Are Brazilian catchments gaining or losing water? The effective area of tropical catchments

Abstract

Similar to most countries, the Brazilian water resources management considers topographically delineated catchment as a territorial unit for policy implementation. Yet, previous studies have shown that catchments are not hydrologically isolated, and topographic limits often neglect the groundwater boundaries. Thus, studies on effective catchment areas are promising for shedding light on inter‐catchment groundwater flow. Here, we investigated the deviation between the topographic and effective areas across Brazil. We applied the effective catchment area index (ECI) to 733 Brazilian catchments and identified the most influencing attributes on the ECI by using principal component and random forest analyses (PCA and RFA, respectively). Further analysis was carried out by contrasting the ECI values against the expected range of the Budyko curve, considering both topographic and effective catchment areas (classic and adjusted framework, respectively). We noted that nearly 32% of the Brazilian catchments presented more than 30% of difference between the effective area and its topographic boundaries. In general, the more arid biomes in Brazil—the Cerrado and Caatinga—are prone to have smaller effective areas while larger effective areas were mostly found in the Atlantic Forest biome, a humid tropical region with a higher mean elevation. Our findings indicate that the aridity index was the main driving factor and negatively correlated with ECI followed by mean slope, precipitation seasonality, and mean elevation. We highlight the potential of adopting a pooling of catchments based on their interconnectivity to minimize management costs while maximizing synergies and lessening trade‐offs between ecosystem functioning and water transfer processes. Our results contribute to a better country‐wide understanding of hydrological connectivity among catchments and highlight the need to consider the effective catchment area to overcome water‐food‐energy security challenges on multiple scales.

The Impact of an Open Water Balance Assumption on Understanding the Factors Controlling the Long‐Term Streamflow Components

“…Similar results were found for 𝐴𝐴 𝐴𝐴𝐵𝐵 . Catchments are prone to gain water through IGF with increasing elevation and lose water with increasing area (Schwamback et al, 2022). Indeed, Brazilian's catchments with larger areas are inclined to present an 𝐴𝐴 𝐴𝐴eff lower than 𝐴𝐴 𝐴𝐴topo (median 𝐴𝐴 𝐴𝐴eff ∕𝐴𝐴topo = 0.82 for the 100 largest catchments).…”

“…10.1029/2022WR032413 6 of 18 (Schwamback et al, 2022). The reasoning of this methodology is described as follows: if we assume the existence of IGF, we can define the continuity equation as 𝐴𝐴 𝐴𝐴𝐴𝐴∕𝐴𝐴𝑑𝑑 = 𝑃𝑃 − ET − 𝑄𝑄 + IGF .…”

“…A direct consequence of the closed water balance assumption is the use of the topographic catchment area 𝐴𝐴 𝐴𝐴topo rather than the effective catchment area 𝐴𝐴 𝐴𝐴eff-an equivalent area proposed by Liu et al (2020) that considers the existence of IGFs by evaluating the deviations between recharge and discharge rates-as the basic spatial unit to compute hydrologic fluxes and to define possible mechanisms that may affect the catchment hydrological response (Wagener et al, 2007). Nevertheless, there is a strong evidence that catchments are "leaky" and cannot be considered as closed hydrologic entities, as they may lose or gain water through different mechanisms such as IGF at different spatial scales, transferring water beyond their topographic boundaries (Fan, 2019;Liu et al, 2020;Muñoz et al, 2016;Schwamback et al, 2022). In fact, given that hydrological connections between catchments do exist, these cases might be the rule rather than the exception (Le Moine et al, 2007).…”

“…Schaller and Fan (2009), when evaluating the exporter/importer condition of 1,555 catchments across the United States found that catchments might have exported (imported) up to 90% (50%) of their flow to other catchments through IGF. Schwamback et al (2022) also reported that nearly 32% of a total of 733 Brazilian catchments showed more than 30% of difference between their 𝐴𝐴 𝐴𝐴topo and 𝐴𝐴 𝐴𝐴eff , concluding on the need to consider inter-catchment connectivity in hydrologic studies. Liu et al (2020) evaluated a global data set with more than 2,700 basins and showed that approximately 35% of the evaluated catchments exhibited an 𝐴𝐴 𝐴𝐴eff either larger than double or smaller than half of their corresponding 𝐴𝐴 𝐴𝐴topo .…”

Most Global Gauging Stations Present Biased Estimations of Total Catchment Discharge

“…The hydrology‐unclosed feature of catchments appears to be more pronounced than expected (Fan, 2019; Kampf et al., 2020). Evidence of such hydrology‐unclosed catchments has come primarily from chemical tracer experiments (Alvarez‐Campos et al., 2022), continuous monitoring of groundwater discharge and streamflow (Jasechko et al., 2021; Käser & Hunkeler, 2016), physical‐based hydrological modeling (Pellicer‐Martínez & Martínez‐Paz, 2014; Zanon et al., 2014), and water budget closing (Schwamback et al., 2022). Intercatchment groundwater flow is not directly observable; however, it plays a nonnegligible role in streamflow generation in many catchments (Frisbee et al., 2016; Schaller & Fan, 2009), especially in karst areas (Liu et al., 2021).…”