INTRODUCTION

Brazil has one of the largest world’s freshwater reserves (ANA, 2019); however, water availability across the country is poorly distributed leading to regions with scarcity and others with relative abundance (Oliveira, Lucas, Godoi, & Wendland, 2021). Due to its continental proportions, Brazil has different climatic conditions that affect water availability. In addition, the availability is highly affected by prolonged droughts, increasing irrigated areas, agricultural expansion, industrial demand, and population growth (Gesualdo et al. , 2021; Mello et al. , 2020). That critical situation is expected to deteriorate once Brazilian water consumption is expected to increase by 24% in the next 30 years (Val et al. , 2019). In turn, agriculture will require more water to meet the projected increase in food demand of 40% (OECD/FAO, 2015). Therefore, there is an urgent need to expand water supply capacity (Mello et al. , 2020) since Brazil plays an important role in the world’s food supply and the sector represents 80% of the total water consumption in the country (Gesualdoet al., 2021).
The topographic catchment area is the unit for implementation of the Brazilian water resources policy as most of the world’s water resources management systems. Over this territorial unit, environmental, social, and economic studies are carried out to develop a relevant and consistent management plan for present and future interests. The national Water Law (from Portuguese Lei das Águas No. 9,433/97) provides orientations for committees composition and the development of water resources management plans, lists the responsibility of public authorities, and standardizes fines and penalties following international guidelines (Veiga & Magrini, 2013; Araújo et al., 2015). Nevertheless, there is still a long journey to completely implement all the instruments (e.g., classification of water bodies, issue of water permits, and charge for the use of water to grant the multiple uses of water). Nonetheless, are topographic catchments isolated in a way to be defined as a single management unit?
Hydrological connectivity studies investigate water-mediated transfer of matter, energy, and/or organisms and have become popular since water is vital for the functioning of ecosystems (Reid, Reid, & Thoms, 2016; Cui et al., 2020). Given the importance of surface and groundwater, previous studies proposed hydrological connectivity indicators and investigated how catchments are inter-connected (Bracken et al. , 2013). These indices are based on integral connectivity scale lengths (ICSL) (Western, Blöschl, & Grayson, 2001), a variation of conductivity in a geologic medium (Knudby & Carrera, 2005), landscape’s information (Borselli, Cassi, & Torri, 2008), relative surface connection function (Antoine, Javaux, & Bielders, 2009), and effective contributing area (Ali & Roy, 2010). Moreover, Liu, Wagener, Beck, & Hartmann (2020) recently proposed the effective catchment index (ECI), which improves the discharge/recharge ratio introduced by Fan & Schaller (2009) by detecting and quantifying the deviation between topographic and effective catchment areas. While most indices cited use soil moisture and topography as input data, the ECI is calculated by the logarithmic ratio between streamflow and the difference of precipitation and evapotranspiration. Although most of the effective boundaries of a catchment are unknown, the ECI provides a quantification of the effective contributing area.
The concept and use of the effective catchment area are of paramount importance for understanding hydrological connectivity, contributing to a more effective intervention on catchment processes than just adopting the topographic catchment area (Bracken et al. , 2013). The effective area considers the inter-catchment groundwater flow, and they are usually significantly smaller or larger than the area given by its topographic boundaries (Aryal, Mein, & O’Loughlin, 2003). Underground water connectivity is even more important from the water management perspective, in which surface water channels are commonly considered to be independent channels that become a unit only and through a topographic encounter (Liu et al. , 2020). Although there is still little research dedicated to the subject, most hydrological models assume no hydrological connectivity between catchments for simulating water flow. This assumption leads to a misunderstanding about the actual hydrological potential of a catchment (Bouaziz et al. , 2018). Therefore, users tend to force hydrological models on isolated catchments during calibration/evaluation while the truth is that those catchments are truly connected.
There are many factors associated with the hydrological connectivity of a catchment such as land cover (Ludwig, Wilcox, Breshears, Tongway, & Imeson, 2005), topography (Poesen, 1984; Hopp & McDonnell, 2009), climate (Bracken, Cox, & Shannon, 2008), and geology (Ali, Tetzlaff, Soulsby, & McDonnell, 2012). In this context, Liu et al. (2020) identified the catchments with the potential to transfer water beyond topographic limits and correlated them with different physiographic factors in the Americas, Europe, and Oceania. Nevertheless, there is still the need for regional studies that consider local characteristics to improve the understanding on multiple scales. Therefore, we used the novel Brazilian dataset of catchment attributes comprising a greater number of catchments and attributes than that analyzed by Liu et al. (2020). Our objective was to investigate the deviation between effective and topographic areas and to assess potential climatic and physiographic attributes explaining that deviation. Additionally, we discussed the implication of our results on the catchments’ potential to lose or gain water and how it affects hydrological connectivity by inferring inter-catchment groundwater flow. Our findings contribute to improving water resources management and allocation mainly in water-scarce environments, which we started to unveil.