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.