Carlos Eduardo Sousa Lima

Forecasts of hydrological variables are extremely important for water resource management in regions that are particularly vulnerable to the effects of climate change and climate variability. The aim of this study was to develop a hybrid neural network capable of producing daily streamflow forecasts on a sub-seasonal scale. This hybrid neural network consists of an artificial neural network with inputs preprocessed by the wavelet transform (WANN). The WANN was tested in the Três Marias, Sobradinho, and Retiro Baixo reservoirs, located in the São Francisco River Basin (SFRB). The obtained results show that WANN was highly accurate in short-term forecasts (7–28 days); however, for long-term forecasts (35 and 42 days), there was a significant drop in performance, especially during the transition periods to the rainy season and in the dry months. The comparison between the performance metrics of the WANN forecasts and the National Electricity System Operator (ONS) operational models for the Três Marias, Sobradinho, and Retiro Baixo basins showed that WANN outperformed all these models. The results obtained show that WANN is a valuable tool for addressing the complex and dynamic challenges of hydrology, making it essential for decision-making on water resource management.

Study RegionGrande River Basin (GRB), Brazil, an important tributary of the São Francisco RiverStudy FocusEvaluate the impact of anthropogenic changes on the onset, development and recovery of hydrological droughts, considering different drought states and their transition probabilitiesNew hydrological insightsA new data-based method was proposed to evaluate the impact of anthropogenic changes on the evolution of hydrological drought called “Drought Roulette Analysis” (DRA). By applying the DRA in the GRB, it was possible to identify significantly slower hydrological recovery during the human-modified period, with increases in the probabilities of transitioning from hydrometeorological to hydrological drought (+5.68%) and of remaining in the hydrological drought state (+22.2%). The DRA provided greater flexibility for drought-propagation analysis in human-modified basins.

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.

The occurrence of subseasonal phenomena, such as dry spells and flash droughts, are potentially detrimental to the agricultural sector and water resources. The impacts on agricultural sector have been well explored by the technical-scientific community, attesting to the importance of subseasonal variability for this sector, especially for low-tech farming areas. However, when it comes to water resources, there is a large gap regarding the hydrological impacts of these phenomena, in a way that pertinent questions about the importance of subseasonal variability for water resources management remain still unexplored or poorly understood. This study evaluated the impacts of dry spells on reservoir inflow in semiarid regions, taking the Orós reservoir (Northeast Brazil) as a case study. The relationship between the dry spell index and the inflow of Orós reservoir during the rainy season was analyzed, as well as the daily time series of precipitation and storage volume during two long-term dry spells. The two formulated hypotheses suggest that dry spells are indeed capable of reducing the reservoir inflow efficiency in semiarid regions during rainy seasons. These hypotheses serve as a starting point for a deeper discussion on the impacts of dry spells on water resource management in arid and semiarid regions.

Brazil has a heterogeneous climate throughout its territory which makes it difficult to outline a single climate risk management strategy. However, this unification can occur within sub-regions that present homogeneous climatic risk. In this context, the aim of this study was to delineate homogeneous climate risk zones in Brazil. For this we used monthly precipitation data for the Brazilian territory provided by the Global Precipitation Climatology Center. These time series were standardized using the Standard Precipitation Index and then their homogeneous clusters were defined using the K-means method. In addition, the joint variability patterns between the clusters’ centroid time series and high- and low-frequency climate variability modes were evaluated using Cross-Wavelet Analysis. The analyses carried out enabled the delineation of 7 homogeneous climate risk zones in Brazil, which showed different variability patterns and were modulated differently by the climate variability modes assessed.

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.

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 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.

PROJEÇÕES DOS IMPACTOS DAS MUDANÇAS CLIMÁTICAS SOBRE AS BACIAS DO HIDROSSISTEMA JAGUARIBE, CEARÁ, BRASIL, UTILIZANDO INFORMAÇÕES DO CORDEX DO MODELO CLIMÁTICO REGIONAL RCA4