G. R. Hancock

Revisiting the hypsometric curve as an indicator of form and process in transport‐limited catchment

Hypsometry has historically been used as an indicator of geomorphic form of catchments and landforms. Yet there has been little work aimed at relating hypsometry to landform process and scale. This paper uses the SIBERIA catchment evolution model to explore linkages between catchment process and hypsometry. SIBERIA generates results that are qualitatively and quantitatively similar to observed hypsometric curves for physically realistic parameters. However, we show that not only does the hypsometry reflect landscape runoff and erosion process, but it is strongly dependent on channel network and catchment geometry. We show that the width to length ratio of the catchment has a significant influence on the shape of the hypsometric curve, though little on the hypsometric integral. For landforms dominated by fluvial sediment transport, the classic Strahler ‘mature’ hypsometric curve is only generated for catchments with roughly equal width and length. Narrow catchments show a hypsometric curve more similar to Strahlers ‘monadnock’ form. For landscapes dominated by diffusive transport, the simulated hypsometric curve is concave-down everywhere, this being consistent with curves reported for some example catchments in France. Because the transition between diffusive dominance to fluvial is scale-dependent, with larger catchments exhibiting greater fluvial dominance, then the hypsometric curve is a scale-dependent descriptor of landforms. Experimental results for simulated landforms from a small-scale rainfall-erosion simulator are reported. It is shown that SIBERIA yields satisfactory fits to the data, confirming its ability to predict the form of the hypsometric curve from a simple model of geomorphic processes.

Medium-term erosion simulation of an abandoned mine site using the SIBERIA landscape evolution model

This study forms part of a collaborative project designed to validate the long-term erosion predictions of the SIBERIA landform evolution model on rehabilitated mine sites. The SIBERIA catchment evolution model can simulate the evolution of landforms resulting from runoff and erosion over many years. SIBERIA needs to be calibrated before evaluating whether it correctly models the observed evolution of rehabilitated mine landforms. A field study to collect data to calibrate SIBERIA was conducted at the abandoned Scinto 6 uranium mine located in the Kakadu Region, Northern Territory, Australia. The data were used to fit parameter values to a sediment loss model and a rainfall–runoff model. The derived runoff and erosion model parameter values were used in SIBERIA to simulate 50 years of erosion by concentrated flow on the batters of the abandoned site. The SIBERIA runs correctly simulated the geomorphic development of the gullies on the man-made batters of the waste rock dump. The observed gully position, depth, volume, and morphology on the waste rock dump were quantitatively compared with the SIBERIA simulations. The close similarities between the observed and simulated gully features indicate that SIBERIA can accurately predict the rate of gully development on a man-made post-mining landscape over periods of up to 50 years. SIBERIA is an appropriate model for assessment of erosional stability of rehabilitated mine sites over time spans of around 50 years.

Use of a landscape simulator in the validation of the SIBERIA Catchment Evolution Model: Declining equilibrium landforms

This paper presents a study to test the ability of the current generation of landscape evolution models to correctly predict landscape form from measured erosion processes. Landscapes generated by the SIBERIA landscape evolution model were compared to experimental model landscapes in declining equilibrium. The model simulations were compared using geomorphologically and hydrologically significant parameters. Comparisons of the simulated landscapes with the experimental landscapes were carried out using the hypsometric curve, width function, cumulative area distribution, and area-slope relationship. These comparisons demonstrate that SIBERIA can correctly simulate the experimental model landscape at declining equilibrium. The simulation showed sensitivity to the spatial distribution of the rainfall, particularly with respect to hypsometry. Using the correct measured distribution of rainfall was necessary rather than using a spatially uniform rainfall distribution. The results also highlighted the importance of digital terrain map (DTM) error in deriving geomorphic statistics. The observed landform had a consistently larger width function than the simulation. Only when the simulations were corrupted with elevation errors with statistical properties of the errors in the experimental landscape DTM, as measured by photogrammetry, did the observed and simulated width functions match.

Goulburn River experimental catchment data set

(651 km 2 ) and Krui (562 km 2 ) subcatchments in the northern half of this experimental catchment with a few monitoring sites located in the south. The data set comprises soil temperature and moisture profile measurements from 26 locations; meteorological data from two automated weather stations (data from a further three stations are available from other sources) including precipitation, atmospheric pressure, air temperature and relative humidity, wind speed and direction, soil heat flux, and up- and down-welling shortand long-wave radiation; streamflow observations at five nested locations (data from a further three locations are available from other sources); a total of three surface soil moisture maps across a 40 km � 50 km region in the north from � 200 measurement locations during intensive field campaigns; and a high-resolution digital elevation model (DEM) of a 175-ha microcatchment in the Krui catchment. These data are available on the World Wide Web at http://www.sasmas.unimelb.edu.au.

An assessment of digital elevation models and their ability to capture geomorphic and hydrologic properties at the catchment scale

Digital elevation model (DEM) data quality is paramount for accurate representation of the land surface and drainage network. This issue was investigated within a small agricultural catchment in the Upper Hunter Valley region of New South Wales, Australia for a DEM created by use of a Differential Global Positioning System (DGPS) and a 25 m DEM from the New South Wales Governments land mapping authority, Land and Property Information (LPI). A hierarchical scaling approach was used to investigate the effect of increasing DEM grid size on a number of geomorphic and hydrologic descriptors (i.e. area–slope relationship, cumulative area distribution, hypsometric curve, width function, Strahler stream order and stream network statistics), as well as addressing the issue of source data accuracy. Results of qualitative and quantitative assessments indicate that as DEM grid size increased, average slope gradients decreased and the drainage network became increasingly simplified. Geomorphic descriptors such as the width function, cumulative area distribution and hypsometric curve appear largely insensitive to DEM scale. The area–slope relationship loses definition in the diffusive region of the curve at large grid scales; however, the fluvial region appears largely insensitive to changes in DEM resolution. A comparison of long-term field soil moisture data with wetness indices derived from DEMs clearly demonstrates that high resolution DEM data are needed to model soil moisture distribution. A 5 m DEM was found to have the minimum resolution required for the current study site in order to accurately capture catchment geomorphology and hydrology and to model the spatial distribution of soil moisture.

The effects of sediment transport, weathering, and aeolian mechanisms on soil evolution

Aeolian-derived soils are found throughout the world. Soil evolution processes in aeolian-dominated landscapes differ from processes in bedrock-weathering landscapes by a number of key aspects including the lack of (1) soil production depth dependency, (2) surface armoring, and (3) grain size self-organization in the soil profile. We use here a soil evolution model (mARM5D) to study the differences between aeolian and bedrock-weathering-dominated landscapes by analyzing soil evolution on a hillslope under various aeolian and bedrock-soil supply settings subject to fluvial and diffusive sediment transport. The model simulates spatial and temporal variation in soil particle size distribution (PSD) and profile depth for each grid cell on the landscape, as a function of physical weathering, aeolian deposition, and diffusive and fluvial sediment transport. Our results indicate that surface armoring plays a major role in soil evolution. Under bedrock-weathering-dominated conditions, armoring reduces soil erosion and in conjunction with depth-dependent soil production, leads to steady state soil grading and depth and a relatively uniform soil distribution. In contrast, aeolian-dominated landscapes tend to have considerable spatial variability in soil depth and PSD. Our results also indicate that in contrast with diffusive transport, which is assumed to be PSD independent, fluvial sediment transport is strongly influenced by the soil production mechanism (aeolian or bedrock weathering). Based on the results presented here, we propose that aeolian-dominated landscapes are more responsive to environmental changes (e.g., climatic and anthropogenic) compared with bedrock-weathering landscapes. We further propose that this sensitivity may help explain the patchy soil distribution that is often observed in aeolian-dominated regions.

Transient landscapes: gully development and evolution using a landscape evolution model

Gullies are a common feature of many landforms and play a significant role in landscape evolution. They are transient landscape features that move along a drainage line incising into soil, alluvium or colluvium disturbing the catchment both headwards and along its banks. It is important to understand gully initiation, development and evolution as while they are transient features they are drivers of landscape change and erode and transport considerable volumes of sediment through the channel network. There has been considerable research into understanding gully development with many numerical predictive models developed. Here a computer-based landscape evolution model (SIBERIA) is examined for its ability to predict gully development in a catchment undisturbed by European agricultural practices. The simulations demonstrate that the SIBERIA model is able to produce gullies in the same position that are qualitatively and quantitatively morphologically similar to field data. The modelling suggests that the whole catchment is at risk of gullying and is in accordance with the field data which demonstrated that the presence of gullies was extensive throughout the catchment. The model also produces erosion rates within that of independently measured field data.

Predicting uncertainty in sediment transport and landscape evolution - the influence of initial surface conditions

Numerical landscape evolution models were initially developed to examine natural catchment hydrology and geomorphology and have become a common tool to examine geomorphic behaviour over a range of time and space scales. These models all use a digital elevation model (DEM) as a representation of the landscape surface and a significant issue is the quality and resolution of this surface. Here we focus on how subtle perturbations or roughness on the DEM surface can produce alternative model results. This study is carried out by randomly varying the elevations of the DEM surface and examining the effect on sediment transport rates and geomorphology for a proposed rehabilitation design for a post-mining landscape using multiple landscape realisations with increasing magnitudes of random changes. We show that an increasing magnitude of random surface variability does not appear to have any significant effect on sediment transport over millennial time scales. However, the random surface variability greatly changes the temporal pattern or delivery of sediment output. A significant finding is that all simulations at the end of the 10,000 year modelled period are geomorphologically similar and present a geomorphological equifinality. However, the individual patterns of erosion and deposition were different for repeat simulations with a different sequence of random perturbations. The alternative positions of random perturbations strongly influence local patterns of hillslope erosion and evolution together with the pattern and behaviour of deposition. The findings demonstrate the complex feedbacks that occur even within a simple modelled system. The effect of DEM surface roughness was examined using a landscape evolution model.Different surface roughness in the DEM produced variability in sediment output.However all simulated landscapes were similar suggesting a geomorphic equifinality.The initial catchment shape exerts a first-order control over landscape evolution.

Does introduced fauna influence soil erosion? A field and modelling assessment.

Pigs (Sus scrofa) are recognised as having significant ecological impacts in many areas of the world including northern Australia. The full consequences of the introduction of pigs are difficult to quantify as the impacts may only be detected over the long-term and there is a lack of quantitative information on the impacts of feral pigs globally. In this study the effect of feral pigs is quantified in an undisturbed catchment in the monsoonal tropics of northern Australia. Over a three-year period, field data showed that the areal extent of pig disturbance ranged from 0.3-3.3% of the survey area. The mass of material exhumed through these activities ranged from 4.3 t ha(-1) yr(-1) to 36.0 t ha(-1) yr(-1). The findings demonstrate that large introduced species such as feral pigs are disturbing large areas as well as exhuming considerable volumes of soil. A numerical landscape evolution and soil erosion model was used to assess the effect of this disturbance on catchment scale erosion rates. The modelling demonstrated that simulated pig disturbance in previously undisturbed areas produced lower erosion rates compared to those areas which had not been impacted by pigs. This is attributed to the pig disturbance increasing surface roughness and trapping sediment. This suggests that in this specific environment, disturbance by pigs does not enhance erosion. However, this conclusion is prefaced by two important caveats. First, the long term impact of soil disturbance is still very uncertain. Secondly, modelling results show a clear differentiation between those from an undisturbed environment and those from a post-mining landscape, in which pig disturbance may enhance erosion.

Landslides in Tertiary Basalts at Murrurundi, Australia

The principal geomorphic feature of Eastern Australia is the Great Dividing Range. It is intermittently capped by Tertiary basalts, which are variously preserved as plateaus or steep-sided ridges. In contrast to most of the Australian landscapes, the Tertiary basalts form steep, topographically immature landforms that are prone to slope instability. This paper looks at the characteristics of the basalts and their associated alteration horizons, which, in combination, make them susceptible to landsliding on a large scale. The very different physical and chemical weathering characteristics of the basalt and the altered materials are contrasted and used to explain a complex landsliding mechanism that is triggered by failure in an amygdaloidal horizon capped by a palaeosol, and then sustained by characteristics of the basalt and its derived soils. Process rates are inferred to range from relatively rapid to very slow, throughout the different stages of instability. The role of anthropogenic factors on slope stability is discussed.