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

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.