Performance of grass and rainforest riparian buffers in the wet tropics, Far North Queensland. 1. Riparian hydrology

Abstract

The long and intense storms of the wet tropics present extreme conditions for testing the effectiveness of riparian buffers. This study presents results of a hydrometric investigation of 4 riparian buffers on 2 commercial banana plantations in Far North Queensland, Australia. It investigates runoff generation and riparian hydrology on hillslopes with differing slopes, contributing areas, and topographic convergence. Both grass and rainforest buffers were examined. Surface and subsurface hydrology were measured for 4 wet seasons (December–April) using paired flumes, piezometers, and tensiometers. All buffers experienced large volumes of surface runoff, with peak discharges ranging from 30 L/s on planar hillslopes to 350 L/s on a highly convergent site. Event runoff : rainfall ratios ranged between 0.01 and 0.65. Grass buffers with smaller contributing areas (<0.3 ha) were able to dissipate the energy of surface runoff under all conditions. In a larger (5 ha), highly convergent hillslope, surface runoff became channelised upslope of the buffer and the vetiver hedges and grass were not able to prevent scouring of a channel through the buffer, reducing its performance. Infiltration occurred in all buffers during small events, and at the convergent buffer during large events, most likely due to the presence of deep soil fill. In contrast, exfiltration occurred in the grass buffers on planar and moderately converging slopes during large events. There, the riparian soil approached saturation and return flow and seepage were measured. Under exfiltration, soil strength may be decreased and riparian buffers are needed to decrease erosion hazard. Localised saturation was observed in the rainforest buffer beneath a planar hillslope during large events, where soils were deeper and dried out more quickly than in the adjacent grass buffer. This study documents the high runoff volumes and peak discharges on cropped slopes in the wet tropics, and evaluates riparian hydrological processes. Infiltration is unlikely to be an important buffer function in this environment, but an additional role of buffers is to reduce the erosion hazard presented by exfiltration.