Hillslope System Research Articles

Unraveling the Relative Contributions of Deterministic and Stochastic Processes in Shaping Species Community Assembly in a Floodplain and Shallow Hillslope System

Understanding the process underlying species coexistence is crucial in ecology. This challenge is relevant in tree communities inhabiting contrasting abiotic conditions, such as lowland floodplain and shallow hillslope karstic systems. We examined the influence of topographic variables and spatial factors on the structure of tree communities in the karstic system in Calakmul, Mexico. We measured 7050 trees (diameter at breast height ≥ 3 cm) in 152 circular plots and generated seven topographic variables from a digital elevation model. We employed redundancy analysis and variance partitioning to test the effects of environmental and spatial factors on tree communities. In addition, we used the null Raup–Crick model to uncover the relative importance of the deterministic and stochastic processes driving community assembly. Our study revealed significant floristic distinction between seasonally flooded and upland forests. The topographic wetness index (TWI) contribution to explaining the floristic differentiation in the studied tree assemblages was greater than that of the other topography-related variables. The explanatory power of the environmental and spatial factors varied slightly between datasets. The null model indicated a predominant influence of deterministic over stochastic processes. Our findings reaffirm the role of seasonal flooding as an abiotic filter. Additionally, the TWI can serve to identify flood-prone conditions within shallow depressions. The preservation of adjacent seasonally flooded and upland forests is relevant for the maintenance of tree diversity in the karst of the Yucatan Peninsula, since flooding drives the distribution of species.

High nitrate variability on an Alaskan permafrost hillslope dominated by alder shrubs

Abstract. In Arctic ecosystems, increasing temperatures are driving the expansion of nitrogen (N) fixing shrubs across tundra landscapes. The implications of this expansion to the biogeochemistry of Arctic ecosystems are of critical importance and more work is needed to better understand the form, availability, and transportation potential of N from these shrubs across a variety of Arctic landscapes. To gain insights into the processes controlling N within a permafrost hillslope system, the spatiotemporal variability of nitrate (NO3-) and its environmental controls were investigated at an alder (Alnus viridis spp. fruticosa) dominated permafrost tundra landscape in the Seward Peninsula, Alaska, USA. Soil pore water was collected from locations within alder shrubland growing along a well-drained hillslope and was compared to soil pore water collected from locations outside (upslope, downslope, and between) the alder shrubland. Soil pore water collected within alder shrubland had an average NO3-N (nitrogen from nitrate) concentration of 4.27±8.02 mg L−1 and differed significantly from locations outside alder shrubland (0.23±0.83 mg L−1; p<0.05). Temporal variation in NO3-N within and downslope of alder shrubland co-occurred with precipitation events where NO3- that accumulated in the soil was likely flushed downslope during rainfall. These findings have important implications for nutrient availability and mobility in N-limited permafrost systems that are experiencing shrub expansion in response to a warming Arctic.

Using StorAge Selection (SAS) functions to understand flow paths and age distributions in contrasting karst groundwater systems

Karst catchments have complex flow paths dynamics due to the unique, marked spatial heterogeneity of karst groundwater systems. We used the StorAge Selection (SAS) function framework to explore variations in age distributions of outflow and evapotranspiration from a small karst catchment in southwest China, as well as two hillslope spring systems within it. Daily hydrogen and oxygen stable isotopes (δD and δ18O) in precipitation and at corresponding discharge points were measured for 3 years in order to calibrate the model parameters. The seasonal variations in flux age of the three karst groundwater systems are similar, which indicates the generally strong hydrological connection between hillslope and depression at seasonal scales. The age distribution of outflow shows a “high-low–high” variation pattern in the whole catchment while a “high-low-low” pattern in hillslope system at the beginning of the wet season. In addition to sinkholes in depression, the distributions of soil and epikarst relating to the system storage capacity, resulting in variations of flow paths under different wetness conditions, is likely the main control on the age distributions in runoff for different karst system. The discharge selection preferences generally shift from for younger water in wet season to for older water in dry season in the whole catchment, while that for younger water in both wet and dry seasons in hillslope system. The dynamics of discharge selection preferences can be linked to the critical zone structure of karst underground system. The results in this study show that both hillslope and whole catchment preferentially release younger water to evapotranspiration in the study area.

Plio-Quaternary landscape evolution in the uplifted Ardennes: New insights from 26Al/10Be data from cave-deposited alluvium (Meuse catchment, E. Belgium)

Despite a wealth of recent studies dealing with the evolution of the drainage network in the uplifted Ardennes massif (E. Belgium), especially from the Middle Pleistocene onwards, the Ardennian landscape evolution and long-term incision rates in the Meuse catchment remain poorly documented over the whole Plio-Quaternary. Alluvium-filled multilevel cave systems represent a relevant setting to unravel the Late Cenozoic history of regional river incision. We present here a dataset of 26Al/10Be concentration data obtained from fifteen pebble samples washed into the Chawresse system, one of the largest multi-level cave systems of Belgium, which developed in Devonian limestones of the lower Ourthe Valley, the main Ardennian tributary of the Meuse. The sample collection spans an elevation difference higher than 120 m and their depleted 26Al/10Be ratios yield burial ages ranging from ~0.25 to 3.28 Ma. After critical assessment of our dataset for intra-karstic reworking issues, the most striking outcome of the obtained burial ages is the acceleration by a factor five of the incision rates (from ~30 to ~150 m/Ma) during the first half of the Middle Pleistocene. Integrating this incision peak and our pre-burial denudation rates, we then revisit the existing framework of Plio-Quaternary denudation and river incision in the Ardennian Meuse catchment. Whilst our 26Al/10Be concentration data shed new light on the temporal and spatial variability of the local river and hillslope system response to coupled tectonic and climatic forcings, it simultaneously highlights sampling issues and the need for further chronological data.

Microbial communities across a hillslope-riparian transect shaped by proximity to the stream, groundwater table, and weathered bedrock.

Watersheds are important suppliers of freshwater for human societies. Within mountainous watersheds, microbial communities impact water chemistry and element fluxes as water from precipitation events discharge through soils and underlying weathered rock, yet there is limited information regarding the structure and function of these communities. Within the East River, CO watershed, we conducted a depth‐resolved, hillslope to riparian zone transect study to identify factors that control how microorganisms are distributed and their functions. Metagenomic and geochemical analyses indicate that distance from the East River and proximity to groundwater and underlying weathered shale strongly impact microbial community structure and metabolic potential. Riparian zone microbial communities are compositionally distinct, from the phylum down to the species level, from all hillslope communities. Bacteria from phyla lacking isolated representatives consistently increase in abundance with increasing depth, but only in the riparian zone saturated sediments we found Candidate Phyla Radiation bacteria. Riparian zone microbial communities are functionally differentiated from hillslope communities based on their capacities for carbon and nitrogen fixation and sulfate reduction. Selenium reduction is prominent at depth in weathered shale and saturated riparian zone sediments and could impact water quality. We anticipate that the drivers of community composition and metabolic potential identified throughout the studied transect will predict patterns across the larger watershed hillslope system.

Experimental and Numerical Investigations of Influence on Overland Flow and Water Infiltration by Fracture Networks in Soil

Preferential flow is common in clay or expansive clay soils, involving water bypassing a large portion of the soil matrix. Dye tracer experiment and numerical modeling are used to simulate the surface runoff and subsurface preferential flow patterns influenced by the soil fracture network of a relatively steep hillslope system (slope angle equals to 10 degrees). The result of the experiments indicates that part of the water is infiltrated through cracks, leading to the delay of the initial runoff-yielding time and reduction of the discharge of the surface runoff. The soil water flow is mainly in the matrix when the intensity of precipitation is low. With the increasing of precipitation, soil water movement may become in the form of preferential flow through cracks. In addition, the nonuniformity of soil water infiltration and the depth of the average water infiltration increase as the precipitation intensity increases. To this end, the complete coupling model was established by using the surface-matrix-crack (SMC) model to simulate water flow within discrete fracture as well as to simulate water flow in the soil matrix based on the concept of dual permeability using the traditional Richards’ equation. In this model, the “cubic law” of fluid motion in cracks within smooth parallel plates and the two-dimensional diffusion wave approximation to Saint-Venant equations with momentum term ignored (two-dimensional shallow water equations) were used. The model divides soil water infiltration into two forms and uses the overall method to calculate the exchange of water between the crack networks and matrix regions as well as the exchange water between surface runoff and infiltration water. Results indicate that the SMC model has better performance compared with the traditional equivalent continuum model when those models are used to simulate the surface runoff movement and the soil water movement in the presence of cracks.

Analysis of sediment from steps in a small catchment in the Polish Carpathians in relation to the transition zone between the hillslope and fluvial system

An investigation to determine the transition zone between the hillslope system and the fluvial system in connection with the degree of differentiation of sediment located upstream and downstream of log steps (LS) and boulder steps (BS) was undertaken in the channel of a stream located in a small mountain catchment area in the Silesian Beskidy Mts. (Polish Carpathians), a stream channel into which large wood was introduced artificially several years ago. Granulometric analyses of 41 samples taken from upstream (21) and downstream (20) of steps were performed to analyze the effectiveness of log steps and boulder steps in the differentiation of mineral sediment.It was demonstrated that log steps have a significant impact on the functioning of the channel by shaping its longitudinal profile and affecting the transport of mineral material, whereas boulder steps retain less sediment, showing the beneficial effects of artificial creation of log steps in mountain stream channels. Results of granulometric analyses suggested the location of the theoretical transition zone between the hillslope system and the fluvial system, suggesting the location and range of this zone in small mountain catchment areas can change depending on the intensity and range of slope processes in the longitudinal profile of the channel.

Influence of topography on soil organic carbon dynamics in a Southern California grassland

Soil erosion and deposition processes play an important role influencing accumulation and long-term stability of soil organic carbon (SOC). Therefore, evaluations of landscape-scale magnitude and variability of SOC across topographically active landscapes will improve understanding about important drivers of SOC dynamics. In this study, we investigated the relationship between terrain attributes and quantity and spectroscopic characteristics of SOC for a hillslope system in Southern California. Thirteen soil cores were sampled at representative landscape positions with a history of intense erosion. We observed a strong relationship between slope, plan and profile curvature, and quantity and quality of SOC accumulation along the hillslope systems. Specifically, moderate slopes (<15%) combined with concave profile and plan curvature led to greater SOC accumulation. Spectroscopic analysis of water extractable organic carbon (WEOC) and bulk SOC shows greater presence of aromatics at eroding sites than at depositional sites, likely due to the contribution of eroded upslope materials. These differences in SOC and WEOC are the result of landscape-scale processes of SOC respiration by soil microflora. They likely reflect the potential for SOC stabilization via interaction with soil mineral surfaces. Our findings emphasize the role of specific terrain attributes rather than hillslope position alone in influencing the dynamics of SOC and its quality, information that can be also relevant in informing management and restoration practices.

The relative role of hillslope and river network routing in the hydrologic response to spatially variable rainfall fields

This paper introduces a new methodology and a rainfall spatial organisation index to examine the relative role of hillslope and channel residence times in the analysis of the significance of spatial rainfall representation in catchment flood response modelling. The relationship between the flood response, the hillslope and channel residence time representation and the spatial organisation of the rainfall fields is obtained by extending the ‘spatial moments of catchment rainfall’ statistics (Zoccatelli et al., 2011) to the hillslope system. The flood prediction error generated by assuming spatially uniform rainfall is related to the spatial organisation of the rainfall fields by means of the scaled spatial moment of order one for the channel network and the hillslope system. The methodology provides a basis for a more general consideration of the relationship between the flood response dependence to spatial rainfall organisation and catchment size. The methodology is illustrated based on data from five extreme flash floods occurred in various European regions in the period 2002–2007. Discharge data are available either from streamgauges or from post-flood surveys for 27 catchments, ranging in size between 36 and 982km2. High space–time resolution radar rainfall fields are also available for the analyses. These data are used to implement a distributed hydrological model simulating the runoff generation by infiltration excess and explicitly representing the surface flow paths across both the hillslopes and the river network. The hydrological model is alternatively forced with spatially-distributed and spatially-uniform rainfall input, to analyse the factors controlling the sensitivity of the model output to the spatial rainfall data. Our results show that the spatial variability of the rainfall can influence the flash-flood hydrographs for catchments as small as 50km2, and that the dependence of flood hydrograph shape to spatial rainfall variability cannot be treated as scale dependent relative to the size of the catchment.The rainfall index can be exploited as similarity index for classifying catchments and flood events according to the hillslopes/channel residence times and to provide guidance on the space and time resolution of the rainfall monitoring system required to predict the flood response.

Laboratory and numerical investigations of hillslope soil saturation development and runoff generation over rainfall events

Summary Runoff is a key process that controls the behaviour of a hillslope hydrological system. The study presented here aims to examine the mechanisms of runoff generation due to saturation excess by investigating the development of the subsurface saturated zone. Experiments were conducted on a hillslope system with a uniform slope (0.2) and a relatively homogeneous and highly permeable medium ( K s = 1.28 cm/min), subjected to high and constant surface recharges (0.51–0.69 cm/min). Two initial conditions prior to the rainfall events were set up in the experiments to represent relatively dry and wet antecedent soil conditions, respectively. Measurements showed that during vertical infiltration, local pressure head and soil moisture remained constant for a certain period, showing a ‘waiting’ behaviour. The saturated area formed initially at the slope toe, quickly rose to the surface and subsequently expanded to the upslope. When propagating in the upslope direction, the wetting front caused the pore-water flow to deflect in areas above the wetting front and at the slope base. With a wetter initial condition, the soil responded to the rainfall more quickly. The initial moisture conditions also altered the relation between the subsurface discharge and pressure head. Under the applied rainfall rates, the system reached a fully saturated condition and produced surface runoff. The rainfall intensity was found to affect the temporal variations and magnitude of surface runoff characteristics; however it did not seem to impose any significant effect on the maximum subsurface discharge rate. These results provide insight into the behaviour of the hillslope system in response to rainfall.

Evaluating the significance of event and post‐event sediment dynamics in a first order tributary using multiple sediment budgets

.Investigations of sediment transfer in upland catchments are rarely conducted over a sustained period of time, consequently a full understanding of the changing nature of sediment supply, storage and yield is often lacking. Three recent sediment budget studies from the Wet Swine Gill headwater catchment in the Lake District, Northern England, UK (a 0.65 km2, first‐order tributary), provide evidence of changes in sediment transfer dynamics over the period 2002–2008. The first sediment budget in 2002 describes the impact of a hillslope debris slide and channelised debris flow event, where the former was the dominant budget component. The termination of the debris flow in the Wet Swine Gill channel meant that the vast majority of slide failure material was not transferred to the downstream fluvial system. However, subsequent modification of exposed hillslope sediment by post‐event erosion processes and gully development resulted in ongoing erosion. A second sediment budget (June 2003–January 2004) demonstrated sediment yield downstream of the in‐channel debris slide deposits far exceeds upstream fluvial sediment delivery by two orders of magnitude (c. 4,000 kg and c. 20 kg, respectively). Erosion of sediment from the exposed hillslope failure scar (c. 1300 kg) was less than channel erosion (c. 3300 kg), and sediment transfers from both the hillslope and channel sediment sources are sensitive to high‐magnitude, low‐frequency trigger events including summer thunderstorms, and winter rainfall/ snow‐melt events. However, linear regression analysis only demonstrates weak or insignificant relations between meteorological conditions and sediment yield. A final sediment budget in April 2008 shows the significance of both hillslope (inclusive of gullying) and channel erosion/ transfer processes over the six‐year monitoring period. In this budget, like the first sediment budget, the hillslope system is marginally more dominant, and therefore demonstrates a further switch in the relative significance of hillslope and channel system components. When interpreting such findings the potential uncertainty in the budget components, particularly in the unmeasured residual components, should be considered, as the magnitude of the error can be large.These results suggest that contemporary event and post‐event sediment flux in small headwater catchments are more complex than short‐term investigations would initially suggest. Furthermore there is a clear need for continued, longer‐term monitoring of sediment system dynamics and associated hydro‐meteorological conditions, in order to develop understanding of how future climate change may impact upland sediment systems.

A similarity framework to assess controls on shallow subsurface flow dynamics in hillslopes

An understanding of the dominant controls on water fluxes through landscapes at scales smaller than the resolution of a hydrologic model is useful for choosing appropriate parameterizations, and for developing a classification system for landscape hydrology. The links between these controls and the parameters in hillslope‐scale models of subsurface lateral saturated flow have been treated implicitly in both physical (e.g., Characteristic Response Function) and empirical formulations, such as power law storage‐discharge relations. In particular, the controls exerted by the boundary condition at the toe of the hilslope and the temporal variability of the recharge have not been integrated with the understanding of the controls of topography and soil properties. In this work, we develop a dimensionless similarity framework for assessing these controls on the subsurface flow dynamics, based on the Boussinesq equation in an idealized hillslope system. Using this framework we demonstrate the relationship between (1) the exponents of the storage‐discharge relations, (2) the boundary conditions and (3) the characteristic storage thickness used in the Characteristic Response Function parameterization. We use the concept of hydrologic regimes to incorporate information about the population of recharge events that drive the system and set up the initial condition of saturated storage for each storm event. The analysis demonstrates that for planar hillslopes with uniform soil properties, exponents range between 0 (when the flow through the hillslope is dominated by the topographic gradients, and is in the early stages of draining) and 2 (when the flow is driven by water table gradients, and the thickness of the aquifer at the seepage face is small compared to the average thickness). The similarity framework can be used to make first‐order predictions of these exponents, and classify hillslopes using a simple typology of subsurface flows based on two components of the behavior: the relative dominance of the topographic and water‐table gradients, and the responsiveness of the hillslope discharge to temporal variability of the recharge inputs.

Late Pleistocene tectonic-geomorphological development within a passive margin — The Cariatá trough, northeastern Brazil

Studies on the geomorphological evolution of the South American passive margin have been based on the pediplanation model, which predicts that its morphology is a response to regional uniform uplift and concomitant development of erosion surfaces. We combined remote sensing, geological mapping, lithostratigraphic and facies analyses, and luminescence dating in the Cariatá trough, northeastern Brazil, in order to determine how brittle tectonics and climate influenced colluviation and the shaping of local landforms in the Quaternary. Our work indicates that Cariatá is an asymmetrical trough ∼ 40 km long, ∼ 25 km wide, 250–550 m deep, and delimited by ENE–WSW-trending faults to the north and south. We recognized an ENE–WSW-oriented compression related to a strike-slip faulting regime, which corresponds to the present-day stress field in the region. This faulting event led to the deposition of colluvial fans, shed from adjacent uplifted crustal blocks, in a tectonically controlled depression. The colluvial succession is ∼ 45 m thick and presents two facies assemblages that filled the southern and, in particular, the northern borders of the trough: non-cohesive debrisflow and mudflow deposits. Optically stimulated luminescence dates of the sedimentary infill yielded ages at 224–128 ka and 45–28 ka, dominated by debrisflow and mudflow deposits, respectively. These ages may be over-estimated due to poor bleaching of colluvium, but they and our field data suggest that the margins of the trough were tectonically uplifted and eroded twice in the Late Pleistocene. The spasmodic colluvial accretion reflects the occurrence of high-magnitude, low-recurrence episodes probably associated with climate shifts in a semi-arid hillslope system. It follows that the present-day low-lying piedmont in which the Cariatá trough occurs is a juxtaposition of surfaces of various ages. This trough may have counterparts across the region. These conclusions do not corroborate the application of the cyclical pediplanation hypothesis in the area.

Modelling overland flow based on Saint‐Venant equations for a discretized hillslope system

AbstractMathematical modelling of overland flow is a critical task in simulating transport of water, sediment and other pollutants from land surfaces to receiving waters. In this paper, an overland flow routing method is developed based on the Saint‐Venant equations using a discretized hillslope system for areas with high roughness and steep slope. Under these conditions, the momentum equation reduces to a unique relationship between the flow depth and discharge. A hillslope is treated as a system divided into several subplanes. A set of first‐order non‐linear differential equations for subsequent subplanes are solved analytically using Chezy's formula in lieu of the momentum equation. Comparison of the analytical solution of the first‐order non‐linear ordinary differential equations and a numerical solution using the Runge‐Kutta method shows a relative error of 0·3%. Using runoff data reported in the literature, comparison between the new approach and a numerical solution of the full Saint‐Venant equations showed a close agreement. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Cesium-134 as a tracer to study particle transport processes within a small catchment with a buffer zone.

The purpose of the study is to use soil particles labeled with the radioactive tracer cesium-134 (134Cs) as a method for studying soil erosion and sedimentation pattern within a small catchment with buffer zones. Cesium is adsorbed to soil particles, and by measuring changes in the 134Cs activity on the soil surface, erosion, sedimentation, and pathways for particles can be traced. A harrowed area was surface-contaminated with 134CsCl, while the buffer zone was left uncontaminated. A grid net in the tilled plot and buffer zone was established for in situ measurements of the 134Cs activity after major runoff events from October 1993 to May 1996. In addition, 134Cs activity and suspended solids in runoff were followed during the events. At the end of the experiment, the vertical distribution of 134Cs in soil profiles and uptake of 134Cs in vegetation within the buffer zone were determined. At the end of the experiment, about 54% of the applied tracer remained at the soil surface. Surface soil erosion occurred relatively uniformly across the hillslope due to sheet flow. Most of the tracer was transported vertically into the soil profile, probably during the first heavy rainfall 3 wk after application when the soil was newly tilled. Sedimentation occurred in the upper part of the buffer zone. The correlation between suspended particles in runoff and 134Cs activity was good (R2=0.76). The study also demonstrates the benefit of utilizing 134Cs2+ tracer for investigating transport pathways for contaminated partic1les within a hillslope system without disturbing the surface soil system.

Stability and damping in a dynamical model of hillslope hydrology

A stability analysis is performed on the two‐state variable, integral‐balance model of hillslope hydrology developed by Duffy [1996]. The motivation for the research is to develop a physically based, low‐dimensional representation of inflow/outflow behavior within a hillslope‐stream setting. Stability criteria are developed for the model equilibria and are evaluated using the results of numerical solutions of Richards' equation for a convex‐concave hillslope geometry. We show that for homogeneous hillslopes of three widely varying soil types the single moisture equilibrium is classified as a stable node for low precipitation rates and a stable spiral for wet conditions. The spiral equilibrium indicates that the hillslope system is lightly damped, and transient oscillations of the state variables are expected for high precipitation rates. The timescale of these oscillations is of the order of several days to weeks for the model hillslopes examined. Furthermore, we demonstrate that the model contains a Hopf bifurcation from a stable static equilibrium to a stable limit cycle. The amplitude, phase, and frequency of the limit cycle are determined analytically using second‐order averaging. However, this behavior is shown to be nonphysical for the particular homogeneous soils and hillslope geometry investigated. Implications of lightly damped behavior in the hillslope system include moisture oscillations in the field under wet conditions and difficulty in numerical solution of Richards' equation.

A chronogram for postglacial soil-landscape change from the Palliser Triangle, Canada

The soil-landscape chronogram summarizes the ways in which five slope-wash erosion cycles followed by two aeolian cycles interrupted postglacial soil formation in an enclosed hillslope system from hummocky moraine terrain. The four slope-wash cycles that followed the early-postglacial cycle occurred between 7.7 and 3 ka BP and caused about half of the total postglacial relief reduction. At the onset of the latter cycles, the locus of erosion switched from its prevalent position on the lower slope to the upper backslope, indicating recurrent fundamental changes in slope hydrology. The ensuing stratigraphic framework has a downslope thickening and fining clastic wedge, grading into the bottomland sequence and subdivided by soil catenas that merge upslope into a single compound mantle on the upland. The aeolian cycles entailed localized loess-dispersal systems, producing minor mantles of sandy-loam loess with saltation-load deposits on the leeward slope and suspension-load deposits elsewhere. Accumulation of long-range calcareous dust during the loess cycles supplied most of the secondary carbonates found in the buried soils. Three radiocarbon dates, together with Mazama tephra, enable partial correlation of these events to the regional Holocene-climate record.

Self-affine scaling and subsurface response to snowmelt in steep terrain

The use of local scaling transformations is proposed as a framework for modeling the geometry, and dynamic response of subsurface flow from steep hillslopes. The study site is the Upper Sheep Creek Watershed, a first-order sub-basin in the Reynolds Creek Experimental Watershed, southwest Idaho. The goal of the research is to gain insight into the evolution of snowmelt, subsurface flow, and runoff generation in this rugged volcanic terrain. In this phase of the study rescaling geometrically proportional (self-affine) surface and bedrock slope profiles provide an analytical framework for defining the shape, volume and dynamic changes of saturated flow for this hillslope system. Rescaling self-affine hillslope trajectories along a stream reach, collapses the flow-domain to a unit cube. A dynamic model for hillslope runoff is proposed, and implications for modeling topographic controls on the snowmelt process are suggested.

Hillslope hydrographs by the finite element method

AbstractSimulation models may be used to explore the implications of making specific assumptions about the nature of a real world system, and then to make predictions of the behaviour of that system under a set of naturally occurring conditions. It is important that understanding generated by the former should be gained before predictive use of the system model. This paper describes and uses a finite‐element model of transient, partially saturated water flow within a hillslope soil mantle overlying an impermeable bedrock, to make an investigation into the effects of parameter variations and initial conditions on the hillslope hydrograph. The results clearly demonstrate that the response of the hillslope system to rainfall is highly non‐linear and that the initial conditions, particularly in the unsaturated zone, are of paramount importance in governing the timing and magnitude of the hydrograph peak. Hillslope convergence appears as the dominant topographic parameter but the non‐linearity of the response and the complex interdependence between the soil and topographic parameters restrict the possibility of further definite conclusions about the relative sensitivity of the simulated hillslope hydrograph to changes in these parameters.