Multi-Objective Calibration of a Pre-Modern Nile Hydrologic Model Using Recovered Records

Hydrologic models are instrumental in understanding the Nile River Basin’s (NRB) behavior. Their effectiveness, however, is often limited by the basin’s complex hydrology and sparse hydrologic records. This study develops a robust hydrological model for the NRB under near-natural flow conditions. We discretized the basin into 34 sub-watersheds with an outlet located at Aswan. The conceptual GR4J model was implemented within the Raven hydrological modeling framework, chosen for its balance between simplicity and compatibility with the relatively sparse gauge observations. Multi-objective calibration was performed using data from the Global Runoff Data Centre (GRDC), supplemented by recovered data through digitization of historical records to improve data coverage across the basin. Stepwise calibration was conducted at 18 sites, focusing on pre-reservoir periods to capture natural flow dynamics. The calibrated model effectively reproduced observed discharges. At the Aswan outlet, the model achieved a Nash–Sutcliffe Efficiency (NSE) of 0.87 (calibration) and 0.80 (validation), with corresponding percent bias (PBIAS) values of 6.1% and 5.0%. Model performance was strongest in the Blue Nile, White Nile headwaters, and Nile main stem. The model also successfully simulated the hydrological step-change observed in Lake Victoria during the 1960s, underscoring its robustness in simulating regional hydroclimate disruptions. This calibrated model provides a valuable tool for reconstructing historical discharge patterns in the NRB and provides a baseline for comparison with the modern Nile.