DISCUSSION
The distribution of water losses and gains across Brazil follows
climatic domains (Cherlet et al. , 2018) and is consistently
supported by the Budyko framework. The Amazon and Pampa biomes presented
both losing and gaining water conditions without a clear tendency for
each condition due to the low density of catchments with observed
hydrometeorological monitoring data in these biomes (Figure 3). It
reveals the heterogeneous distribution of gauged catchments among the
Brazilian regions and biomes (ANA, 2019) and the lack of basic observed
data systematically collected over time and space (Marengo, 2006).
Especially in the Amazon biome, the sparse monitoring network is due to
the difficulty in access for local operations.
Most arid catchments located in the Caatinga and Cerrado biomes
presented effective areas smaller than half of their topographic
boundaries, indicating a losing water condition (dark red circles in
Figure 4). We observed that these catchments have higher evaporative and
aridity indices when all catchments were plotted on the classic Budyko
framework (Figure 4a), i.e., the framework which considers the
topographic area. In fact, topographic catchments with a strong
deviation from their effective areas (Aefftopo or
Aefftopo) were closer to theoretical water and energy
limits. Catchments that are close or exceed these limits support the
hypothesis of inter-catchment connectivity (e.g., groundwater fluxes)
(Bouaziz et al. , 2018). Therefore, considering a closed water
balance in those Brazilian catchments with larger deviation possibly
increases uncertainty in hydrological studies, leading to inconsistent
results.
The effective area represents the subsurface fluxes and processes
(Figure 4b) by estimating possible gains or losses from the relationship
between Q and P-ET. The effective catchment area provided a better fit
of catchments in the adjusted Budyko framework as expected. Nonetheless,
the arid catchments with effective areas smaller than half of their
topographic areas were outside the Budyko range (dark red circles,
Figure 4b). This scenario suggests other hydrological processes not
captured by the use of the effective area. Those arid catchments are
located along the northeast coast of Brazil (Caatinga and Atlantic
Forest biomes) and share particular characteristics that influence the
surface and subsurface hydrological processes. They have a complex
network of reservoirs (Nascimento & Neto, 2017; ANA, 2021), which
alters the local water cycle by reducing flow downstream and increasing
ET losses. In the ECI estimation, this disturbance exacerbates the
deviation of the effective area from its corresponding topographic area.
Consequently, even small deviations can represent a large offset between
streamflow change and aridity, primarily in arid regions (Berghuijs,
Gnann, &Woods, 2020). The hydrological disturbances caused by a series
of reservoirs may violate the hypothesis of Budyko about the climate
aridity control over the precipitation partitioning. Furthermore,
considering heavy hydrological disturbances in approaches that
investigate catchments effective areas and inter-catchment connectivity.
Inadequate measures of P, ET, and Q can also be considered as sources of
uncertainty not only in the Budyko framework but also in the ECI
estimation. We made use of the best database available in order to make
our findings reliable, but there are still some uncertainties such as
low density of precipitation monitoring stations, mathematical
limitations to represent the ET processes, and possible
non-representative rating curves for discharge estimations. Despite the
uncertainties associated with the ECI estimate, we achieved a better
adjustment of the catchments using the effective area within the Budyko
framework (Figure 4b). Therefore, we can assume that these uncertainties
in the ECI estimates are lower than those associated with studies using
the topographic area. Nonetheless, hydrological disturbances should be
carefully investigated. Overall, the Budyko framework corroborates the
water losing and gaining conditions assumed from ECI as an alternative
to comply with the assumptions of a closed water balance.
The ECI most influencing
attributes
The climatic and physiographic attributes found in the Brazilian biomes
support the effective area indices found and contribute to a better
country-scale understanding of hydrological processes and
inter-catchment connectivity. Liu et al. (2020) significantly
contributed to understanding how physiographic factors and some
catchment location aspects could explain the deviation between
topographic and effective areas at a global scale. Our study takes
further steps towards downscaling their global study and investigating
other factors that, in turn, were relevant influencing the variability
of ECI in Brazil. Furthermore, we bring some practical implications of
our findings to water resources management in Brazil.
The strong negative
correlation between the aridity index and ECI found in the semiarid
region is closely related to its hydrological characteristic of
intermittent rivers and ephemeral streams. This region is characterized
by shallow soils formed on crystalline bedrock with a minimum
contribution of baseflow from deeper groundwater to the surface flow.
Moreover, the semiarid has great spatiotemporal variability of
precipitation, with a mean annual amount of less than 600 mm (Silva,
Santos, and Santos, 2018; Toledo & Alcantara, 2019). The effective
precipitation (P-ET) is larger than the surface runoff in losing water
catchments (negative ECI) and therefore contributes to the subsurface
flow, which may not return as baseflow in the same draining catchment
(Figure 6). The absence of rainfall precipitation in most part of the
year limits the ET losses, which corroborates the correlation between
aridity and the losing water condition (negative ECI) in the semiarid
when combined with intermittent flow.