<p>Effective management of water ecosystem services is critical for sustainable water resource planning, particularly in regions with limited data availability. This study evaluates the performance and applicability of the InVEST Seasonal Water-Yield (SWY) model in the data-scarce Upper Indus Basin (UIB), a major water source supporting nearly 200 million people. The model was evaluated across four representative years (2005, 2010, 2015, and 2020) to assess its ability to capture hydrological variability under varying land use/land cover (LULC) and climatic conditions in a complex, data-scarce, high-altitude environment. Model performance was evaluated by comparing basin wide water-yield outputs (Quick Flow (QF) + Base Flow (BF)) with observed streamflow at the annual and monthly scales. At the annual scale, Invest-SWY showed strong agreement with observations, achieving a maximum coefficient of determination (<i>R</i>²) of 0.64 and a minimum RMSE of 6.5 × 10⁸ m³/year, indicating reliable long- term performance. Monthly-scale evaluation revealed lower accuracy, with <i>R</i>² values up to 0.54 and RMSE reaching 15.8 × 10⁸ m³/month, indicating greater uncertainty in short-term simulations. Monthly QF results exhibited pronounced seasonal patterns, with peak runoff consistently occurring during the summer months (June–August), corresponding to snowmelt and monsoonal precipitation, while winter months showed minimal runoff. Model performance was generally higher during wetter years and in lower-elevation regions, whereas snow-dominated and high-altitude areas contributed less to both QF and local recharge. Overall, the results demonstrate that Invest-SWY is appropriate for annual and basin-scale water ecosystem service assessments in data-scarce mountainous regions, while caution is warranted when interpreting the monthly scale outputs. This study provides quantitative evidence supporting the use of Invest-SWY as a practical decision-support tool for long-term watershed management under data constraints.</p>