<p>The limited availability of hydrometeorological records in mountainous regions severely constrains the technical management of water resources in river basins. In response to this limitation, this study presents a methodological framework for estimating both surface and groundwater discharges in scarce-data basins, applying an integrated hydrological numerical model, based on the finite element method, to the Casacay River catchment in south-western Ecuador. The MELEF-<i>FSW</i> model was employed to simulate the coupled flows of groundwater and surface water under transient conditions, incorporating processes of overland flow, infiltration, soil water content, groundwater recharge, actual evapotranspiration and water management. The Casacay catchment is characterised by steep topography, pronounced seasonal variability, and intensive water abstraction facilities for human consumption. Given the dispersed nature of the available data, multiple information sources were integrated, including field observations, satellite-derived, hydromorphological analyses, and GIS-based tools. The model configuration involved the definition of high-resolution meshes in critical zones, assigning calibrated hydrogeological parameters, and applying realistic boundary conditions and scenarios of water consumption extraction and recharge, during a 3 years calibration period. No validation period was performed due to the scarcity of actual observed data. The local analysis of the results indicates a limited soil water storage capacity, a cumulative decline in groundwater reserves, and a strong seasonal variability in streamflow. Therefore, the proposed methodology demonstrates technical robustness and potential replicability in hydrological catchments with similar characteristics, providing a valid alternative for local water resource assessment in the absence of conventional monitoring networks.</p>