Assessing Precipitation Memory and Operational Controls on National-Scale Reservoir Storage and Hydropower Dynamics in Türkiye

Abstract Reservoir storage dynamics emerge from the tightly coupled interaction of precipitation-memory forcing and institutional operational management, yet disentangling these distinct controls remains challenging at national scales. This study investigates the multivariate relationships among precipitation, macro-scale reservoir storage, and hydropower generation in Türkiye using a data-efficient framework combining Seasonal–Trend Decomposition using Loess (STL), an optimized precipitation-memory index (PM_t) , and a Vector Autoregressive model with exogenous inputs (VARX). The optimized precipitation-memory structure identified a 12-month temporal lag window with a memory decay parameter of τ= 1.25. Despite the extended memory horizon, climate forcing remained strongly concentrated at short lags, with approximately 91% of the cumulative memory weight occurring within the first three months. The precipitation-memory index was identified as a significant contributor to both reservoir-storage and hydropower-capacity-factor dynamics. Impulse-response analysis revealed positive responses of both reservoir storage and hydropower capacity factors to precipitation-memory innovations, although the responses occurred over different characteristic timescales. Forecast Error Variance Decomposition (FEVD) demonstrated that, although precipitation memory contributes a stable, non-trivial share to system variability, long-term forecast uncertainty is governed predominantly by endogenous hydropower-system dynamics, with smaller but persistent contributions from precipitation memory and reservoir-storage variability. Forward-looking scenario simulations further showed that sustained precipitation deficits generate progressive, non-linear storage anomalies through time, with a 30% precipitation deficit producing a cumulative storage drawdown of approximately 6.8 percentage points relative to baseline conditions after 24 months. The analytical outcomes indicate that hydro-climatic forcing and operational management jointly govern national-scale reservoir dynamics. Hydro-climatic variability provides the primary source of water-availability fluctuations, while institutional regulation controls how these signals persist and propagate through the system over time. The proposed framework provides a transferable and data-efficient approach for assessing reservoir resilience and water–energy interactions under increasing hydro-climatic variability.