Identification of the degree of impact of dams on the hydrologic regime of semi-arid rivers: an evaluation of the DHRAM method

Abstract

The flow regulation that results from the implantation of dams causes consequences to the river ecosystems due to the modification on the characteristics of the hydrologic regime. The investigation of these changes become relevant, mainly in semi-arid regions where there is a great amount of these hydraulic structures and lack of such analyzes. Considering the above, this paper aims to evaluate the Dundee Hydrological Regime Alteration Method (DHRAM) through the classification of the degree of impact of dams located on rivers Itapicuru, Paraguaçu and their tributaries, verifying the adequacy of its use to represent the semi-arid hydrologic regime. Thereby, the DHRAM was applied in three versions: considering the thresholds that define the scores to classify the degree of impact in its original set (accordingly to Black et al. (2005)); with the adjustment of those thresholds to local conditions; and, with the regrouping of variables and adjustment of thresholds. The results showed that the method in its original set is applicable to semi-arid rivers, however it tends to be very restrictive against the high natural hydrologic variability characteristic of these rivers, and it ends up pointing to a high degree of alteration for dams that are known for not causing a very siginifcant flow regulation. The DHRAM with the regrouping of variables and the adjustment of thresholds presented the classification that approached the most to the known characteristics of the studied dams, being useful for the evaluation of the impact of dams still in project, and also to guide the adoption of operating rules that minimize the most significant hydrologic alterations that are identified.Keywords: Hydrologic indicators; Classification of the degree of impact; Semi-arid. RESUMOA regulação de vazões que ocorre a partir da implantação de barragens provoca consequências sobre ecossistemas fluviais devido às modificações nas características do regime hidrológico. A investigação dessas modificações ganha relevância, principalmente em regiões de semiárido onde há grande presença dessa obra hidráulica e carência de tais análises. Diante disso, o presente artigo teve como objetivo avaliar o Dundee Hydrological Regime Alteration Method (DHRAM) a partir da classificação do grau de impactos de barragens localizadas nos rios Itapicuru, Paraguaçu e seus afluentes, verificando a adequação de seu uso para representação de regime hidrológico de semiárido. Aplicou-se, então, o DHRAM em três versões: considerando os limiares que definem a pontuação para a classificação do grau de impacto na sua forma original (conforme Black et al. (2005)); com ajuste desses limiares às condições locais; e com reagrupamento das variáveis e ajuste nos limiares. Os resultados mostraram que o método em sua forma original é aplicável para rios de semiárido, entretanto tende a ser muito restritivo frente a alta variabilidade hidrológica natural característica desses rios, e acaba por apontar um grau de alteração elevado para barragens reconhecidas como não regular...