A case study in the use of threshold measures of hydrologic loading in the design of stream buffer strips

Abstract

In forestry, a buffer area has logging excluded in order to protect stream values. In Victoria (Australia) the usual method is to define a stream and mark the boundary a set distance from the stream. However, this does not give any protection to watershed convergences that may not carry a definable stream, and it is easy to show major variations in the hydrologic loading of such areas. In contrast a buffer designed using measures of convergence can (in principle at least) give protection to such hollows and be independent of whether a ‘stream’ had been defined. These buffer limits would be set at a value consistent with the ‘natural threshold of erosion’. This paper explores the buffer properties given by two such measures–the specific area and the slope index. Both the parameters are closely related, but the slope index incorporates the local slope. It was found that the buffers given by a simple cut-off were not necessarily associated with streams, and that most protection was allocated to areas of watershed convergence (hollows) because these have high hydrologic loadings. In contrast, as one moved downstream the hydrologic loading tended to be so small that this method gave little or no buffering. Use of a ‘regional value’ on synthetic logging coupes showed a wide variation in the level of protection given in such areas. This reflects that many logging areas tend to be divergent and, in reality, have little need for buffer protection compared to their more convergent areas. The geometric pattern of buffers differed substantially from that associated with fixed buffer width protection, and shows that a major deficiency of the ‘fixed width’ approach is under-protection of source areas and overprotection of downstream slopes contributing to a waterway.