The growing impact of human activities on the environment has increased their influence on the planet’s natural cycles, especially in relation to the hydrological cycle of watersheds. The fundamental processes for its water and energy balance have been affected, which influences water availability and surface streamflow. This study sought to evaluate the anthropogenic impacts on the hydrological cycle of the São Francisco River Basin (SFRB), Brazil, between 1985 and 2015. The study area comprised SFRB and 10 sub-basins for general and specific analyses, respectively. Analyzed data consisted of Land Use and Land Cover (LULC), precipitation, streamflow, and temperature. The methodology incorporated: (i) assessment of LULC dynamics; (ii) trend analysis with the Mann–Kendall method and Sen’s Slope; and (iii) decomposition of total streamflow variation via Budyko’s hypothesis and climate elasticity of streamflow. As a result, it was possible to detect an anthropic modification of SFRB, which is the main component of its streamflow variation, in addition to increased streamflow sensitivity to climate variations. In addition, the divergent behavior in the trends of hydrological variables suggests a change in the streamflow response to precipitation. Therefore, the results allowed us to identify and quantify the impacts of anthropic modifications on the hydrological cycle of the SFRB.

Resumo As projeções futuras divulgadas pelos relatórios do Painel Intergovernamental sobre Mudanças Climáticas, atentam para a ocorrência de variações e mudanças no clima. Tais alterações, tendem a impactar especialmente o ciclo hidrológico global, uma vez que são projetadas modificações nos padrões de precipitação em larga escala e elevação na temperatura média da atmosfera. A Bacia Hidrográfica Estendida do Rio São Francisco (BESF) está localizada entre as regiões Sudeste e Nordeste do Brasil. O índice de Aridez (IA) pode contribuir no planejamento da segurança hídrica de um território. O objetivo deste estudo é analisar a aridez para a BESF através das precipitações e de temperaturas mínimas e máximas, obtidas por intermédio dos modelos climáticos do CMIP6 e do CORDEX. Calculou-se o índice de Aridez (IA) e realizou-se a comparação dos resultados do IA obtidas dos modelos climáticos com os dados de referência da Unidade de Pesquisa Climática (CRU). Ambos modelos, CMIP6 e CORDEX, apresentaram boa performance em representar o IA, quando comparados com o CRU. Os modelos do CMIP6 e CORDEX indicam indicam uma alteração na condição de aridez de na BESF, embora que os dados do CORDEX sejam mais recomendáveis para a BESF, em função de sua resolução espacial.

Climate change in the coming decades could intensify extreme events such as severe droughts. Combined with the possible increase of water demands, these changes exert a great deal of pressure on the water systems. In order to confirm this assumption, a set of scenarios was proposed in this study to consider the combined impact of climate changes and the increase in water demand on the main multiple-use reservoirs of São Francisco River, Brazil. For this purpose, five CMIP6 climate models were used, considering two greenhouse gas emissions scenarios: the SSP2-4.5 and SSP5-8.5. The affluent natural flows and regulated flows were estimated to the adopted reservoirs considering all existing, new and projected demands. The combination of scenarios indicated an increase in Potential Evapotranspiration; possible significant reduction in water availability, with a decrease in precipitation with a magnitude of −15% in the most pessimistic scenarios and a decrease in flows with a magnitude varying between −5% and −40% in the Sobradinho and Três Marias reservoirs; growth in water demand, mainly for irrigation, with annual rates of 6.80%, 7.42%, 10.99% and 9.29% for Itaparica, Sobradinho, Três Marias and Retiro Baixo, respectively; and a substantial reduction in the performance of the evaluated reservoirs, mainly for the Retiro Baixo and Itaparica reservoirs, which showed a high vulnerability index and a low sustainability index.

This study aimed to analyze the variability of average annual streamflow time series of the National Interconnected System (NIS) (Brazil) and create a projection model of future streamflow scenarios from 3 to 10 years using wavelet transform (WT). The streamflow time series were used and divided into two periods, namely, 1931–2005 and 2006–2017, for calibration and verification, respectively. The annual series was standardized, and by the WT, it was decomposed into two bands plus the residue for each base posts (BP) for later reconstruction. Then, an autoregressive (AR) model per band and residue was made. The projection was obtained by adding the AR models. For performance evaluation, a qualitative analysis of the cumulative probability distribution of the projected years and an analysis of the likelihood were performed. The model identified the probability distribution function of the projected years and obtained a likelihood ratio of > 1 in most SIN regions, indicating that this methodology can capture the mediumrange variability.

Climate change in the coming decades could intensify extreme events such as severe droughts. Combined with the possible increase of water demands, these changes exert a large pressure on the water systems. In order to confirm this assumption, a set of scenarios was proposed in this study to consider the combined impact of climate changes and the increase in water demand on the main multiple-use reservoirs of São Francisco River, Brazil. For this purpose, five CMIP6 climate models were used considering two greenhouse gas emissions scenarios: the SSP2-4.5 and SSP5-8.5. The naturalized and withdraw streamflows were estimated to the adopted reser-voirs considering all existing, new and projected demands. The conjunction of scenarios indicat-ed an increase in Potential Evapotranspiration, a possible significant reduction in water availa-bility, a growth in water demand mainly for irrigation and a substantial reduction in the perfor-mance of the evaluated reservoirs.

The present study addressed the effects of water scarcity on the deterioration of water quality in 155 reservoirs located in the state of Ceará, Brazil, using the drought index SPI (standardized precipitation index) calculated for the accumulation time scales of 3, 6, and 12 months. Based on this, a comparison was made with the variations in the trophic state index of these reservoirs, between 2008 and 2021. The study pointed to the occurrence of an intense series of dry events between 2012 and 2018, highlighting the year 2013, when the SPI assumed a value of -2.43 identifying an extremely dry year. Also, during this period, the amount of water stored by the state's reservoirs achieved only 8% of the total storage capacity, with the existence of eutrophic and hyper-eutrophic dams accounting for 68% of the total in 2016. In this context, it was observed that, during the transition from a dry to a wet period, the average trophic state index of the reservoirs tended to increase, rising from 62.1 (2008–2012) to 65.1 (2013–2017), before decreasing to 61.5 with the return of the wet period (2018–2021). In addition, to assess the influence of human activities on water quality, land use and occupation data from three basins in Ceará were analyzed. It was observed that the basin with the largest area occupied by agriculture experienced the most significant increases in total phosphorus and chlorophyll-a concentrations.