Surface and subsurface flow in eucalyptus plantations in north-central Portugal

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

Modelação da erosão do solo e sua verificação numa área florestal recentemente ardida em Açores, centro-norte de Portugal

This study aims to assess the suitability of two empirical erosion models, for mapping the risk of erosion in post-fire situation at slope-scale, based on Geographical Information Systems. It was developed within the scope of EROSFIRE project, which aims to evaluate the advantages and disadvantages of field rainfall experiences, to map the risk of erosion, by providing required data for model calibration.O presente trabalho tem como objectivo avaliar a adequabilidade de dois modelos empíricos de erosão, para o mapeamento do risco de erosão pós-fogo à escala da encosta, com base em Sistemas de Informação Geográfica. Foi desenvolvido no âmbito do projecto EROSFIRE, que pretende avaliar as vantagens e desvantagens de experiências de campo de simulação de chuva para mapear o risco de erosão, fornecendo dados necessários à calibração de modelos

Post-fire soil erosion mitigation: a review of the last research and techniques developed in Portugal

Post-fire soil erosion risk assessment is not exempt of uncertainties. In many parts of the world post-fire soil erosion can have devastating effects over forest ecosystems, infrastructures and human life. However, in the Mediterranean countries, it has been defined as “low” and it was attributed to a long history of intense land use. This review paper integrates the last research assessing post-fire runoff and soil erosion as well as mitigation techniques focussing inPortugal. In the case of north-centralPortugal, recent research assessing wildfire and ground preparation effects leads to the conclusion that post-fire soil losses was as high as 5-10 Mg ha-1yr-1, while ground preparations soil losses doubled these figures. Since those values are higher than the estimates for tolerable soil losses, further considerations must be taken in order to guarantee a sustainable land use. The advantages of post-fire soil erosion control with straw mulching are well known worldwide. Recently, other treatments were assessed in recently burnt eucalypt and pine plantations in Portugal: 1) the eucalypt chopped bark mulch; 2) the slash logging mulch (unchopped twigs, stems and leaves); 3) the hydromulch, a mixture of water, organic fibres, seeds, nutrients and a surfactant with good performance in cut slopes rehabilitation; and 4) the polyacrylamides (PAM), a chemical agent with good performance in agricultural soil erosion control and high potential due to its low application rate. The more effective treatments were those increasing the ground cover, one of the most important key factors for soil erosion. For this reason, the eucalypt chopped bark mulch and the hydromulch reduced the runoff in 50% and the soil erosion in 80-90%, while the PAM did not achieved the required reduction effect. In areas where the ground cover was high (due to the presence of a needle carpet) the slash logging mulch was not effective since soil erosion was already low in the untreated areas.Existe una gran incertidumbre en la cuantificación del riesgo erosivo tras un incendio forestal. En algunas partes del mundo la erosión post-incendio puede tener efectos devastadores sobre los ecosistemas forestales, las infraestructuras y también sobre las vidas humanas. Sin embargo, en los países del litoral mediterráneo la erosión post-incendio suele ser catalogada como “baja”, especialmente debido a una larga historia de intenso uso del suelo. Este trabajo de revisión integra los últimos estudios realizados en Portugal sobre la evaluación del riesgo de erosión y de escorrentía post-incendio así como las técnicas para su mitigación. En el caso del centro-norte de Portugal, estudios recientes han demostrado que los incendios forestales pueden resultar en pérdidas elevadas (5-10 Mg ha-1año-1), mientras que algunas actividades de preparación del suelo que se llevan a cabo en la región pueden duplicar estos valores. Todas estas figuras sobrepasan las estimativas de pérdidas de suelo tolerables, por lo que se deben tomar medidas que garanticen la sostenibilidad de los usos del suelo. A nivel mundial, se ha constatado que el mulch de paja es el método más efectivo para reducir la erosión del suelo tras un incendio de alta severidad. Recientemente, otros tratamientos han sido testados en eucaliptales y pinares incendiados de Portugal: 1) el mulch de cortezas de eucalipto trituradas; 2) el de restos forestales no triturados (ramas, palos, hojas); 3) el hydromulch, una variante del mulch compuesto por agua, fibras orgánicas, surfactantes, nutrientes y semillas utilizado en la restauración de taludes y canteras; y 4) las poliacrilamidas (PAM), un agente químico utilizado con éxito en la reducción de la erosión en terrenos agrícolas y con alto potencial debido a su baja tasa de aplicación. Los tratamientos más efectivos fueron aquellos que afectaron al factor determinante de la erosión: la cubierta del suelo. Por esta razón el mulch de cortezas de eucalipto trituradas y el hydromulch redujeron la escorrentía a la mitad, y la erosión en un 80-90%, mientras que las poliacrilamidas no tuvieron ningún efecto. Los restos forestales no triturados aplicados en incendios con alguna cubierta sobre el suelo (agujas de pino) no fueron efectivos debido al bajo riesgo erosivo de esas áreas.

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

Assessing the role of pre-fire ground preparation operations and soil water repellency in post-fire runoff and inter-rill erosion by repeated rainfall simulation experiments in Portuguese eucalypt plantations

This study addressed the impacts of contrasting pre-fire ground preparation operations on post-fire runoff and inter-rill erosion in six eucalypt plantations in north-central Portugal, with a special emphasis on the role of soil water repellency in the seasonal patterns of overland flow generation. To this end, a down slope ploughed, a contour ploughed and a terraced site were compared with three unploughed sites. Runoff and erosion data were collected in the field by carrying out rainfall simulation experiments (RSEs) with two intensities (45-50 and 80-85mmh-1) at six occasions during the first one to two years following wildfires in 2005 and 2006.Overall runoff coefficients varied markedly amongst the six study sites and between the two intensities (7 to 55%). While runoff figures were comparable to those of prior RSE studies in recently burnt areas, overall sediment losses were comparatively low (7-155gm-2) but contained a substantial organic matter fraction (29-74%). Apparently, the inter-rill erosion rates were essentially sediment-limited, fitting in with the long history of intensive land use that is typical in the Mediterranean Basin. The hydrological and erosion impacts of the three pre-fire ground preparation operations were minor, probably because these operations took place several years before the latest wildfire. Overall, the two rainfall intensities produced the expected differences but this effect was only statistically significant for simultaneous RSEs. Furthermore, the effect of rainfall intensity varied markedly between the study sites and, occasionally, between the two plot pairs at the same site. This impeded an erosion risk ranking of the six study sites that was consistent for both rainfall intensities. Runoff and erosion rates did not decrease in a simple or pronounced manner with time-since-fire. These temporal patterns could in part be attributed to changes in soil water repellency but other factors were involved as well. Removal of the protective soil cover by litter in particular appeared to play a key role in the increase in sediment losses following logging and wood extraction

Short-term effects of post-fire salvage logging on runoff and soil erosion

Research has been undertaken on the hydrological and erosional impact of forest fires, but remarkably little work has been conducted on salvage logging operations that often follow them. We assessed the effects of mechanical salvage logging following wildfire on soil physical properties, ground cover, and runoff and erosion response on a eucalypt stand in Portugal. We compared two levels of mechanical disturbance, skid trails by a wheeled forwarder tractor (“skid_low”) and skid trails by the same tractor followed by a tracked feller-buncher (“skid_high”) with control conditions (no mechanical disturbance). Three plots (16 m2) by level of soil disturbance were installed after a moderate intensity fire and subsequent logging operations, and monitored during the first post-fire year. In two control and two skid_low plots runoff was also measured. Soil bulk density was higher with increasing mechanical disturbance. Soil compaction reduced porosity 7–16%, but as the control porosity was 70%, the reduced porosity still was 60–65%. Soil resistance to penetration significantly increased for the most disturbed area, whereas soil shear strength was significantly reduced in the intermediate disturbance level. Mechanical disturbance did not induce significant effects on soil moisture or ground cover. Initial greater coverage by bare soil in the disturbed plots compared to the control was compensated by a more rapid vegetation recovery in those plots. Annual runoff amount was not higher at the skid-low than at control plots. Absence of runoff difference was due to high soil porosity of 65%, even after tractor trafficking, and a greater surface roughness on the skid_low plots compared to the control. Sediment production increased with increasing soil disturbance. The mean sediment rate was 1.6–3 times greater for the disturbed than the control plots. This increase could be explained by the higher soil compaction and reduced soil shear strength on the mechanically disturbed plots and initial differences in ground cover. Organic matter content of the eroded sediments was higher on the control than the disturbed plots, due to initial higher ash cover on the control plots. Overall, sediment production was significantly related to rainfall intensity and reduced by vegetation regrowth. Sediment rates tend to decrease with time since the initial fire and logging disturbance, highlighting the importance of a rapid implementation of erosion control measures. Specific management practices are needed to minimize the impacts of logging in burnt soils which are already subject to greater erosion risk and soil degradation.publishe

Assessing the role of pre-fire ground preparation operations and soil water repellency in post-fire runoff and inter-rill erosion by repeated rainfall simulation experiments in Portuguese eucalypt plantations

This study addressed the impacts of contrasting pre-fire ground preparation operations on post-fire runoff and inter-rill erosion in six eucalypt plantations in north-central Portugal, with a special emphasis on the role of soil water repellency in the seasonal patterns of overland flow generation. To this end, a down slope ploughed, a contour ploughed and a terraced site were compared with three unploughed sites. Runoff and erosion data were collected in the field by carrying out rainfall simulation experiments (RSEs) with two intensities (45-50 and 80-85mmh-1) at six occasions during the first one to two years following wildfires in 2005 and 2006.Overall runoff coefficients varied markedly amongst the six study sites and between the two intensities (7 to 55%). While runoff figures were comparable to those of prior RSE studies in recently burnt areas, overall sediment losses were comparatively low (7-155gm-2) but contained a substantial organic matter fraction (29-74%). Apparently, the inter-rill erosion rates were essentially sediment-limited, fitting in with the long history of intensive land use that is typical in the Mediterranean Basin. The hydrological and erosion impacts of the three pre-fire ground preparation operations were minor, probably because these operations took place several years before the latest wildfire. Overall, the two rainfall intensities produced the expected differences but this effect was only statistically significant for simultaneous RSEs. Furthermore, the effect of rainfall intensity varied markedly between the study sites and, occasionally, between the two plot pairs at the same site. This impeded an erosion risk ranking of the six study sites that was consistent for both rainfall intensities. Runoff and erosion rates did not decrease in a simple or pronounced manner with time-since-fire. These temporal patterns could in part be attributed to changes in soil water repellency but other factors were involved as well. Removal of the protective soil cover by litter in particular appeared to play a key role in the increase in sediment losses following logging and wood extraction

Surface and subsurface flow in eucalyptus plantations in north-central Portugal

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