CONCLUSIONS

In this study, we were driven by the question “are topographic catchments isolated in a way to be defined as a single management unit?”. In order to investigate it, we assessed the effective area of 733 Brazilian catchments and the deviation from their respective topographic area by adopting the effective catchment index (ECI). Our findings indicated that 31% of the studied catchments have a significant mismatch between their topographic and effective areas (i.e., effective areas either smaller than half or larger than double of their topographic areas). We also inferred the hydrological connectivity through flow-process to better understand the groundwater flow and runoff patterns.
We evaluated the ECI estimates by contrasting our results against the expected range of the Budyko curve considering both topographic and effective catchment areas (classic and adjusted framework). Most catchments were between the theoretical water and energy limits in the Budyko space for both classic and adjusted Budyko frameworks. We found that catchments better fitted the Budyko curve when considering the effective area (the adjusted Budyko Framework). Additionally, catchments that gain water are closer to the upper water limit while catchments that lose water are mostly concentrated below the Fu curve, and an opposite behavior was observed on the classic framework.
The ECI values are consistent with the biome’s climate dynamics as indicated in the classic Budyko framework. We observed a clear signal of water gain (ECI > 0) in catchments located in the Atlantic Forest biome (humid tropical zone) and water loss (ECI < 0) in the Cerrado and Caatinga biomes (Savannah and semi-arid zones), which present higher evaporative index. A lack of a clear pattern in the other biomes may be due to the low density of gauged catchments. We also identified the aridity index, mean slope, seasonality, and mean elevation as the four most influencing catchment attributes on the ECI. The aridity index showed the most influence in the ECI, increasing as the ECI decreases, indicating that smaller effective areas are found in the aridest biomes. Besides, ECI was positively correlated to mean slope and mean elevation supporting the highest presence of catchments with larger effective areas in the Atlantic Forest biome, a region characterized mostly by a mountainous landscape.
The use of the ECI index allows us to infer from catchment connectivity; therefore, it should be considered in future hydrologic studies at a catchment scale, water-related services, and water resources management. Here, we highlight the potential of pooling of catchments based on their interconnectivity (gaining or losing water potential given by ECI) and topographic location and extension. According to this concept, catchments can be grouped by their common interests, minimizing management cost while maximizing synergies and lessen trade-offs of water transfer processes.
Our results contribute to a better country-scale understanding of hydrological connectivity among catchments to overcome regional challenges related to unbalanced water availability and demand. Our contributions are the first steps towards more robust water, food, energy, and ecosystem services management for making Brazilian water security more resilient to climate variability.
ACKNOWLEDGEMENTS  
The authors acknowledge the Graduate Program in Hydraulic Engineering and Sanitation – PPGSHS (USP-EESC), the Graduate Program in Civil and Environmental Engineering - PPGCA (UNESP) and Graduate Program in Environmental Technologies – PPGTA (UFMS-FAENG) for the scientific support. This study was supported by grants from the Ministry of Science, Technology, and Innovation (MCTI) and National Council for Scientific and Technological Development (CNPq) (grants numbers 441289/2017-7 and 309752/2020-5) and São Paulo Research Support Foundation (FAPESP, grants numbers 2015/03806-1 and 2019/24292-7). This study was also financed in part by the Coordination for the Improvement of Higher Education Personnel (CAPES) - Finance Code 001 and CAPES PrInt.