Soil Water Repellency Severity and its Spatio-Temporal Variation in Burnt Eucalypt Plantations in North-Central Portugal

Soil water repellency (SWR) is a dynamic soil property that influences soil hydrology. The main goal of this work was to determine in situ spatial and temporal variations in SWR in six recently burnt eucalypt stands with different pre- and post-fire management. The severity of SWR was measured in the field using the “molarity of an ethanol droplet test” during 1–2 years, at intervals of 1–2 weeks. Measurements were taken for the ash layer, soil surface and three different soil depths (2–3, 7–8 and 14–16 cm). The volumetric soil moisture content (SMC) was measured at the three deepest layers. The results showed that ploughing operations carried out several years before the fire did not have major impacts on the severity of SWR. The ash layer was predominately wettable, whereas the other soil layers were dominated by strong to extreme severity SWR. Furthermore, and starting at a soil depth of 3 cm, the severity of repellency decreased with increasing depth. The spatial variability within a slope was significant on half of the sampling days, indicating that wettable patches were present under median severely repellent conditions, possibly increasing hydrological connectivity within the slopes. Temporal SWR patterns were inversely related to antecedent rainfall and SMC. The SWR patterns were similar between sites, as well as for the different sampling depths within each site. Major changes in the median levels of repellency were registered in periods as short as 1–2 weeks, but measured rainfall and/or soil moisture conditions were insufficient to explain these changes