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Brazil's Northeast region (BRN), especially the state of Ceará (CE), has dealt historically with severe drought events since the late 1800s, which commonly led to catastrophic impacts of mass migration and deaths of thousands of people. Throughout the last century, the "Droughts Polygon" region experienced an intense infrastructural development, with the expansion of a dense network of reservoirs. This paper presents a parsimonious hydrologic modeling approach to investigate the 100‐year (1920–2020) evolution of the hydrology of the 24,500 km2 Upper Jaguaribe Basin, throughout the development of a dense reservoir network. We aimed at reproducing the hydrology at the basin scale and analyzed the outcomes of reservoir expansion in terms of water fluxes and water security. Our model's structure captured the growth in reservoir count and storage capacity, which was then confronted with an evolving water demand, allowing us to estimate how water security (i.e., proportion of demand being met) varied over the 100‐year period. Significant streamflow reduction at the basin's outlet and increase in evaporation losses, associated with a decrease in streamflow at varying exceedance frequencies were observed at the end of the study period. While reservoir expansion allowed for the transition from complete vulnerability to meteorological droughts to increased levels of water security, drought impacts had, in the meantime, disproportionally intensified, especially in reservoirs of medium to small capacities. Smaller reservoirs are suggested to have played the role of distributing water resources throughout the region, while larger reservoirs were more efficient as tools to promote water security. Plain Language Summary: Brazil's Northeast region is known to suffer from severe droughts since colonial times. The state of Ceará is perhaps the best representation of how hydraulic interventions were used in the last century to mitigate water scarcity in Brazil's semi‐arid. Recently, the debate on how widespread construction of reservoirs can lead to unintended (negative) consequences in terms of water provisioning has been the brought to the attention of the scientific community. Studies to quantitatively characterize such phenomena are thus necessary to create basic understanding on how humans interact with water systems, with the potential of promoting more sustainable water management practices. In this study, we investigate a large region in the state of Ceará that experienced a dramatic increase in storage capacity through the last century, spurred by the construction of dams of varying sizes led by both local population as well as funded by the authorities. Our study tries to characterize how this complex system has evolved over time in terms of the positive and negative effects reservoir expansion might have brought to humans living in that region. Key Points: A hydrologic model with an evolving structure was developed to capture 100 years of reservoir expansion in a large semi‐arid basinDrought intensity and duration evolved differently through the system, with smaller reservoirs becoming more affected over timeWater retention in the basin reduced streamflow at the basin outlet and increased evaporation losses [ABSTRACT FROM AUTHOR]

Peninsular India is a unique region with major mountain ranges that govern regional atmospheric circulation and precipitation variability, the monsoons, and regional geology at range of timescales and process scales. However, the landscape and climatic feature controls on streamflow variability at a regional scale using flow duration curves (FDCs) – compact descriptions of streamflow variability that offer a window into the multiple, interacting processes that contribute to streamflow variability – have received little attention. This study examines the suitability of the partitioning of (1) an annual streamflow FDC into seasonal FDCs and (2) a total streamflow FDC into fast- and slow-flow FDCs to unravel the process controls on FDCs at a regional scale, with application to low-gradient rivers flowing east from the Western Ghats in Peninsular India. The results indicate that bimodal rainfall seasonality and subsurface gradients explain the higher contribution of slow flow to total flow across the north–south gradient of the region. Shapes of fast and slow FDCs are controlled by recession parameters, revealing the role of climate seasonality and geological profiles, respectively. Systematic spatial variation across the north–south gradient is observed, highlighting the importance of the coherent functioning of landscape–hydroclimate settings in imparting a distinct signature of streamflow variability. The framework is useful to discover the role of time and process controls on streamflow variability in a region with seasonal hydro-climatology and hydro-geological gradients. [ABSTRACT FROM AUTHOR]

Monitoring Earth's terrestrial water conditions is critically important to many hydrological applications such as global food production; assessing water resources sustainability; and flood, drought, and climate change prediction. These needs have motivated the development of pilot monitoring and prediction systems for terrestrial hydrologic and vegetative states, but to date only at the rather coarse spatial resolutions (∼10–100 km) over continental to global domains. Adequately addressing critical water cycle science questions and applications requires systems that are implemented globally at much higher resolutions, on the order of 1 km, resolutions referred to as hyperresolution in the context of global land surface models. This opinion paper sets forth the needs and benefits for a system that would monitor and predict the Earth's terrestrial water, energy, and biogeochemical cycles. We discuss six major challenges in developing a system: improved representation of surface-subsurface interactions due to fine-scale topography and vegetation; improved representation of land-atmospheric interactions and resulting spatial information on soil moisture and evapotranspiration; inclusion of water quality as part of the biogeochemical cycle; representation of human impacts from water management; utilizing massively parallel computer systems and recent computational advances in solving hyperresolution models that will have up to 109 unknowns; and developing the required in situ and remote sensing global data sets. We deem the development of a global hyperresolution model for monitoring the terrestrial water, energy, and biogeochemical cycles a “grand challenge” to the community, and we call upon the international hydrologic community and the hydrological science support infrastructure to endorse the effort.

As each catchment responds uniquely, even if they appear similar, formulating generalizable hypotheses and using routinely used signatures of catchment similarity to examine streamflow variability can be difficult. Flow Duration Curve (FDC), a concise portrayal of streamflow variability at a specific gauging station, can provide insights into hydroclimatic and landscape processes occurring at a wide range of space and time scales that govern flow regimes in a region. This study explores the suitability of partitioning of annual streamflow FDC into seasonal FDCs, and total streamflow FDC into fast and slow flow FDCs to unravel the process controls on FDCs at a regional scale, with application to low-gradient rivers flowing east from the Western Ghats of Peninsular India. The focus is on investigation of the controls of common regional landscape features (in space) and seasonal climatic (in time) variations on regional variations of the FDC. Findings of the study indicate that bimodal rainfall seasonality and higher fraction of moderate to good groundwater potential zones explains the higher contribution of slow flow to total flow across north-south gradient of the region. Shapes of fast and slow FDCs are controlled by recession parameters revealing the role of climate seasonality and geologic profiles, respectively. A systematic spatial variation across north-south gradient is observed– highlighting the importance of coherent functioning of landscape-hydroclimate settings in imparting distinct signature of streamflow variability. The framework is useful to discover the role of time and process controls on streamflow variability in a region with seasonal hydroclimatology and hydro-geologic gradients. [ABSTRACT FROM AUTHOR]

Water reallocation decision-making is a challenge faced by most river basins around the world. In this study, a system thinking framework was developed to structurally unfold the complex interactions of water reallocations with societal, economic and ecological subsystems in the Heihe River Basin in China. The results indicate that ecological degradations appeared much later than economic development. Slow-changing societal values and limited considerations of technological development and government regulations towards environmental protection contributed to the weak and untimely responses of water reallocations to ecological degradation. This framework can assist in strategic water reallocation decision-making in river basins. [ABSTRACT FROM AUTHOR]

While conflict-and-cooperation phenomena in transboundary basins have been widely studied, much less work has been devoted to representing the process interactions in a quantitative way. This paper identifies the main factors in the riparian countries' willingness to cooperate in the Eastern Nile River basin, involving Ethiopia, Sudan, and Egypt, from 1983 to 2016. We propose a quantitative model of the willingness to cooperate at the national and river basin scales. Our results suggest that relative political stability and foreign direct investment can explain Ethiopia's decreasing willingness to cooperate between 2009 and 2016. Further, we show that the 2008 food crisis may account for Sudan recovering its willingness to cooperate with Ethiopia. Long-term lack of trust among the riparian countries may have reduced basin-wide cooperation. While the proposed model has some limitations regarding model assumptions and parameters, it does provide a quantitative representation of the evolution of cooperation pathways among the riparian countries, which can be used to explore the effects of changes in future dam operation and other management decisions on the emergence of conflict and cooperation in the basin. [ABSTRACT FROM AUTHOR]