Abstract—This study presents an integrated hydrological assessment of the contributing basin to the city of Asunción, corresponding to the Middle and Lower Paraguay Basin, using satellite products and in situ measurements during the 2003–2022 period. The main variables analyzed include precipitation (IMERG), evapotranspiration (ET), groundwater storage (GWS), and streamflow at the Asunción station (control point). The results highlight the hydrological dynamics and the basin’s sensitivity to interannual climate variability. Evapotranspiration remained relatively stable, while GWS exhibited high variability, reflecting seasonal recharge and depletion patterns. Precipitation showed strong seasonality, consistent with the basin’s climatic regime. The analysis of monthly mean streamflow from 1931 to 2021 revealed extreme hydrological events—both floods and droughts—correlated with ENSO phases. Notably, 2015 and 2016 were identified as persistently wet years, whereas 2020 and 2021 marked widespread hydrological stress across all variables. Standardized anomaly analyses revealed synchronicity between surface and subsurface hydrological components. The calculation of the water balance and GWS anomalies allowed for spatial identification of vulnerable zones. A classification scheme based on the Standardized Groundwater Index (SGI) was applied to map dry (SGI < -1), intermediate (-1 ≤ SGI ≤ 1), and wet (SGI > 1) zones. Cross-correlation analysis between the residual water balance (P – ET – Q) and ΔGWS indicated that groundwater storage responds with a one-month lag relative to surface hydrological fluxes. This finding highlights the buffering capacity of the subsurface and supports the use of GRACE data as an indicator of integrated hydrological response. Overall, the study emphasizes the need for continuous groundwater monitoring and streamflow control of river tributaries, as well as their interaction with climate-driven surface water processes in the region.