Hydrological and sedimentological changes following the 2010-forest fire in the Nahal Oren Basin, Mt. Carmel Israel–a comparison to pre-fire natural rates

The present study compares pre-fire and post-fire hydrological and sedimentological data at the mountainous, Upper Nahal Oren basin (18 km²), in Mt. Carmel, northwestern Israel, and traces its recovery after 4–6 years. The study summarizes 15 years − 2001–2016 of monitoring rainfall, streamflow and sediment transport during which, the 2010-fire consumed about a third of the basin vegetation cover. Sediments were monitored in a reservoir and at a gaging station. The results indicate that: (a) The threshold rainfall amounts for flow generation during the period 2010–2013 decreased by about < 50% indicating higher hydrological sensitivity. Since 2014 – 4 years after the fire, these values almost returned to their pre-fire background rates. (b) The annual average number of flow events significantly increased from 4.4 flows per year pre-fire to 8 flows per year post-fire (an increase of 180%). (c) No changes in peak discharges between pre- and post-fire, were documented. (d) Flow duration significantly increased with increasing storm rainfall; the slope of the regression line for the post-fire periods is significantly steeper indicating stronger basin response. (e) Post-streamflow events base flows significantly shortened from 7.2 days to 1.8 days (in average), due to the decrease in deep infiltration and recharge to the shallow perched aquifers along the channel resulting in decrease of spring discharge. During 2010–12, almost no base flows were documented. (f) Post-fire suspended sediment concentrations (SSC) significantly increased 2–3 orders of magnitude. The slope of regression of the post-fire period is significantly steeper than the pre-fire period. These, together with the increase in number of sedimentary events explains the increase in annual sediment weight by about 35%. Since 4th-6th year, post-fire SSC values decreased to almost pre-fire rates and annual sediment weight to even lower (73%) than pre-fire rates. Event sediment weight significantly increase with storm rainfall. The various, basin scale hydrological and sedimentological parameters documented at the present study indicate that the major impact of the fire continues 2–3 years, thereafter (4–6 years) they almost return to the pre-fire values, probably due to the recovery of the vegetation cover.