Gregory R. Hancock

Effect of Catchment Aspect Ratio on Geomorphological Descriptors

In recent years several geomorphic measures have been used to test the ability of landscape evolution models to simulate field catchments. These are the hypsometric curve, area-slope relationship, width function and cumulative area distribution. The geomorphic modelling community is only now developing the statistical tools to assess whether these measures are sufficiently discriminatory for use in comparisons between model and field data. To examine this question and the effect of catchment geometry (and especially catchment aspect ratio), a series of synthetic catchments with varying aspect ratio and area matching that of a well understood field catchment were used. This field catchment, for which a set of calibrated hydrology and erosion parameters had been determined, has been used in previous studies to test the ability of the SIBERIA landscape evolution model to simulate field catchments. These synthetic catchments were then allowed to evolve using the calibrated SIBERIA model and the hypsometric curve, area-slope relationship, width function and cumulative area compared with field catchment data. Results show that for a good match to be obtained between simulation and field data using these descriptors as tools of comparison, all that is needed is a catchment with an aspect ratio matching that of the field data (given calibrated erosion parameters). The findings highlight the strengths and weaknesses of the above geomorphic measures when used as a means of comparison between model and field data.

A framework for the quantitative testing of landform evolution models

Recent years have seen the development of a number of physically based computer models simulating the evolution of landforms under the action of erosion. To date, comparisons with field data have been largely qualitative so that only subjective assessments of their adequacy have been performed. To be able to rely on the quantitative predictions of these models methodologies for assessing their quantitative reliability are needed. Key problems are (a) the lack of repeatability of field measurements, (b) the sensitive dependence of models on initial conditions combined with the inherent unknowability of initial conditions in the field, (c) identification of measures for assessing model adequacy that can distinguish differences arising out of the physics, from random effects and unknown inputs, and (d) development of an objective, statistical methodology that can reject an inadequate model. This paper addresses these problems by proposing a statistical methodology based on Monte-Carlo simulation using the landform evolution model being tested. The principles are presented in a series of examples that compare the SIBERIA catchment evolution model with a natural undisturbed site in Arnhem Land, Northern Territory, Australia. One model calibration, based on an eye fit of field data, is shown to be deficient. An improved method for determining drainage density, based on fitting to the slope of the cumulative area diagram, is proposed. While the examples presented are by no means comprehensive, it is concluded that SIBERIA model does an adequate job of simulating the field landform. A formal probabilistic framework for model testing is developed, together with a methodology for objectively assessing the value of data for model testing. This methodology allows for model and input uncertainty, and correlation of statistics.

The fate of suspended sediment and particulate organic carbon in transit through the channels of a river catchment.

Particulate organic matter (POM) transiting through rivers could be lost to overbank storage, stored in‐channel, added to by erosion or autochthonous production, or turned over to release greenhouse gases to the atmosphere (either while in the water column or while stored in the channel). In the UK, a net loss of POM across catchments has been recorded, and the aim here was to investigate the balances of processes acting on the POM. This study considered records of suspended sediment and POM flux in comparison to stream flow, velocity, stream power, and residence time for the River Trent (English Midlands, 8,231 km2). We show that for the lower two thirds (106 km) of the River Trent, 2% is lost to overbank storage; 10% is lost to the atmosphere in the water column; and 31% is turned over while in temporary storage. Permanent in‐channel storage is negligible, and for the lower course of the river, material stored in‐channel will have a residence time of the order of hundreds of days between the last flood hydrograph of one winter and the first winter storm of the next winter (usually in the same calendar year).When considered at the scale of the UK, 1% POM in transit would be lost to overbank sedimentation; 5% turned over in the water column, and 14% turned over while in temporary storage. In the upper third of the study river channel, there is insufficient stream power to transport sediment and so in‐channel storage or in‐channel turnover over to the atmosphere dominate. The in‐channel processes of the River Trent do not conform to that expected for river channels as the headwaters are not eroding or transporting sediment. Therefore, the source of sediment must be lower down the channel network.