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In the Baixo Vouga region of north-central Portugal, forests occupy half of the territory, of which two thirds are Eucalypts plantations. The hydrological implications of this large-scale introduction of eucalypt are unknown and the aim of this exploratory study, realized in the Caramulo Mountains, was to describe overland flow (OLF), subsurface flow (SSF) and stream flow (Q) in a catchment dominated by Eucalyptus plantations. The main conclusions are that annual OLF rate is low, spatially heterogeneous between 0.1% and 6% and concentrated during the wet season as saturation excess, particularly as return flow. Infiltration-excess OLF due to the strong soil water repellence (SWR) is dominant during dry season, but produces residual runoff amount. SSF is the principal mechanism of runoff formation. It originates from matrix flow and pipe flow at the soil-bedrock interface, principally during the wet season. Matrix flow is correlated with soil moisture (SM) content, with a threshold of 25 %. Pipe flow starts with saturation of soil bottom but without saturation of the entire soil profile, due to a large network of macropores. Stream flow response is highly correlated with matrix flow behaviour in timing and intensity. SWR induces a very patchy moistening of the soil, concentrates the fluxes and accelerates them almost 100 times greater than normal percolation of the water in the matrix.

This paper investigates water repellency and soil moisture under 4 different Eucalyptus globulus plantations in Portugal. On 8 occasions over a 16-month period, measurements were made at 3 depths (surface, 0.10 and 0.20m) at 60 points on four 10 by 18m grids. The main results are: (i) at all sites and depths, spatial frequency of repellency (defined as percentage of repellent grid points) followed a moisture-related seasonal cycle, its amplitude being greatest for the longest established site, where surface repellency was contiguous in dry late-summer conditions, but was entirely absent after wet winter conditions; (ii) at a few points at 2 sites, repellency persisted during winter; (iii) repellency severity was dichotomously distributed regardless of season (i.e. soils were generally either wettable or highly repellent); and (iv) at the longest established site, when soil moisture was 27% soils were wettable. This may either support the existence of a 'transition zone', or be an artefact of the different scales of repellency and soil moisture assessments. Reasons for the observed changes in repellency and their relationship with soil moisture and antecedent rainfall are explored and soil hydrological implications discussed. Additional keywords: water repellence, repellency severity, %Ethanol method, hydrophobicity, Eucalyptus globulus, overland flow, time domain reflectometry, transition zone., Introduction The spatial and temporal dynamics of soil water repellency can be expected to cause considerable variations in soil hydrological response. In particular, the time taken for repellency to break [...]