Influence of soil moisture content on water repellency after a forest burnt

A wildfire occurred in 2012 in one of the protected relic laurel forests of Europe (the National Park of Garajonay, Canary Islands). Soils from unburnt and burnt areas were studied and compared on its water repellence level at different soil moisture content from 2004 till 2023. 32 study sites and more than 100 soils were prepared under saturation conditions (sprayed of distilled water on the surface of each sample). Starting from saturation till oven-dried conditions, lower soil moisture contents were established in successive increments after the end of the WDPT test. The petri dishes were weighted at each step to determine the gravimetric soil water content (g g ^-1) by the thermogravimetric method at the end of the sequence. To describe the influence of soil moisture content on water repellency, three phases were distinguished. Phases I and II corresponded with the air-drying phase and phase III to the oven-drying phase.

Results of the study highlight that the water repellency of in both unburned and burnt sites strongly depended on the soil moisture content. After 11 years from the fire, the infiltration capacity of the soils showed improved levels of water repellency and in some cases total recovery. In order to reproduce the soil hydrophobic behavior under naturally occurring drying conditions (phases I and II), the time required for infiltration was modelled as a function of gravimetric moisture content during air-drying. Variability (standard deviation) increased with increasing times to infiltration (i.e. decreasing moisture content), which can be attributed to evaporation and soil hydraulic effects influencing the results during longer tests.