The influence of cascade dams on multifractality of river flow

Abstract

Abstract Using multifractal analysis, we investigated in this study hydrological alterations in São Francisco River basin, Brazil, caused by the construction of a cascade of dams and reservoirs. We applied Multifractal Detrended Fluctuation Analysis (MFDFA) to daily streamflow time series recorded during 1929–2016 at locations upstream and downstream of cascade dams and for periods before and after dam construction. We calculated multifractal spectra f(α) and analyzed the complexity parameters: the position of α0 of the spectrum maximum, which represents the overall Hurst exponent H, the width of the spectrum W that represents the degree of multifractality, and asymmetry parameter r that represents the dominance of small (large) fluctuations. It was found that the changes in the values of complexity parameters obtained from multifractal spectrum of streamflow series after dam construction revealed that after the construction of Sobradinho dam, located in the Sub-Middle São Francisco region, streamflow dynamics shifted towards the regime with uncorrelated increments H∼0.5 and stronger multifractality (larger W, with the dominance of small fluctuations r>1. The superimposed influence of all cascade dams induced the streamflow regime (downstream of all dams in the Lower São Francisco region) with uncorrelated increments H∼0.5 , weaker multifractality (smaller W) and the dominance of large fluctuations (r<1). The novelty of this work is the moving window MF-DFA analysis as a tool to investigate temporal evolution of stream flow multifractality. This approach revealed other (otherwise hidden) aspects of hydrological alterations in São Francisco River such as the influence of Itaparica reservoir for which regularized discharge was adopted to be in synchronization with upstream Sobradinho reservoir. These results provide evidence that multifractal analysis can be used to assess the intricacy of hydrological alterations caused by human activities.