Catchments in the Cerrado e Caatinga — more arid biomes — were prone
to present a losing water condition that deteriorates the imbalance
between water availability and demand in these regions (Gesualdoet al. , 2021). This situation is exacerbated by the long-term
conversion of natural vegetation to different agricultural land uses
(e.g., sugarcane, soybean, and corn), which have been responsible for
more than half of the national grain production (Spera, 2017) mainly in
the Cerrado. Furthermore, the increase in irrigated areas for food
production in the Cerrado and Caatinga biomes has been recently related
to an increase in evapotranspiration and baseflow reduction in these
regions (Oliveira et al ., 2020; Lucas et al ., 2021).
Therefore, quantifying the effective catchment area is key to better
understand synergies and trade-offs of land use changes and the increase
in irrigated areas. The observed losing water condition suggests a
strong inter-catchment hydrological dependency among the catchments in
these biomes as a substantial part of precipitation contributes to the
subsurface flow.
Our ECI results were
positively correlated with mean slope and mean elevation corroborating
the gaining water condition found in 72% of the catchments located in
the Atlantic Forest biome, which presents the highest mean elevation and
slope (Almagro et al., 2020). Indeed, low and high slopes were
associated with smaller and larger effective areas, respectively (Figure
5b). Different from Liu et al. (2020), we did not note high
variability of the effective area — i.e., either positive or negative
ECI — in flatter regions (Figure 5d). On the other hand, ECI only
presented low variability with increasing elevation. For instance, the
catchment in the Pantanal biome, characterized by a complex hydrological
dynamic, presented a substantial deviation between its effective and
topographic area, indicating a losing water condition. Thus, the
observed losing water condition corroborates the characteristics of a
flat lowland area, where rivers flood the plains and feed an intricate
seasonal drainage system (Ivory, McGlue, Spera, Silva, & Bergier,
2019). Additionally, the topographical catchment delineation is more
susceptible to errors in complex topographies since the effect of
Digital Elevation Models accuracy is not well understood, leading to a
mismatch between topographic and effective areas (Zandbergen et al.,
2011).
Catchments with a
well-defined precipitation seasonality were associated with gaining
water conditions as in the southern part of the Cerrado and the entire
Atlantic Forest biome. Nonetheless, the precipitation seasonality index
has some limitations when applied in Brazil due to its climate
characteristics. Low thermal amplitude characterizes the climate in the
north and northeast regions so that it is difficult to determine whether
precipitation occurs during the summer, the winter, or throughout the
year. The WTD and HAND had little influence on ECI probably due to the
low spatial resolution of the available data. Carrying out large-scale
studies involving groundwater is still challenging since high-resolution
products at large scales are scarce (Gleeson, Cuthbert, Ferguson, &
Perrone, 2020). Besides, monitoring the groundwater table is associated
with high levels of uncertainties, frequently limited to developed
regions (Fan, Li, & Miguez-Macho, 2013). Similarly, the soil texture
data used in this study also have a low spatial resolution (250 m),
which may have compromised the RFA performance in identifying its
influence on ECI estimates (Supplement S3). Even though this attribute
presents the lowest influence, it plays an important role in the
catchment’s water cycle and groundwater flow in terms of soil water
storage and percolation.
Implications for the understanding of hydrological
connectivity and potential advances in water resources
management
In this paper, we assessed the concept of interconnected catchments
through the investigation of their effective areas. The hydrological
connectivity was inferred from the flow-process perspective, defined by
Bracken et al. (2013) as the understanding of runoff patterns and
processes on hillslopes. From this perspective, we spatially assumed the
connection between catchments and their influencing attributes. Based on
the ECI results, we can state that catchments are sub-superficially
interconnected. Although the scientific community agrees on
inter-catchment connectivity, this is still a recent concept and little
explored by water management decision-makers.
Incorporating the knowledge of the effective area and its influencing
factors in the water resources management, the groundwater boundaries
and processes may be reasonably considered. These processes are often
neglected when the topographic area is solely used as a management unit.
Therefore, the inclusion of the magnitude of the effective area would
improve the comprehension of more reliable hydrological processes on a
catchment scale. Besides, understanding the deviation between
topographic and effective areas copes with the lack of clear and
detailed information about aquifer properties and limits (Hirata,
Kirchheim, & Manganelli, 2020), which are important to have integrated
water management (Samani, 2021).
The ECI is a relevant tool for tackling water vulnerabilities and
inequalities in allocating and managing water. In the semiarid region,
Brazilians are exposed to high levels of water insecurity and inequality
exacerbated by recurrent and prolonged droughts (Gesualdo et al. ,
2021). By knowing the magnitude of the deviation between the topographic
and effective areas, the influencing climatic and physiographic
attributes, and its underlying hydrological processes, decision-makers
can assemble groups of nearby connected catchments. Based on this, water
resources management would be settled into a pooling of
catchments — a combination of the interests and needs of a group of
catchments — considering their interconnection. The exclusive use of
topographic delineation is a limiting factor although the water
resources management practice is already designed for a large group of
catchments (e.g., as in the São Francisco River Basin (one of the 12
hydrographic units in the country).
Efforts on water-related preservation and conservation have been made in
contributing surface areas upstream reservoirs and pumping points even
though these are often much smaller or larger than the actual
contributing areas. Therefore, advances in understanding and identifying
effective areas play a key role in synergistically managing watershed
services related to water yield and provision. For instance, it would
imply a greater environmental responsibility by water-gaining catchments
benefiting from water ecosystem services provided by water-losing
catchments. We emphasize the paramount importance of integrating
hydrological processes and water ecosystem services relevant for
catchment management as alterations in flow processes impact water
provision and vice-versa (Grizzetti, Lanzanova, Liquete, Reynaud, &
Cardoso, 2016). Thus, managing a pool of catchments can increase
synergies and lessen trade-offs of water transfer processes. Sustainable
inter-basin water transfer is an alternative for addressing the
imbalance in water availability and demand mainly in the Caatinga and
Cerrado biomes (Gesualdo et al., 2021). Hence, advancing the
understanding of hydrological connectivity between semiarid and arid
catchments better copes with water scarcity. In this context, the ECI
guides possible solutions to a question encompassed by Guswa et
al. (2014): “What parcel of land is the highest priority for
conservation?”. The effective area deviation can support
decision-makers in identifying catchments with the highest priority for
conservation and best management practices implementation.
Considering the inter-catchment connectivity contributes to
investigating the extent of groundwater pollution and projecting
efficient water use in activities such as agriculture. In the
agriculture sector, the quantification of effective catchment areas
would allow decision-makers to strategically manage the increase in
water-fed irrigation areas in Caatinga and Cerrado biomes and understand
how this disturbs the water balance in these regions. Although the land
cover was not one of the most important attributes to identifying the
effective catchment area, it has a major role in unveiling the
mechanisms of movement and storage of water at a catchment scale.
Nevertheless, anthropogenic impacts on water fluxes are still poorly
understood (Neupane & Kumar, 2015) such as land use and land cover
changes and inter-basin transfers.
There are future research opportunities for addressing surface and
groundwater integration, such as:
- How water-fed irrigation area disturb the water balance?
- What are the results and uncertainties from adding the variable “land
use and land cover changes and inter-basin transfers” to ECI
computation?
- The ECI indicates the deviation between the topographic and effective
catchment areas, so what are the physical boundaries of the effective
area of a catchment?