The geometry of a constant buffer-loading design method for humid watersheds

doi:10.1016/S0378-1127(98)00275-8

Forest Ecology and Management

Volume 110, Issues 1–3, 5 October 1998, Pages 113-125

https://doi.org/10.1016/S0378-1127(98)00275-8 Get rights and content

Abstract

Riparian buffer strips are used in forestry to protect streams from possible adverse effects of forest harvesting or other land uses. For any given stream reach, a buffer loading can be defined as the contributing watershed area per unit area of buffer. The study used a large (66 km²) mountain watershed as a prototype. To allow accurate computation this was divided into facets by forming a flow net to the maximum accuracy of the 1 : 25 000 contour coverage. With fixed width buffers, the buffer loading was both highly variable and also independent of the Strahler order of the stream. Thus, the rationale of having larger buffers on larger streams does not seem justified. The study considered a buffer-strip design in which each element of stream buffer had exactly the same ratio of upslope-to-buffer area, giving a constant buffer loading. Computation of the buffer for each facet used an iterative procedure to achieve a satisfactory shape and position of the buffer boundary within each facet. The method gave a much more substantial protection to (convergent) channel sources and less protection to divergent areas than a fixed-width buffer design. The buffers defined also were highly asymmetric and discontinuous. The buffers defined reflected the topography, and were strongly influenced by small facets close to the stream. In cases where flow lines run close to and approximately parallel to the streams, the buffers defined were also non-intuitive. The method is predicated on the subsurface hydrology flow paths being close to those given by surface flow lines and this is not always true. Of importance is the finding that, relative to the mean protection offered, fixed-width buffers tend to underprotect slope convergences at the heads of streams, and overprotect divergent areas found along streams of increasing order.

Introduction

The question of “how should buffer strips be designed” often initiates debate in forestry matters around the world. This study examines aspects of the riparian environment, hydrologic loading and buffer-strip geometry in a mountainous, forested country. This is used to examine, firstly, the problems of a fixed-width buffer-strip design and, secondly, the feasibility and applicability of a topographically-based design procedure that ensures a constant ratio of upslope contributing area to buffered riparian area. The methodology is applied to the West Tarago River Watershed in the Central Highlands region of Victoria, Australia. The ultimate purpose of the paper is not to suggest a `better' buffer-strip design method but, rather, to add to the knowledge of formalised design methods for a rough, complex country and to develop a knowledge of the advantages and deficiencies of various methods. It is intended that subsequent papers will extend this theme. Although there has been extensive literature searching, we could find few papers on this issue in the literature. At the very outset, it is acknowledged that any buffer design will be a compromise involving many factors, and that no single-process methodology can meet all needs.

Section snippets

The concept of a buffer strip

A buffer is defined as an area of land along a stream, retained from the watershed land-use practice, to protect the stream from up-slope impacts. The role of buffer strips is to provide an undisturbed area of land adjacent to streams that acts to `filter' sediment and other stream pollutants, perhaps to protect the stream from direct insolation, and to provide areas of undisturbed `habitat'. In some cases, the question of width has become a political bargaining chip in complex land-use

A constant buffer-loading design method

Fig. 1 compares the constant buffer-loading design with the more conventional fixed-width design. The concept of a constant buffer-strip loading design is that each unit area of stream buffer has the same contributing watershed area. Thus, if 10% of the watershed was given over to buffer, then for each 100 m² of riparian area acting as buffer there would be 900 m² contributing to, and only to, that 100 m². In this way, more rigorous and uniform stream protection can be achieved. To do this, the

The study area

The watershed studied was that of West Tarago River in southeast Victoria, Australia (Fig. 5), with an overall area of 65.4 km². The predominant geology of the area is granite from the Gembrook batholith (Douglas and Ferguson, 1976). This watershed has no agricultural land. Annual precipitation is ca. 1400 mm per annum, with rainfall being the only significant precipitation. Altitude ranges from ca. 200 to 900 m. The area is steep with slopes of 30° being common. There are commonly areas of

Watershed faceting

Most work was carried out on a Sun^® 10/30 work station using the Arc/Info^® GIS. A 1 : 25 000 contour-line coverage of the mapsheets was purchased from the State mapping agency. Notwithstanding a knowledge of errors in the map (see Bren, 1995), it was assumed that the map was a perfect representation of the terrain, and that the contour lines corresponded to flow equipotential lines. The question of absolute accuracy of such maps is an interesting one (see for instance Fryer et al., 1994). The

Results

Faceting of a large area on a map proved to be a tedious procedure. To form a practical net many approximations had to be made because of the difficulties associated with long, closely spaced and almost parallel arcs; the difficulty lies in getting the stream lines more or less orthogonal to the contours, as noted by Dawes and Short (1994). Care was required with use of arc and node-snapping because of the large number of closely spaced arcs and nodes, the propensity these showed to

Discussion

A `pure' application of the constant buffer-loading method introduces a number of related philosophic difficulties which should be discussed. The first is illustrated by Fig. 12, showing a 10% buffer ratio applied to a hypothetical slope running into a stream junction. Although the buffer ratio is constant, the method would logically allow logging very close to streams near the confluence because there is only a small upslope area of generated facets. Computation of buffers on random coupes

Conclusions

The application of a constant buffer-loading method of buffer-strip design aimed at giving a substantially uniform buffer hydrologic loading for forestry work in rough, steep mountain country requires accurate maps and, if it is to be done accurately, sophisticated GIS facilities.

The required boundaries can be computed reasonably accurately using a simple method for smaller buffer percentages. Study of the buffer loading of a large natural watershed suggests that the usual fixed-width buffers

Acknowledgements

This work was supported by Project B2 of the Cooperative Research Centre for Catchment Hydrology. The assistance of Peter Hairsine and Rob Vertessy of that organisation is acknowledged.

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The geometry of a constant buffer-loading design method for humid watersheds

Abstract

Introduction

Section snippets

The concept of a buffer strip

A constant buffer-loading design method

The study area

Watershed faceting

Results

Discussion

Conclusions

Acknowledgements

For. Ecol. Manage.

The role of topography in controlling throughflow generation

Earth Surf. Proc.

Hydrologic effects of a stretch of forest road

Aust. For. Res.

The significance of topology for modeling the surface hydrology of fluvial landscapes

Water Resour. Res.