Soil water repellency is now known to occur in diverse soils in various parts of the world. One of the possible adverse effects of soil water repellency is that it can reduce infiltration capacity and hence, on sloping terrain, enhance overland flow and soil erosion. The main aim of the present work is to assess the effects of soil water repellency on surface runoff production in the inner coastal dune areas of central Portugal. This was done for a pine and a eucalypt forest stand and, within each stand, for 2 slopes with contrasting aspect and somewhat different slope angles. Overland flow was measured for 4 pairs of unbounded plots of about 5 m² at fortnightly intervals from February to October 2001. Over the same period, soil water repellency at and immediately below the soil surface was measured next to the plots at monthly intervals. The runoff–repellency relationship was also studied by carrying out rainfall simulation experiments on 0.24-m² plots and associated repellency measurements.

The effect of soil water repellency was most clearly demonstrated by statistically significant higher runoff coefficients under strong-to-extremely than under none-to-slightly hydrophobic conditions immediately below the soil surface. Such a difference in runoff over the measurement period was, however, restricted to 2 unbounded plots, both of which were located on the eucalypt slope with a southerly aspect and the greater slope angle. At the scale of these plots, the increase in runoff coefficient due to soil water repellency is moderate, when integrated over the entire period of strong–extremely repellent conditions, but can be quite substantial for individual 2-weekly periods. With respect to the observed differences in runoff between plots, be it plots on the same slope or not, it has proved difficult to distinguish the effect of soil water repellency from that of other factors likely to affect overland flow generation.