Use of beryllium-7 to study the effectiveness of woody trash barriers in reducing sediment delivery to streams after forest clearcutting

Reliable information on the effectiveness of control measures in reducing soil erosion and sediment delivery to streams resulting from clearcutting operations is needed to support the development of best management practices for soil and water conservation in forest areas. To address this need, an investigation of the efficiency of barriers (constructed by piling up woody trash along contours) in reducing soil loss and sediment delivery was undertaken in an area experiencing prolonged highly erosive rainfall events immediately after final forest harvest. The study sites selected were located in south-central Chile and are characterized by a wet autumn and winter and mean annual rainfall of ∼2300 mm. Using a newly developed approach for employing 7Be measurements to quantify soil redistribution over an extended wet season, soil redistribution was documented at two sites located ∼2 km apart, El Monumento (EM) and San Germán (SG), the latter exhibiting less compacted soil. For an autumn period of 82 d in 2006 associated with 989 mm rainfall at EM, the observed net soil loss from two plots bounded at the top and bottom by trash barriers with a 15 and 30 m spacing, respectively, was −0.4 ± 0.1 and −0.75 ± 0.08 kg m−2 and the sediment delivery ratio 34 and 92%. Results obtained for the SG site from three plots with 10, 15 and 30 m spacing, for a 58-d autumn period with 664 mm rainfall in 2008, were −1.0 ± 0.1, −0.7 ± 0.1 and −1.8 ± 0.1 kg m−2 net soil loss, with associated sediment delivery ratios of ∼60, 55 and 88%, respectively. These results suggest that the shorter barrier spacings (∼15 m) are more effective in reducing soil loss. At SG during a 149-d period in 2008, with an extremely high precipitation total of 2089 mm, the net erosion varied only between −5.6 ± 0.4 and −6.5 ± 0.4 kg m−2 and the sediment delivery ratio between 70 and 86% for 10, 15 and 18 m barrier spacings. These results suggest that under conditions of extreme rainfall and unconsolidated soils, the woody trash barriers are unable to retain the large amounts of sediment mobilized upslope and that much of this sediment passes through the barrier, increasing the net soil loss and sediment delivery ratio. Nevertheless, in the absence of the lower woody barrier the soil loss increases. These results emphasize the benefits of installing barriers, and the need to improve their design to maximize trapping efficiency.