Avaliação da retenção de água em terraços na bacia hidrográfica do Rio São Francisco, em Minas Gerais

Abstract

In Brazil, the water erosion is considered the most important cause of degradation of agricultural lands, doing expressive social and environmental harms to society. The soil erosion reduces the productive capacity of lands, increasing the production costs and, consequentemente, a reduction in the profit gotten by the farmers. Although being one old practice, the terracing still presents relative difficulties to its use, being its efficiency dependent of the correct sizing of the space between terraces, the blade of draining, its transversal section and mainly of the difficulty in keeping its constructive uniformity. The objective of this work was to evaluate the water holding capacity in terraces located in level, as for its volumetric capacity of necessary and effective storage. Terraces type Nichols were studied, of average base, constructed in two distinct areas in sub-basins of the North of Minas Gerais in Icaraí de Minas, Pintópolis and Ubaí-MG cities, in the scope of the Program of Revitalization of Watersheds of the São Francisco River. It was determined in the medium curve of each terracing system, the average of four transversal sections of the terrace, that associated to its length it was gotten its volume of real storage. By means of the section gotten in the quota lowest of the crest of the ridge, combined with the quota lowest of its extremity, its volume of effective storage was determined. From the results gotten through the relation between the effective and necessary volume, it was possible to observe that for Icaraí de Minas area 01 and 02, the efficiency varied from 76.4 to 44.8%; for Pintópolis, from 9.0 to 63.5% of the volumetric potential of water holding in the terraces, not being possible to evaluate the efficiency for the terraces of Ubaí city. These results evidence troubles in the efficiency of terraces built by the Program of Revitalization, for the North of Minas Gerais conditions, in the soil conservation and water holding.