Les Basher

Characterization and quantification of suspended sediment sources to the Manawatu River, New Zealand.

Knowledge of sediment movement throughout a catchment environment is essential due to its influence on the character and form of our landscape relating to agricultural productivity and ecological health. Sediment fingerprinting is a well-used tool for evaluating sediment sources within a fluvial catchment but still faces areas of uncertainty for applications to large catchments that have a complex arrangement of sources. Sediment fingerprinting was applied to the Manawatu River Catchment to differentiate 8 geological and geomorphological sources. The source categories were Mudstone, Hill Subsurface, Hill Surface, Channel Bank, Mountain Range, Gravel Terrace, Loess and Limestone. Geochemical analysis was conducted using XRF and LA-ICP-MS. Geochemical concentrations were analysed using Discriminant Function Analysis and sediment un-mixing models. Two mixing models were used in conjunction with GRG non-linear and Evolutionary optimization methods for comparison. Discriminant Function Analysis required 16 variables to correctly classify 92.6% of sediment sources. Geological explanations were achieved for some of the variables selected, although there is a need for mineralogical information to confirm causes for the geochemical signatures. Consistent source estimates were achieved between models with optimization techniques providing globally optimal solutions for sediment quantification. Sediment sources was attributed primarily to Mudstone, ≈38-46%; followed by the Mountain Range, ≈15-18%; Hill Surface, ≈12-16%; Hill Subsurface, ≈9-11%; Loess, ≈9-15%; Gravel Terrace, ≈0-4%; Channel Bank, ≈0-5%; and Limestone, ≈0%. Sediment source apportionment fits with the conceptual understanding of the catchment which has recognized soft sedimentary mudstone to be highly susceptible to erosion. Inference of the processes responsible for sediment generation can be made for processes where there is a clear relationship with the geomorphology, but is problematic for processes which occur within multiple terrains.

Testing the hillslope erosion model for application in India, New Zealand and Australia

The hillslope erosion model (HEM) was developed to describe erosion and sediment yield on rangelands and is based on mathematical relationships among sediment yield, runoff, hillslope characteristics, and a relative soil erodibility value. It is available on the web site, http://www.eisnr.tucson.ars.ag.gov/HillslopeErosionModel. Currently, HEM has had limited application outside the USA. Our aim was to test the utility of the model with data from (a) a sandy loam at Hyderabad, India; (b) a clay loam at Pukekohe, New Zealand; and (c) a heavy red clay soil in northern Australia. Calibration showed that derived relative soil erodibility values for Indian and Australian locations differed from those determined for the USA datasets, however the default value appeared to be applicable for the New Zealand data with some variability. Our testing suggests that further calibration and analysis are necessary before default values can be identified for all sites. We also suggest however, that cautious use with derived soil erodibilities is possible at these locations, as further model testing occurs.

Integrating Environmental and Socio-Economic Indicators of a Linked Catchment–Coastal System Using Variable Environmental Intensity

Can we develop land use policy that balances the conflicting views of stakeholders in a catchment while moving toward long term sustainability? Adaptive management provides a strategy for this whereby measures of catchment performance are compared against performance goals in order to progressively improve policy. However, the feedback loop of adaptive management is often slow and irreversible impacts may result before policy has been adapted. In contrast, integrated modelling of future land use policy provides rapid feedback and potentially improves the chance of avoiding unwanted collapse events. Replacing measures of catchment performance with modelled catchment performance has usually required the dynamic linking of many models, both biophysical and socio-economic—and this requires much effort in software development. As an alternative, we propose the use of variable environmental intensity (defined as the ratio of environmental impact over economic output) in a loose coupling of models to provide a sufficient level of integration while avoiding significant effort required for software development. This model construct was applied to the Motueka Catchment of New Zealand where several biophysical (riverine water quantity, sediment, E. coli faecal bacteria, trout numbers, nitrogen transport, marine productivity) models, a socio-economic (gross output, gross margin, job numbers) model, and an agent-based model were linked. An extreme set of land use scenarios (historic, present, and intensive) were applied to this modelling framework. Results suggest that the catchment is presently in a near optimal land use configuration that is unlikely to benefit from further intensification. This would quickly put stress on water quantity (at low flow) and water quality (E. coli). To date, this model evaluation is based on a theoretical test that explores the logical implications of intensification at an unlikely extreme in order to assess the implications of likely growth trajectories from present use. While this has largely been a desktop exercise, it would also be possible to use this framework to model and explore the biophysical and economic impacts of individual or collective catchment visions. We are currently investigating the use of the model in this type of application.

Modelling loess landscapes for the South Island, New Zealand, based on expert knowledge

Abstract In New Zealand, occurrence of loess often determines the spatial pattern of soil depth, and influences droughtiness, leaching potential, organic matter accumulation, nutrient retention, and natural plant‐species distribution. Understanding loess distribution is therefore a major prerequisite for soil and land resource management. Although New Zealands soil scientists have accumulated a good empirical knowledge of loess distribution through several decades of field investigation, only some of this knowledge is recorded in papers and reports. This study estimates loess thickness and percent cover, and provides loess landscape models for the internal loess distribution of all land units in the South Island based on expert knowledge. We derived loess depth classes and percent cover classes and assembled land units with similar loess distribution patterns. The soil sets underpinning the map units of the New Zealand Land Resource Inventory (NZLRI) were classified according to loess depth, loess cover, and loess pattern. New loess maps of the South Island were produced from those classifications, displaying loess coverage, thickness, loess pattern, and loess landscapes. These maps present our current knowledge of the coarse‐scale loess distribution and provide a framework for fine‐scale loess landscape modelling.

Development of a landslide component for a sediment budget model

Most erosion models focus on overland-flow erosion with fewer incorporating landslide erosion although it is common on hillslopes. Landslide models are typically dynamic, spatially distributed simulations with large data requirements for parameterisation and are often computationally intensive. The Australian SedNet model represents a middle ground between process-based and empirical models and is being modified for New Zealand conditions by incorporating shallow landsliding.We describe a method for implementing a model within SedNetNZ to provide the long-term annual average sediment contribution from shallow landsliding and its spatial distribution. The mass of soil eroded over a defined period is calculated from the landslide probability for each slope class, slope class area, failure depth, soil bulk density, and sediment delivery ratio. Landslide probability is derived from mapping a time series of landslides intersected with DEM-derived slope. The conceptual approach and methodology for parameterisation are suitable for landslide modelling where rainfall-triggered shallow landslides occur. A method for modelling long-term shallow landsliding within a sediment budget model.Time series of historical landsliding mapped from aerial photography.Landslide-slope relationships derived from landslide distribution and slope.Landslide-slope relationships used to spatially model sediment generation.Applicable to any landscape subject to rain-triggered shallow landsliding.

Towards understanding river sediment dynamics as a basis for improved catchment, channel, and coastal management: the case of the Motueka catchment, Nelson, New Zealand

ABSTRACT This paper brings together work in the Motueka catchment that has focused on both suspended sediment data and bedload transfers to provide a more holistic understanding of sediment dynamics in the catchment to inform effective river management. The annual suspended sediment load averages 349,000 t and shows considerable temporal variability (49,000 t to 1.7 Mt). Event yields may increase by an order of magnitude in response to single high magnitude storm events. Much of the sediment is generated from high rainfall areas of the catchment under indigenous forest and grassland. Short-term studies show pasture areas have a higher specific sediment yield than production forest, but that forest harvesting leads to a short-term increase in yield. Bedload transfers assessed via morphological budgeting from digital elevation model (DEM) differencing in selected reaches of the upper Motueka reveal similarly highly variable transfers at an annual scale, reflecting the magnitude and frequency of competent flow events. Longer term mean bed-level (MBL) changes demonstrate a high degree of spatial variability in the upper Motueka. Overall, DEMs of difference and longer term MBL changes both reveal a net channel degradation and export of bedload in the mainstem of the upper Motueka. Suspended sediment data also suggest an overall reduction in sediment yield from the catchment, suggesting a catchment-wide limitation of sediment supply, or a period of lower flows reducing sediment mobilization. This understanding has informed on issues such as the role of river channel management and catchment land use on in-stream ecosystems, coastal erosion, and shelf water quality and fisheries. Future river management, if it is to be effective, needs to recognize the history of this system, its likely longer term trajectory, and its linkages with the coast.