Long-Term Correlations in São Francisco River Flow: The Influence of Sobradinho Dam

Abstract In this work we study the influence of the Sobradinho dam construction on daily streamflow of São Francisco River, Brasil, by analyzing long-range correlations in magnitude and sign time series obtained from streamflow anomalies, using the Detrended Fluctuation Analysis (DFA) method. The magnitude series relates to the nonlinear properties of the original time series, while the sign series relates to the linear properties. The streamflow data recorded during the period 1929-2009, were divided in the periods pre-construction (1929 to 1972) and post-construction (1980 to 2009) of Sobradinho dam and analyzed for small scales (less than 1 year) and for large scales (more than 1 year). In post-construction of Sobradinho dam, DFA-exponents of magnitude series increased at small scales (0.895 to 1.013) and at large scales (0.371 to 0.619) indicating that the memory associated with nonlinear components becames stronger. For sign series, the DFA-exponent increased at small scales (0.596 to 0.692) indicating stronger persistence of flow increments direction, and decreased at large scales (0.381 to 0.259) indicating stronger anti-persistence (positive increments are more likely to be followed by negative increments and vice versa). These results provide new evidence on the hydrological changes in the São Francisco River caused by human activities.摘要：本研究针对巴西圣弗朗西斯科河（São Francisco River）的索布拉迪尼奥大坝（Sobradinho Dam）建设对每日径流的影响展开研究，通过采用去趋势波动分析（Detrended Fluctuation Analysis, DFA）方法，对径流异常值衍生的幅值与符号时间序列的长程相关性进行分析。其中，幅值序列关联于原始时间序列的非线性特性，而符号序列则对应其线性特性。研究采用1929—2009年的径流观测数据，将其划分为索布拉迪尼奥大坝建设前（1929年至1972年）与建设后（1980年至2009年）两个时段，并分别在小尺度（小于1年）与大尺度（大于1年）下开展分析。结果显示，在大坝建成后，幅值序列的DFA指数在小尺度下从0.895升至1.013，大尺度下从0.371升至0.619，表明非线性分量相关的记忆性显著增强。对于符号序列，其DFA指数在小尺度下从0.596升至0.692，意味着径流增量方向的持续性更强；而在大尺度下则从0.381降至0.259，表明反持续性显著提升（即正增量之后更大概率伴随负增量，反之亦然）。上述研究结果为人类活动引发的圣弗朗西斯科河水文变化提供了新的实证依据。