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- Temporal changes on the effect of rock fragments in interrill soil loss: a simulation experiment and a simple descriptive modelPublication . Figueiredo, Tomás de; Ferreira, Alfredo Gonçalves; Gonçalves, Dionísio; Poesen, JeanSoils with rock fragments have been studied under several aspects in the last years. Research shows that a single pattern in the erosional response of these soils to rainfalls is not always found. In order to contribute to the understanding of this topic, an experiment was carried out, simulating interrill areas covered by rock fragments. This paper specifically aims at presenting and discussing temporal changes on sediment exported from such areas, also introducing a simple descriptive model to represent soil loss temporal evolution. Small bottom perforated boxes, 612 cm2 area, were filled with a silt-loam fine earth, very poor in organic matter, covered with simulated rock fragments and leaned at 10% slope gradient. The experiment comprised the exposure to 240 mm natural rainfall of 48 boxes corresponding to selected combinations, 4 replicates each, of rock fragments cover (0, 17, 30 and 66%), size (2, 4 and 10 cm), form (rectangular and circular) and position (resting on top and embedded). During the experiment boxes were kept under near saturation soil water conditions. Water and soil exported from the boxes as infiltration, runoff, wash and splash were measured after each period of precipitation. Recorded values of soil loss plotted against precipitation, both expressed in cumulative terms, follow a sigmoid curve. This pattern of response was interpreted as a result of crust formation on soil surface exposed to rainfalls, a hypothesis suggested by observations during the experiment and confirmed at its end. Parameters of this model were related with rock cover and characteristics. The effect of rock fragments on soil loss varies with time, a conclusion that must be taken into account when interpreting either results from experiments with different durations or the evolution of stoniness on eroding surfaces. Os solos pedregosos vêm sendo estudados sob diversos aspectos nos últimos anos. No que respeita à perda de solo, os resultados publicados mostram todavia a dificuldade em estabelecer um padrão único de resposta destes solos às precipitações erosivas. Com vista a aprofundar conhecimentos sobre este tópico, foi instalado um ensaio experimental, simulando superfícies com variável pedregosidade sujeitas a erosão interssulcos. Constitui objectivo deste trabalho apresentar e discutir a evolução temporal da perda de solo nessas superfícies, propondo a sua representação num modelo descritivo simples. O ensaio compreendeu a exposição a 240 mm de chuva natural de um conjunto de tabuleiros com 612 cm 2 de área e 10% de declive, contendo terra fina franco-limosa, muito pobre em matéria orgânica, coberta por elementos grosseiros simulados. Os tabuleiros mantiveram-se próximo da saturação de água. Para além do solo nu, testaram-se tratamentos com 4 repetições cada, correspondendo a combinações específicas de 3 fracções de cobertura (17, 30 e 66%), 3 dimensões (2, 4 e 10 cm), 2 formas (rectangulares e circulares) e 3 posições (pousados à superfície, semi-aflorantes e aflorantes). A infiltração e o escoamento, e as perdas de solo neste e por salpico, foram medidas ao longo do ensaio, na sequência de períodos de precipitação. A perda de solo acumulada representada em função da precipitação acumulada ao longo do ensaio segue uma curva sigmóide. Este modelo de resposta foi interpretado como resultando da formação da crosta superficial do solo exposto, hipótese sugerida pela observação no decorrer do ensaio e confirmada no final. Os parâmetros da curva sigmóide correlacionaram-se com a fracção de cobertura, tendo sido também explorada a relação com outros parâmetros descritivos da pedregosidade. A conclusão de que a relação entre perda de solo e pedregosidade é temporalmente variável, traz consequências para a interpretação quer de resultados de ensaios com diferente duração, quer da evolução temporal da pedregosidade em superfícies erodidas.
- Runoff and soil loss from steep sloping vineyards in the Douro Valley, Portugal: rates and factorsPublication . Figueiredo, Tomás de; Poesen, Jean; Ferreira, Alfredo Gonçalves; Gonçalves, DionísioThe Douro Region,, NE Portugal, where the grapes for Port Wine are produced, faces a high potential erosion risk due to its natural setting, for long tackled with heavy labour inputs. These allowed the stabilization of steep vineyard covered hill-slopes by means of traditional terracing, manually built and following models that changed through time. Alternative vineyard installation and cultivation techniques were tested and progressively adopted, namely row plantation perpendicular to the contour, nowadays an integral yet small part of the plantation schemes accepted under the rules of the World Heritage status, granted by UNESCO in 2001. This paper investigates the effects of vine plantation schemes on long-term (10 years) runoff and soil loss data recorded in the Douro Region using a set of meso-scale erosion plots installed in vineyards planted in rows perpendicular to the contour. It also aims at identifying, and quantitatively deriving the significance of erosion factors helping to interpret the results obtained. Runoff and soil loss were measured between 1978 and 1988 using five runoff plots, 32.1 m long and 5.2 m wide, on a 45 % slope, over a silt loam soil containing 60 % rock fragments by mass, under Mediterranean climatic regime with less than 600 mm mean annual rainfall. Vine plantation schemes tested correspond to three different plant densities (3571, 4808, 5917 plants ha-1) on paired plots (except for the intermediate plant density, tested in a single plot), which provided a mean vegetation cover of 50, 60 and 75 %. The 10 year data analyzed correspond to 167 runoff events. Mean annual runoff and soil loss, computed for the 10 years of records, were 22 mm and 361 kg ha-1 respectively. These relatively low soil loss rates are explained under the prevailing conditions of rainfall erosivity, soil, topography and vegetation cover, by the very high rock fragment cover, protecting the topsoil from wash by runoff. Soil loss was very much concentrated in a few erosive events, 3 of them being responsible for ca.75 % of the total soil loss recorded in 10 years. These events occurred in spring and summer, whereas most erosive events in Mediterranean conditions occur in autumn and winter. Results also show that increasing the vine plant density by 66 % resulted in a reduction of mean annual runoff by 19 % and of mean annual soil loss by 57 %, demonstrating the significant effect of this erosion control measure in permanent crops such as vineyards.
- Efeito das características dos elementos grosseiros na erosão inter-sulcos: um contributo para a sua modelaçãoPublication . Figueiredo, Tomás de; Ferreira, Alfredo Gonçalves; Gonçalves, Dionísio; Poesen, JeanO papel da pedregosidade tem vindo a merecer, nos últimos anos, crescente interesse no quadro dos estudos sobre erosão dos solos. Se por um lado é reconhecida a influencia das características dos elementos grosseiros na perda de solo, a incorporação desse efeito na modelação dos processos erosivos em solos pedregosos é ainda incipiente. Com o objectivo de testar o efeito de várias características dos elementos grosseiros na erosão inter-sulcos, foi conduzido um ensaio experimental, à micro-escala. Os resultados do ensaio permitiram calibrar modelos explicativos da influência dessas características na perda de solo por salpico e por escoamento difuso.