Subsídios à determinação de vazões ambientais em cursos d’água não regulados: o caso do ribeirão Pipiripau (DF/GO)

Abstract

In river basins with strong urban and agricultural development, such as the highlands of Central Brazil, high water demands and extraction rates can lead to unwanted environmental and social impacts, particularly during low flow periods. Although the Brazilian legislation has provisions for the establishment of minimum flows in rivers, they are often underestimated, and therefore unable to provide the necessary ecosystem services and other instream values. The objective of this work was develop a suitable methodology for the establishment of environmental flows and water management rules for unregulated rivers, and apply it to a river in the Federal District. The method chosen was a modified version of New South Wales’ (Australia) Macro Water Sharing Plans – MWSP (NSW, 2006; Harris et al., 2006). The modification was required because the Brazilian legislation does not allow for water trading, and because some of the indicator data are not available. Therefore, an environmental flow potential (PVA) for a given management unit (river, reach, etc.) was defined as the product of 3 indicators: i) the hydrologic risk (Eh); ii) the instream value (Vec); iii) the economic dependence (De). Each indicator has 3 levels (low, medium, and high, corresponding to the scores 1, 2 e 3), and a number of different parameters. The environmental flow potencial (PVA) is divided into 3 classes (low, medium, and high), based on the scores of the indicador product. The higher the value of PVA, the higher the restrictions to access to water (existing and future licenses), and the higher the need for compensation mechanisms for optimizing water-use. We illustrate usefulness of the method by discussing its application to Pipiripau river. Two expert panels were asked to calculate the indicators, and the final score of (PVA) has medium, requiring the minimum flow in the river to be increased by 25%. Results of this work show that the method is useful in establishing preliminary environmental flows in sensitive areas, and as a tool for water planners, water licensing agencies, and river basin commitees.