Distributed assessment of contributing area and riparian buffering along stream networks

We present a simple approach for quantifying the local contributions of hillslope area and riparian area along a stream network based on gridded digital elevation data. The method enables one to compute catchment characteristics such as the distribution of riparian and hillslope inputs to the stream network, the variation of riparian‐area percentage along the stream network, and subcatchment area distributions. We applied the technique to the 280‐ha Maimai research area in New Zealand. We found that 85% of the catchment area contributed to streams with a local catchment area of <20 ha, whereas only 28% of the riparian area was found along these small streams. The potential of riparian zones to buffer hillslope runoff depends partially on the size of the riparian zone relative to the adjacent hillslope or upland area. Our approach enables calculation of a spatially distributed measure of riparian to hillslope area ratios. At the 280 ha Maimai research area we found that the ratio between riparian and hillslope area was 0.14. When we calculated this “buffer capacity” for each 20 m stream reach along the stream network, the values were below 0.14 for 75% of the stream length and the median was 0.06. Using the catchment‐wide ratio would thus significantly overestimate the “effective” riparian‐to‐hillslope‐area ratio.