Within-in flume sediment deposition in a forested catchment following wildfire and post-fire bench terracing, north-central Portugal

Abstract

Wildfires increase plot-scale soil erosion rates across the world. However, very few studies have monitored post-fire erosion losses at the catchment scale, especially in the Mediterranean region. The FIRECNUTS project was originally set out to address this research gap by selecting a recently burnt catchment and instrumenting its outlet with two flumes with maximum discharge capacities of 120 and 1700 l s-1. Six months after the fire, however, this experimental catchment was bench terraced by bulldozer. Furthermore, the smallest flume suffered from regular deposition of sediments from the onset of its construction. This study was therefore an attempt to explore whether quantifying this deposition of sediments could provide further insight into the sediment yield of a recently burnt and terraced catchment. To this end, the deposited sediments were removed and weighted on a total of 101 occasions during a three year period. The results revealed that: (i) the amounts of sediment deposited over the three hydrological years corresponded to relatively small erosion rates, varying between 0.030 and 0.046 Mg ha-1 yr-1; (ii) while bench terracing is generally viewed as a soil and water conservation technique, it did not produce a clear decrease in sediment deposition, as deposition per mm of streamflow was the same before and after terracing (0.8 kg mm-1); (iii) daily sediment deposition could be explained reasonably well by daily rainfall volume and maximum daily streamflow volume.