Surface and subsurface flow in eucalyptuslantations in north-central Portugal

Abstract

In the Baixo Vouga region of north-central Portugal, forests occupy half of the territory, of which two thirds are Eucalyptslantations. 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 Eucalyptuslantations. 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,articularly as return flow. Infiltration-excess OLF due to the strong soil water repellence (SWR) is dominant during dry season, butroduces residual runoff amount. SSF is therincipal mechanism of runoff formation. It originates from matrix flow andipe flow at the soil-bedrock interface,rincipally 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 soilrofile, due to a large network of macropores. Stream flow response is highly correlated with matrix flow behaviour in timing and intensity. SWR induces a veryatchy moistening of the soil, concentrates the fluxes and accelerates them almost 100 times greater than normalercolation of the water in the matrix.