Evaluating Hydrological Alterations Due to Climate Change: Insights from HEC-ResSim Simulations in the Indus river fed Tarbela Reservoir

Pakistan, endowed with substantial water and glacier resources in the Upper Indus Basin (UIB), relies heavily on the Tarbela Reservoir for hydropower generation, irrigation, and flood control. As the world's largest earth-fill dam, Tarbela spans a 169,600 km² catchment with an average annual inflow of 79 billion cubic meters, supplying approximately 33% of Pakistan's energy needs. However, climate change-induced variations in precipitation, temperature, and melt patterns pose risks to its reliability. This study assesses the reservoir's response for hydropower generation using the Hydrologic Engineering Center's Reservoir System Simulation (HEC-ResSim) model, integrating historical hydrological data to simulate operations under observed conditions.

Using the data sourced from the Water and Power Development Authority (WAPDA), three modules  of HEC-ResSim's were used: Watershed Setup for defining stream alignments, reservoirs, and computational points; Reservoir Network for configuring physical parameters (e.g., elevation-storage-area relationships) and operational rules (e.g., flood control, conservation, and inactive levels); and Simulation for running daily computations with HEC-DSSVue for data management and visualization. Alternatives incorporated time-series inputs from DSS files, with Excel used for supplementary calibration and analysis.

Simulations replicated reservoir behavior, revealing seasonal dynamics: low inflows and power generation in winter (December-February) due to reduced melt, peaking in monsoon (June-September) from rainfall and snowmelt. Annual power generation fluctuated, with notable dips (e.g., around 2013) attributed to water scarcity or operational constraints, despite consistent capability. Inflow-outflow comparisons highlighted storage roles in regulating flows. Model accuracy was validated for 2012-2014 against observed data, yielding Nash-Sutcliffe Efficiency (NSE) of 0.987, Index of Agreement of 0.99, and R² of 0.99, confirming robust simulation of power outputs relative to inflows.

Results underscore climate vulnerabilities, with flow variations over decades impacting generation efficiency amid unpredictable weather, floods, and droughts. The ongoing Tarbela 5th Extension (adding 1,530 MW via three 510 MW units, increasing total capacity from 4,888 MW to 6,418 MW) promises enhanced utilization. Conclusions emphasize HEC-ResSim's utility for real-time decision-making and scenario evaluation. Future studies could employ advanced versions, incorporate climate projections (e.g., from CMIP6), and compare pre- and post-extension scenarios to optimize sustainable hydropower amid UIB's evolving hydrology.