Development and application of a P Index for Sete Cidades watershed in São Miguel Island

C. Neves¹, J. Pinheiro, L. Matos & J. Madruga

RESUMO

A Bacia Hidrográfica da Lagoa das Sete Cidades localiza-se no maciço com o mesmo nome, na região Nordeste da Ilha de São Miguel. Possui uma área de 445,5 ha e é constituída por duas sub-unidades, a Lagoa Azul (357,3 ha) e a Lagoa Verde (88,2 ha). A maior parte da bacia hidrográfica é ocupada principalmente por pasta-gens permanentes. Devido a problemas ligados ao uso agrícola desta bacia (carga animal e uso excessivo de adubo) a sua situação trófica tem vindo gradualmente a deteriorar-se. O índice de P (Lemunyon & Gilbert, 1993) foi criado para identificar especificamente as áreas onde é maior o risco de perda de fósforo do solo para as águas superficiais numa bacia hidrográfica. A sua aplicação permite referenciar no terreno as fontes difusas de poluição, uma vez que se baseia na avaliação por parcela dos factores (de origem e de transporte) considerados relevantes para a mobilização de nutrientes para a massa de água. Neste trabalho foram considerados como factores de origem os valores de P Olsen e as quantidades de adubo aplicadas (kg P2O5 ha^-1 ano^-1) e como factores de transporte o potencial de escoamento (estimado a partir do declive), distância da parcela à linha de água e distância da parcela à massa de água (m). Com a adopção desta metodologia pretendeu-se identificar possíveis áreas de risco de perda de P e assim contribuir para o desenvolvimento de programas e/ou de planos de gestão adequados à bacia hidrográfica das Sete Cidades.

ABSTRACT

The Sete Cidades catchment is located in the geological site with the same name, in the Northeast of the São Miguel Island. It has an 444,5 ha area and is formed by two sub units, the Blue Lagoon (357,3 ha) and the Green Lagoon (88,2 ha). The most part of the watershed is occupied by permanent pastures. Due to agriculture land use (animal pressure and excessive fertilization), its trophic condition is increasingly deteriorating. The P index was created by Lemunyon & Gilbert (1993) to identify the areas which most contributes to phosphorus losses from soil to the water bodies. Its application helps locating diffuse polluting sources, since it is based on the evaluation on a parcel basis of source and transports factors considered as relevant to the nutrient mobilization to the water mass. In this study are considered as source factors the P Olsen and applied fertilizer (kg P2O5 ha^-1 year^-1), and as transport factors the leaching potential (as estimated from slope), distance from field to the water line and from field to the water body (m). With this methodology we intended to identify the risk of P loss areas and thus contribute to the development of programs and/or management plans addressed to the Sete Cidades watershed.

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