Search

Study region: The Congo River basin (CRB), the world's second-largest river system, is subject to extreme hydrological events that strongly impact its ecosystems and population. Study focus: Here we present an improved 40-year (1981–2020) hydrological reanalysis of daily CRB discharge and analyze the spatiotemporal dynamics of recent major CRB floods and droughts, and their teleconnection with El Niño-Southern Oscillation (ENSO), the dominant driver of tropical precipitation. We employ a large-scale hydrologic-hydrodynamic model (MGB) with lake storage dynamics representation and a data assimilation (DA) technique using in-situ and remote sensing observations. New Hydrological Insights: The MGB model demonstrates satisfactory performance, with Kling-Gupta efficiency metric of 0.84 and 0.71 for calibration and validation, respectively. Incorporating lake representation substantially enhances simulations, increasing the Pearson correlation coefficient from 0.3 to 0.63. Additionally, DA yields a ∼13% reduction in discharge errors via cross-validation. We find that the 1997–1998 flood impacting the south and central CRB is statistically linked to a major El Niño event during that period. However, no such association is found for the 2019–2020 flood. Severe droughts in 1983–1984 and 2011–2012, affecting northern and southern CRB respectively, exhibit strong correlation with preceding El Niño and La Niña events, with a ∼10–12 months lag. This study advances understanding of the intricate interplay between spatiotemporal hydrological variability in CRB and large-scale climate phenomena like ENSO.

Study region: The western Amazon basin at Tamshiyacu gauging station (near the Iquitos City) hosts floodplain agriculture that can be affected by the sudden reversal in direction of water levels known as “repiquetes” that produce intermittent flooding. Study focus: This study assesses repiquete flooding risk in riparian crop areas based on statistical analyses of repiquete events registered from 1996 to 2018, hydraulic modeling to estimate flooded extension, and assessment of climatological characteristics during the formation of repiquetes. New hydrological insights: Floods (≥ 20 cm) produced by repiquetes in riparian crop areas between 83.00 and 88.00 m above sea level (masl) occur 1.8 times per year. However, not all elevation ranges have the same flooding risk to crops. Terrain elevations between 85.31 and 87.00 masl have a reduced flooding risk of 0.35 per year. Likewise, areas with elevations between 87.00 and 88.00 masl (43% of the total area) were not affected by repiquetes. Extreme repiquetes (study cases of 2002 and 2008) have been influenced by the increase of atmospheric moisture flux convergence and precipitation over both the northern Ucayali and Marañón basins through the six previous days. Flood impacts from the extreme event of 2002 (2008) could have reached 40% (25%) of the available area for agriculture at the initiation of the repiquete.

Study region: Upper Madeira Basin (975,500 km2) in Southern Amazonia, which is suffering a biophysical transition, involving deforestation and changes in rainfall regime. Study focus: The evolution of the runoff coefficient (Rc: runoff/rainfall) is examined as an indicator of the environmental changes (1982–2017). New hydrological insights for the region: At an annual scale, the Rc at Porto Velho station declines while neither the basin-averaged rainfall nor the runoff change. During the low-water period Rc and runoff diminish while no changes are observed in rainfall. This cannot be explained by increase of evapotranspiration since the basin-averaged actual evapotranspiration decreases. To explain the decrease of Rc, a regional analysis is undertaken. While the characteristic rainfall-runoff time-lag (CT) at Porto Velho basin is estimated to 60 days, CT is higher (65–75 days) in the south and lower (50 days) over the Amazon-Andes transition regions. It is found that 1) the southern basin (south of 14 °S) best explains low-level Porto Velho runoff, 2) in the south, rainfall diminishes and the frequency of dry days increases. Both features explain the diminution of the runoff and the Rc in Porto Velho. Moreover, the increasing dryness in the south compensates for the rainfall and frequency of wet days (>10 mm) increase north of 14 °S and explains the lack of basin-averaged rainfall trends of the upper Madeira basin. Keywords: Runoff coefficient, Rainfall trends, Atmosphere and land surface interactions