Hydrological changes caused by the construction of dams and reservoirs: The CECP analysis

We investigated the influence of the construction of cascade dams and reservoirs on the predictability and complexity of the streamflow of the Sao Francisco River, Brazil, by using complexity entropy causality plane (CECP) in its standard and weighted form. We analyzed daily streamflow time series recorded in three fluviometric stations: Sao Francisco (upstream of cascade dams), Juazeiro (downstream of Sobradinho dam), and Pao de Acucar station (downstream of Sobradinho and Xingo dams). By comparing the values of CECP information quantifiers (permutation entropy and statistical complexity) for the periods before and after the construction of Sobradinho (1979) and Xingo (1994) dams, we found that the reservoirs' operations changed the temporal variability of streamflow series toward the less predictable regime as indicated by higher entropy (lower complexity) values. Weighted CECP provides some finer details in the predictability of streamflow due to the inclusion of amplitude information in the probability distribution of ordinal patterns. The time evolution of streamflow predictability was analyzed by applying CECP in 2 year sliding windows that revealed the influence of the Paulo Alfonso complex (located between Sobradinho and Xingo dams), construction of which started in the 1950s and was identified through the increased streamflow entropy in the downstream Pao de Acucar station. The other streamflow alteration unrelated to the construction of the two largest dams was identified in the upstream unimpacted Sao Francisco station, as an increase in the entropy around 1960s, indicating that some natural factors could also play a role in the decreased predictability of streamflow dynamics.