Reconstruction of a long series of Natural Discharges: an application to the Tiber River basin

Natural discharges are crucial for the quantitative assessment and sustainable management of water resources, enabling the assessment and prediction of water availability and the impact of climate change. The aim of this study is to present a simple methodology for the reconstruction of natural discharges and identify direct and indirect withdrawals. It is based on groundwater recharge and runoff data provided from the BIGBANG v.6 database (ISPRA), which collects national-scale maps of the main hydrological variables in the period 1951-2022. Specifically, the framework is based on the calibration of 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. The procedure was applied to the Tiber River basin, closed at the Ripetta station. This methodology enables the reconstruction of long natural flow time series at monthly scale, which are fundamental to catchment and water resource management policy.  Furthermore, the methodology is able to effectively identify dynamic differences between surface and subsurface flows. The results demonstrate the procedure's high accuracy in reproducing natural flow rates, with minimal errors. The developed approach provides a valuable tool for watershed-scale water resource management, supporting policy planning, evaluation, and implementation.