Hydrologic assessment of woody biomass removal for biofuel production

Abstract

Wood-based energy markets have been proposed as a means to ensure sustainable forests, enhance ener- gy security, promote environmental quality and realize social benefits. An important issue among stake-holders is that collecting small-diameter woody biomass may significantly alter the water budget of post-timber harvesting landscapes. However, little is actually known about the hydrologic impacts that additional biomass removal and post-harvest land treatments may have on the water budget. Climate data and volumetric water content measurements at depths of 10, 20, 30 and 100 cm were collected from 28 one-acre plots near Eugene, OR, USA, subject to seven different land treatments. This information was analysed and used to calibrate and validate a site-specific water balance model (UNSAT-H) to evaluate a null hypothesis that changes in biomass removal do not impact subsurface environment. Results showed a positive correlation between post-harvest land treatments involving soil compaction and evaporation with compacted sites producing over 30% more evaporation than non-compacted, harvested sites and 20% more than non-compacted, non-harvested sites. Furthermore, non-compacted sites subject to increased biomass removal had higher infiltration rates than both unharvested sites and harvested, compacted sites. In terms of changes to runoff and sediment production, maximum impact coincides with the period immediately after track construction and harvesting. However, these effects decrease significantly over the 5-year time frame, well within the inter-logging cutting cycle of 30–40 years in this region.