Temporal variation in topsoil water repellency in two recently burnt eucalypt stands in north-central Portugal

The aim of this study is to improve our knowledge of the temporal and spatial variations of soil water repellency following wildfire, in particular for the eucalypt stands that now dominate the landscape of north-central Portugal.Topsoil water repellency was monitored on 21 occasions over a 10-month period, starting in September 2005, six weeks after a moderately severe wildfire. This was done, on mostly alternating dates, in two neighbouring commercial eucalypt plantations, one with an undisturbed and one with a ploughed soil profile, in the foothills of the Gralheira Massif in north-central Portugal. Water repellency severity was measured in situ at soil depths of 2–3 and 7–8 cm using the ‘Molarity of an Ethanol Droplet’ (MED) test, and accompanied by soil moisture measurements using a ThetaProbe™ or, at a few occasions, sample analysis in the laboratory for gravimetric content.The results show a broadly seasonal pattern of overall very high water repellency in dry periods and reduced or no repellency following prolonged rainfall. This was more pronounced at the undisturbed compared to the ploughed site, as the latter exhibited strong to extreme water repellency at almost all sampling dates. Significant changes in repellency severity, including major increases, occurred within periods as short as 6–7 days, suggesting that the sampling intervals used here may have not captured the full dynamics of topsoil repellency. Repellency severity was consistently lower at greater soil depth, in particular when considering the whole study period. Soil moisture was found to relate to the temporal variations in repellency. As found in previous studies, however, soil moisture alone was not sufficient to predict the temporal variations in water repellency.