Impacts of wildfire and post‐fire land management on hydrological and sediment processes in a humid Mediterranean headwater catchment

Abstract

The extensive afforestation of the Mediterranean rim of Europe in recent decades has increased the number of wildfire disturbances on hydrological and sediment processes, but the impacts on headwater catchments is still poorly understood, especially when compared with the previous agricultural landscape. This work monitored an agroforestry catchment in the northwestern Iberian Peninsula, with plantation forests mixed with traditional agriculture using soil conservation practices, for one year before the fire and for three years afterwards, during which period the burnt area was ploughed and reforested. During this period, continuous data was collected for meteorology, streamflow and sediment concentration at the outlet, erosion features were mapped and measured after major rainfall events, and channel sediment dynamics were monitored downstream from the agricultural and the burnt forest area. Data from 202 rainfall events with over 10 mm was analysed in detail. Results show that the fire led to a notable impact on sediment processes during the first two post-fire years, but not on streamflow processes; this despite the small size of the burnt area (10% of the catchment) and the occurrence of a severe drought in the first year after the fire. During this period, soil loss at the burnt forest slopes was much larger than that at most traditionally managed fields, and, ultimately, led to sediment exhaustion. At the catchment scale, storm characteristics were the dominant factor behind streamflow and sediment yield both before and after the fire. However, the data indicated a shift from detachment-limited sediment yield before the fire, to transport-limited sediment yield afterwards, with important increases in streamflow sediment concentration. This indicates that even small fires can temporarily change sediment processes in agroforestry catchments, with potential negative consequences for downstream water quality.

How do large wildfires impact sediment redistribution over multiple decades?

“…Interestingly, this difference in sediment processes persists after the wildfire disturbance is negated by vegetation regeneration, as sediment eroded from burnt areas and deposited in the first post-fire years can be available for reentrainment in years without wildfire-enhanced erosion. This process corresponds to earlier research (Inbar et al, 1998;Mayor et al, 2007;Wittenberg & Inbar, 2009), although in the Águeda headwater catchments Nunes et al (2020) found that the large rainfall rates can flush these sediments in a few years.…”

“…Therefore, the simulated erosion by LAPSUS was calibrated for a headwater catchment located within the Águeda catchment (i.e. Macieira de Alcôba), which experienced a small fire (~10 ha) in 2011 (Nunes et al, 2020). This smaller headwater catchment contains similar land use, slope, and soil conditions as the Águeda catchment (Nunes, Naranjo Quintanilla, et al, 2018, 2020; Tavares Wahren et al, 2016).…”

“…To calibrate LAPSUS, the lumped K es and P es factors (Equations 4 and 5) for the whole catchment and the m, n, and p parameters (Equations 1 and 2) were adjusted. Unfortunately, the measured which experienced a small fire ($10 ha) in 2011 (Nunes et al, 2020).…”

“…In particular, a better understanding is needed of the importance of occasional severe post‐fire erosion events in comparison with long‐term background erosion processes (Nunes, Doerr, et al, 2018; Shakesby, 2011; Shakesby & Doerr, 2006). Furthermore, investigating a larger catchment scale can help identify locations with post‐fire erosion risk to help delineate intervention strategies, as well as examine the transport pathways of sediments from burnt areas which can have important negative impacts on water quality (Nunes, Bernard‐Jannin, et al, 2018, 2020; Shakesby, 2011).…”

Wildfire and hydrological processes

“…Wilson et al (2021) explored how streamflow and sediment delivery from hillslopes to streams is affected by stream channel networks and watershed connectivity, studying stormflow responses to the 2012 High Park Fire in Colorado. Nunes et al (2020) explored the impacts of wildfire and post‐fire management on watershed responses in a catchment in the Mediterranean Rim region of Iberia in southwestern Europe. In an agricultural catchment that was ~10% burned in a wildfire, sediment generation greatly increased, though streamflow did not, in the years directly following the fire.…”