Post-fire short- and long-term soil erosion monitoring – The impact of consecutive storm events on R factor and erosion rates

Wildfires have a significant impact on soil erosion. Most studies emphasize on the "disturbance window", which typically ranges from 3 to 10 years. Studies on the long-term effects of fire on soil erosion are relatively few, especially when it comes to studies that go beyond 20 to 30 years after the fire.

This study carried out at Seich Sou, a suburban forest in Thessaloniki city, North Greece. A wildfire in 1997 destroyed half of the forest, and another one occurred in 2021. This study focuses on investigating the long-term (1997 wildfire) and short-term (2021 wildfire) post-fire impacts on erosion in relation to rainfall intensity and rainfall erosivity (R factor). Field plots using silt fences were installed, to quantify soil erosion in both burned and unburned regions.

Regarding the short-term effects of the wildfire in 2021 on soil erosion, the findings indicated that vegetation is the primary factor influencing annual erosion rates. Soil erosion in burned plots is significantly influenced by rainfall intensity, particularly when it surpasses 6–7 mm/30 min. However, in burned plots it was revealed that soil erosion did not significantly increase when the rainfall intensity increased beyond 10 mm/30min. On the other hand, in the unburned plots, soil erosion was considerably increased beyond a certain threshold of rainfall intensity (>10 mm/30 min).

For the first time in literature, it was revealed that when two consecutive and very intense storms occurred, the second, more intense rainfall generated noticeably less erosion rates than the first. An average 20% reduction in soil erosion (both in burned and unburned plots) was observed after the second storm, when the R factor increased by 690%. The main reason for this behavior is the quick depletion of the available sediments caused by the high-intensity consecutive rainfalls, which decreased the erosive effect of the second consecutive storm.

 We also found that since both major erosive episodes were so close to one each other in time, the considerable rise in R factor in the second post-fire year did not significantly increase soil erosion. These results demonstrate that the R factor in RUSLE, which is used to determine the annual erosion rate in burned and unburned regions, without the appropriate reference to the corresponding field data, which used to validate the model, has potential significant errors that may lead to inaccurate erosion rate estimations. Before implementing the erosion model into practice, researchers and stakeholders that utilize the R factor in erosion modeling should thoroughly investigate the precise dates of the significant erosive events.

Concerning the long-term effects of the 1997 wildfire, the findings from the "natural reforestation" plots showed that, 25 years after the wildfire, erosion rates are three times higher (0.062 t/ha/year) than those of the "control" plots (0.023 t/ha/year). The forest ecosystem has not significantly recovered, and it seems that the "window of disturbance" in the reforested area has not been closed. Depending on site quality, geomorphology, and meteorological conditions, it may take more than 20 years to return soil erosion rates to normal levels in Mediterranean environments, where soils are typically thin and rocky.