role of watershed attributes and storm characteristics in Hg transport and bioavailability. Copyright © 2008 John Wiley & Sons, Ltd.

Physically‐based modelling of double‐peak discharge responses at Slapton Wood catchment

AbstractHeavy winter rainfall produces double‐peak hydrographs at the Slapton Wood catchment, Devon, UK. The first peak is saturation‐excess overland flow in the hillslope hollows and the second (i.e. the delayed peak) is subsurface stormflow. The physically‐based spatially‐distributed model SHETRAN is used to try to improve the understanding of the processes that cause the double peaks. A three‐stage (multi‐scale) approach to calibration is used: (1) water balance validation for vertical one‐dimensional flow at arable, grassland and woodland plots; (2) two‐dimensional flow for cross‐sections cutting across the stream valley; and (3) three‐dimensional flow in the full catchment. The main data are for rainfall, stream discharge, evaporation, soil water potential and phreatic surface level. At each scale there was successful comparison with measured responses, using as far as possible parameter values from measurements. There was some calibration but all calibrated values at one scale were used at a larger scale.A large proportion of the subsurface runoff enters the stream from three dry valleys (hillslope hollows), and previous studies have suggested convergence of the water in the three large hollows as being the major mechanism for the production of the delayed peaks. The SHETRAN modelling suggests that the hillslopes that drain directly into the stream are also involved in producing the delayed discharges. The model shows how in the summer most of the catchment is hydraulically disconnected from the stream. In the autumn the catchment eventually ‘wets up’ and shallow subsurface flows are produced, with water deflected laterally along the soil‐bedrock interface producing the delayed peak in the stream hydrograph. Copyright © 2007 John Wiley & Sons, Ltd.

Wildfire disturbance and shallow landsliding in coastal British Columbia over millennial time scales: A numerical modelling study

An algorithm for wildfire occurrence is introduced for incorporation into a numerical model of drainage basin evolution. Within the model, fire return intervals are determined using a stochastic rule set and fire sizes are assigned according to a pareto distribution. A Weibull distribution was fit to millennial-scale lake charcoal data for calibration of fire return intervals in this study, while data for the Olympic Peninsula were used to determine the fire size distribution. Loss of tree root strength resulting from stand-replacing wildfires has been found to be comparable to loss after timber harvesting. Hence, data for increased shallow landslide activity associated with logging were used to calibrate transport equations for years following wildfire incidence. A weathering relation is incorporated in the model, which drives the supply of sediment. When sediment supply is depleted in the model, the shallow landsliding module shuts off until new material is available for transport. Time since last fire analysis indicates that the Weibull parameters defined for the Clayoquot charcoal data produce results that are weighted too heavily towards recent fires. Sensitivity analysis was undertaken, suggesting alternative values may be more reasonable for further model runs. The introduction of a weighted probability factor for aspect, whereby south-facing slopes have a higher likelihood of fire initiation, was not found to significantly affect patterns of fire age in the landscape. Patterns of net topographic change after 6000 years of model run time indicate that greatest net erosion is found on steep, upper slopes, with the highest deposition occurring in hillslope hollows and low-order channels. Depositional grid cells in interfluves show a variable response to the inclusion of wildfire in the model. Net erosion occurs over a relatively low proportion of hillslope area, with wildfire model runs showing a strong erosional signal in these locations. For stream grid cells, the additional deposition due to the inclusion of wildfire in model runs is about 9–10% for 1st to 3rd order streams. Overall, deposition rates increase with decreasing stream order, reflecting greater hillslope/channel coupling in low-order streams.

A mass movement origin for cirques

AbstractThe glacial process of cirque initiation, whereby small initial hillslope hollows grow by nivation until snow can form glacier ice, and ice motion then enlarges the hollow to a fully developed cirque, appears to have difficulty explaining the creation of large cirques in the time available during Quaternary glaciations, at the rates at which glaciers are reported to erode rock, and in rapidly uplifting mountain ranges. It also has difficulty explaining the striking proliferation of cirques in Fiordland, South Island, New Zealand, an area of harder rock and less glaciation than the nearby cirque‐poor area of South Westland. Here we show that cirques can be initiated as large, deep‐seated, often coseismic rock slope failure source area depressions in which snow may accumulate to form cirque glaciers, which can then remove detritus from, smooth, and enlarge the cirque. We present an example of a classically shaped cirque that has never held a glacier. We show that many similarities between the locations, sizes and shapes of rock slope failure source area depressions and cirques are understandable on this basis, as is the occurrence of cirques in presently aseismic intraplate locations and their relative paucity in actively uplifting ranges. The extent to which cirques may be of mass movement origin has implications for their value as palaeoclimatic indicators. Copyright © 2006 John Wiley & Sons, Ltd.

A field‐based study of soil water and groundwater nitrate release in an Adirondack forested watershed

Nitrate (NO3−) movement was studied using a combination of isotopic, chemical, and hydrometric data within the 135 ha Archer Creek watershed in the Adirondack Mountains of New York from January 1995 to December 1996. This research was conducted to identify sources of stream water NO3− and the mechanisms that deliver NO3− to the stream to test two hypotheses: (1) Soil water NO3− concentrations are highest after dry periods and subsequently lower with each storm. (2) Stream water NO3− concentrations are controlled by groundwater during growing season low flows and by soil water during the dormant season and during storms. Antecedent moisture conditions and season had little effect on mean soil water NO3− concentrations before storms (range of 1.1–5.1 μmol L−1 throughout the study). High soil water NO3− concentrations (up to 136 μmol L−1) were found only at the watershed ridge top during the 1996 snowmelt and early summer. Results from isotopic hydrograph separations and chemical end‐member mixing analysis showed that soil water and till groundwater dominated stream base flow and storm flow during six monitored storms. Near‐stream wetland groundwater and event water contributed little to streamflow during most conditions. Near‐stream groundwater contributions to streamflow were significant only during very low base flow (<0.05 mm h−1) during the summer and fall. Highest stream water NO3− concentrations coincided with peaks in the till groundwater contribution according to isotopic hydrograph separations using δ18O and chloride as conservative tracers. A conceptualization of streamflow generation and watershed NO3− release is described in which hillslope hollows are the principal zones of soil water and till groundwater mixing in the watershed and till groundwater is the main source of stream water NO3− during both base flow and storms.

Geomorphic effects of large debris flows on channel morphology at North Fork Mountain, eastern West Virginia, USA

Extreme rainfall in June 1949 and November 1985 triggered numerous large debris flows on the steep slopes of North Fork Mountain, eastern West Virginia. Detailed mapping at four sites and field observations of several others indicate that the debris flows began in steep hillslope hollows, propagated downslope through the channel system, eroded channel sediment, produced complex distributions of deposits in lower gradient channels, and delivered sediment to floodwaters beyond the debris-flow termini. Based on the distribution of deposits and eroded surfaces, up to four zones were identified with each debris flow: an upper failure zone, a middle transport/erosion zone, a lower deposition zone, and a sediment-laden floodwater zone immediately downstream from the debris-flow terminus. Geomorphic effects of the debris flows in these zones are spatially variable. The initiation of debris flows in the failure zones and passage through the transport/erosion zones are characterized by degradation; 2300 to 17 000 m3 of sediment was eroded from these zones. The total volume of channel erosion in the transport/erosion zones was 1·3 to 1·5 times greater than the total volume of sediment that initially failed, indicating that the debris flows were effective erosion agents as they travelled through the transport/erosion zones. The overall response in the deposition zones was aggradation. However, up to 43 per cent of the sediment delivered to these zones was eroded by floodwaters from joining tributaries immediately after debris-flow deposition. This sediment was incorporated into floodwaters downstream from the debris-flow termini causing considerable erosion and deposition in these channels. © 1998 John Wiley & Sons, Ltd.

When is a Depression Wetland a Carolina Bay?

90Southeastern Geographer Lowrance, R., Sharpe, J. K., and Sheridan, J. M. 1986. "Long-Term Sediment Deposition in the Riparian Zone ofa Coastal Plain Watershed," Journal ofSoil and Water Conservation , Vol. 41, pp. 266-271. Lowrance, R., Mclntire, S., and Lance, C. 1988. "Erosion and Sediment Deposition in a Field-Forest System Estimated using Cesium- 137," Journal ofSoil and Water Conservation , Vol. 43, pp. 195-199. Markewich, H. W., Pavich, M. J., and Buell, G. R. 1990. "Contrasting Soils and Landscapes of the Piedmont and Coastal Plain, Eastern United States," Geomorphology, Vol. 3, pp. 417-448. NCDEM (North Carolina Division of Environmental Management). 1994 et seq. TarPamlico River Basinwide Water Quality Management Plan (Raleigh, NC: Division of Environmental Management). Phillips, J. D. 1993. "Pre- and Post-Colonial Sediment Sources and Storage in the lower Neuse River basin, North Carolina," Physical Geography, Vol. 14, pp. 272-284. Phillips, J. D., Wyrick, M. J., Robbins, J. G., and Flynn, M. 1993. "Accelerated Erosion on the North Carolina Coastal Plain," Physical Geography, Vol. 14, pp. 1 14-130. SCS (Soil Conservation Service). 1985. Haw River Erosion Report. Deep River Erosion Report (Raleigh, NC: United States Department of Agriculture). SCS (Soil Conservation Service). 1989. Soil Survey of Craven County, North Carolina (Raleigh, NC: United States Department of Agriculture). Slattery, M. C, Burt, T. P., and Boardman, J. B. 1 994. "Rill Erosion along the Thalweg ofa Hillslope Hollow: A Case Study from the Cotswold Hills, Central England," Earth Surface Processes and Landforms, Vol. 19, pp. 377-385. Trimble, S. W. 1974. Man-Induced Soil Erosion in the Southern Piedmont, 1700-1970 (Ankeny, IA: Soil Conservation Society of America). Southeastern Geographer Vol. XXXVII, No. I, May 1997, pp. 90-98 WHEN IS A DEPRESSION WETLAND A CAROLINA BAY?1 Robert F. Lide Are sites such as those shown in Figures 1 and 2 the same type of geomorphic feature? Does the term "Carolina bay" have a specific geomorphological definition , or is it simply a colloquial term that may be used to describe any depression wetland found on the southeastern Atlantic Coastal Plain? The purpose ofthis research note is: (1) to illustrate that, despite a seemingly precise definition, the term "Carolina bay" often is loosely applied in the scientific literature and (2) to Mr. Lide is a research technician at the Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802. VOL. XXXVII, NO. 1 91 argue that the term should be limited to geomorphic features that have a more or less elliptical, ovate, or circular outline. Carolina bays are a common type of depression wetland found on the Atlantic Coastal Plain of the southeastern United States (Fig. 3). Their size is highly variable , ranging from 50 meters in length to 8 kilometers (Sharitz and Gibbons, 1982). Although generally characterized by internal drainage, many have natural overflow outlets that may allow the establishment of temporary connections to stream and drainage networks during wet periods. Some Carolina bays form the headwaters of perennial streams, and a few have small creeks flowing into them. These wetlands increasingly are recognized as critical habitat for the maintenance of biodiversity. In recent years, many have been the subject of wetland preservation and restoration efforts. Historically, Carolina bays have been defined in the scientific literature by several specific geomorphological criteria: (1) being elliptical, ovate, or circular; (2) having a long-axis orientation that usually is northwest-southeast; and (3) often having a sand rim that is best developed along the eastern and southeastern perimeter (e.g., Melton and Schriever, 1933; Johnson, 1942; Prouty, 1952; Price, 1968; Savage, 1982). Many sites meet these geomorphological criteria (Fig. 1). In recent years, however, some published reports have used the term "Carolina bay" loosely to include coastal plain wetlands that lack some or all ofthese specific criteria (e.g., Norris, 1963; Pechmann et al., 1989; Schalles et al., 1989; Gibbons, 1 990; Mahoney, Mort, and Taylor, 1 990; Newman and Schalles, 1 990; Pechmann et al., 1991). Furthermore, wetlands with highly irregular outlines sometimes are identified as "Carolina bays" (Fig. 2). Although a few notes about circular or elliptical "bays" were recorded by early scientists (e.g., Tourney, 1848; Glenn, 1895), modern scientific investigation of Carolina bays began...

Distribution of caesium‐137 in soils across a hillslope hollow

AbstractThe distribution of soil 137Cs in relation to selected soil and landform properties was studied across a 16 ha hillslope hollow in the Hunter valley, New South Wales, Australia. The hillslope was used as grazing for cattle. Caesium‐137 was not significantly related to the amount of sand, silt, or clay, the bulk density, the organic matter content, the slope angle or the relative distance downslope. However, 137Cs was significantly related to the thickness of the soil A horizon. Spatial variations in 137Cs were compared with topographic units and a six‐element hillslope model, but there was little correspondence. It was thought that the effects of microtopography could have masked potential interrelationships between 137Cs and broader scale landform parameters.

Rill erosion along the thalweg of a hillslope hollow: A case study from the cotswold hills, Central England

AbstractRill erosion is an important erosional form on agricultural soils in England, causing large losses of soil, particularly on cultivated slopes. This paper describes a rill system that developed in a small agricultural catchment in north Oxfordshire during the winter of 1992–93. The rill system comprised two components: a system of ‘feeder rills’ along the valley‐side slopes, which were the result of flow concentration and erosion along wheelings, and a thalweg rill, which formed along a dry valley bottom as a result of surface runoff concentration from the feeder rills. Total volumetric soil loss from the rill system was 32·28 m3, equivalent to 3·01 m3, ha−1 for the rill catchment area, or 3·91 t ha−1. Mean discharge for the thalweg rill and feeder rills, calculated during a storm event, was 31·101s−1 and 1·171s−1, respectively. All flows were fully turbulent and supercritical. We emphasize the need for a spatially distributed approach to the study of runoff and erosion at the catchment scale.

A model of transient, topographically driven, saturated subsurface flow

A two‐dimensional, finite difference model of saturated subsurface hillslope flow, capable of handling varied topography, is developed. The model uses rainfall, elevation data, saturated conductivities, and available soil porosity to predict saturated thicknesses in time and space over the hillslope. The model is tested against piezometric data collected in a steep hillslope hollow in the Cascade Range of Washington state during the winter wet season. The model closely tracks the timing and magnitude of piezometric response.

A Comparison of Piezometric Response in Unchanneled Hillslope Hollows: Coastal Alaska and Japan

Piezometric response in several hillslope hollows was compared for small and large storms at sites in coastal Alaska and Japan. During smaller storms with dry antecedent conditions there was evidence of a wedge of groundwater accumulating from the base of the slope. For wetter antecedent conditions and larger storms, piezometric response was less affected by slope position, possibly because of the contribution of preferential pathways in the soil to groundwater fluctuations. Piezometric response was more rapid and generally higher in more dissected hollows.

On the generation of delayed peaks in stream discharge

Where deep permeable soils overlie impermeable bedrock and where steep hillslopes border a narrow floodplain, subsurface stormflow dominates the hydrograph. In such cases delayed peaks in stream discharge may occur several days after rainfall, providing the major volumetric response and possibly also the highest peak discharge. The general occurrence of such delayed peaks is considered, making reference to field data from two small drainage basins in southwest England, and to the results of computer simulations using a distributed hillslope model. The importance of soil-water convergence into hillslope hollows is stressed. The relative roles of rainfall and antecedent soil moisture in controlling the size and timing of the delayed peak is discussed. The implications of the results for hillslope runoff modelling are briefly considered.

Erratum. Spatial variation in solutional denudation and soil moisture over a hillslope hollow

Erratum. Spatial variation in solutional denudation and soil moisture over a hillslope hollow

Spatial variation in solutional denudation and soil moisture over a hillslope hollow

AbstractA spatial pattern of relative solutional denudation is described for a hillslope hollow and adjacent spurs at Bicknoller Combe, Somerset. The pattern was obtained from a network of micro‐weight loss rock tablets emplaced in the soil. The results show that the hollow is the main locus of solutional denudation. The soil moisture distribution over the hollow indicates that it is a transmission zone for acid soil water percolating from the adjacent spurs to the saturated wedge at the base of the hollow. The wetter acid soils in the hollow are responsible for the relatively higher solutional denudation taking place in the hollow.

Modelling Hillslope Soil Water Status during Drainage

Modelling Hillslope Soil Water Status during Drainage