Performance of grass and rainforest riparian buffers in the wet tropics, Far North Queensland. 2. Water quality

Riparian lands have the potential to buffer streams from hillslope sediment and nutrient transport. Most research on buffers has been conducted under laboratory or manipulated field experimental conditions. Few quantitative data exist on buffer performance under natural field conditions. This study reports measured soil loss and evaluates the effectiveness of riparian buffers on planar and convergent slopes under field hydrological conditions in Far North Queensland. The conditions are extreme for testing the effectiveness of riparian buffers as the land is steep, intensely cropped and receives high intensity rainfall. Hillslopes cropped with bananas were monitored using paired flumes. Runoff, bedload, and suspended loads were measured leaving the crop (upper sites) and leaving the riparian buffers (lower sites). Highly variable hillslope soil losses of <1 to >70 t/ha per wet season were recorded. High rates of hillslope soil loss were from areas of steep gradient with little ground cover experiencing high rainfall intensity. On planar slopes, even with high soil loss, grass buffer strips were able to trap >80% of the incoming bedload. Total N (TN), total P (TP) and suspended sediment (SS) loads were reduced between 25 and 65% by the planar slope grass buffer and within the first 15 m of the moderately convergent grass buffer. Loads leaving the moderately convergent buffer were often higher than those delivered from the crop, due to seepage after prolonged or high frequency rainfall. Under these conditions the buffer's main function is to prevent erosion rather than trap sediment and nutrients. Results from a highly convergent 5-ha hillslope, suggest that for buffers to be more effective in such topography, they should also be placed at the end of the crop rows, where contributing areas are smaller. Flow was able to concentrate within the crop and on at least one occasion was able to scour a 30-cm-wide channel through the entire width of the buffer releasing previously trapped material and making the buffer ineffective. A remnant rainforest buffer, receiving runoff from a planar slope, acted as a temporary store of sediment and nutrients that were reworked during subsequent events. This study demonstrates both a need for managed buffer strips on sloping tropical cropped land and identifies limitations on their potential effectiveness.