From Natural Flow to Anthropogenic Pressure: Quantifying Water Withdrawals in the Tiber River Basin Over 70 Years

Quantifying long-term water withdrawals at the river basin scale remains a major challenge due to scarce direct observations and the interaction between natural hydrological processes and human activities. This study introduces a semi-distributed modelling framework to reconstruct natural discharge and infer net water withdrawals from observed streamflow records. Net water withdrawals are estimated as the difference between simulated natural and observed discharges. Natural discharge is simulated by partitioning precipitation (BIGBANG v.8 database, ISPRA) into surface runoff and groundwater recharge through an infiltration-based scheme. Groundwater contributions are represented using a linear reservoir to capture delayed baseflow response. The model is governed by three parameters, which are (i) the infiltration coefficient, (ii) the ratio between the hydrogeological and catchment area and (iii) storage coefficient of the linear reservoir model.  Calibration is performed over 1954–1965, assumed minimally impacted by withdrawals, by maximizing Nash–Sutcliffe Efficiency and minimizing volume bias, with Kling–Gupta Efficiency as an additional metric. The approach is applied to the Tiber River basin closed at Ripetta station (central Italy) using data spanning 1954–2023. The calibrated model reproduces observed discharge dynamics satisfactorily. The reconstructed natural discharge series is then extended to 2023 to proceed with the calculation of net withdrawals. The resulting time series shows a clear long-term linear increasing trend, with significant interannual variability. Statistical tests (Chi-square and t-tests) on residuals confirm normality and a mean not significantly different from zero, supporting the robustness of the inferred trend. This approach enables spatially coherent reconstruction of water withdrawals using commonly available hydrological data, providing a valuable tool for assessing anthropogenic pressures where direct measurements are lacking. Results for the Tiber River basin reveal progressive intensification of human influence on water resources over seven decades, offering insights for water management, policy development, and hydrological forecasting in human-modified catchments.