In this study, the sustainability status of surface and groundwater resources in the Zayandehrood River Basin was evaluated using the Multivariate Water Resources Index (MWRI). This index integrates three key parameters—average precipitation, available surface water volume, and exploitable groundwater volume—calculated through the entropy weighting method. To provide a more detailed analysis of sustainability, three functional components of the system reliability, reversibility, and vulnerability were calculated for each sub-basin, from which the initial sustainability index was derived. The novelty of this research lies in the incorporation of the temporal trend of the water resources index, assessed using the Mann-Kendall test, into the base sustainability model, leading to the development of a modified sustainability index. Utilizing 20 years of data on precipitation, surface flow volume, and groundwater level fluctuations, a composite water resources sustainability index was formulated. The analysis included five sub-basins: Buein-Miandasht (upstream of the dam) and four downstream sub-basins Lenjanat, Najafabad, Esfahan-Borkhar, and Kuhpayeh-Segzi. Results indicated that only the Buein-Miandasht sub-basin, with a modified sustainability index of 0.31, is in an acceptable condition, while the other sub-basins demonstrated critically low values (less than 0.001), indicating unsustainable conditions. These findings are consistent with field observations and the significant reduction in water resources downstream of the Zayandehrood Dam, underscoring the urgent need for a revision in regional water resource management strategies. The application of the MWRI in this study provides an effective framework for the integrated assessment of water resource sustainability at the basin level.