Catchments better fitted the Budyko curve when considering the effective
area (the adjusted Budyko Framework, Figure 4b). Moreover, we noted a
different behavior of those catchments regarding the long-term
evaporative index (P-Q)/P. Catchments that gain water are closer to the
upper water limit while catchments that lose water are mostly
concentrated below the Fu curve by comparing the classic and adjusted
framework. The increase in the evaporative index of catchments with
larger effective areas reflects the correction of Q (i.e., the
difference between P and Q increases as Q is corrected by an effective
area larger than the topographic area). Conversely, lower evaporative
indices were observed in catchments that lose water as P-Q decreases.
Furthermore, three catchments that gain water exceeded the energy limit
probably due to errors in the ET estimation. These catchments are
geographically close to each other and both belong to a mountainous
region. Most catchments that deviated from the Budyko curve range are
those which lose water. These deviations may be attributed to the
framework limitations regarding the assumption of a closed water balance
and the climate aridity control of catchment hydrological processes
(e.g., the long-term partitioning of precipitation into streamflow
(Budyko, 1951) and evapotranspiration).
Identifying Influencing
Attributes
From the subsequent PCA
phase, three attributes — distance to coast, baseflow index, and the
Strahler order — were excluded from the Random Forest analysis. Most
catchments studied are more densely distributed along the Brazilian
coast (Figure 3) and share similar branching complexity. Thus, those
attributes may not contribute to the variance of the entire dataset
(Supplement S2). Based on the RFA, the aridity index was the most
influencing factor and negatively correlated with ECI (Pearson
coefficient of -0.6 and p-value < 0.05). The global study of
Liu et al. (2020) (Figure 5a) also showed a strong correlation
between the ECI and the aridity index. Most catchments with effective
areas smaller than half of their topographic areas are located in the
aridest biomes: the Cerrado and Caatinga. On the contrary, larger
effective areas were mostly found in the Amazon and Atlantic Forest
biomes, which are characterized by lower aridity indices.