An enhanced river routing scheme for the closure of global water budget

Large-scale river routing schemes are essential to close the hydrological cycle in fully coupled Earth System Models (ESMs). The availability of a realistic water flow is a powerful instrument to evaluate modeled land surface, a crucial component of the global climate whose properties are often simplified by heavy parameterization, due to lack of process knowledge and validation data. We built up a new concept of river routing model, named HYDROS (HYdro-Dynamic ROuting Scheme), that replaces the present scheme embedded in the CMCC-CM2 global coupled model. The new scheme aims at overcoming one of the current major limitations, that is the use of time-independent flow velocities parameterized as a function of topography. Through the imposition of hydraulic equations, HYDROS defines a time-varying flow velocity associated with the amount of lateral runoff generated by the ESM's land component and the flow through the river system. Compared to the scheme currently in place, HYDROS show improvements in the simulation of mean annual discharge phase, especially for the Arctic rivers and the Amazon. In the Mississippi case, an extreme flood episode is better caught by the new representation, indicating that the improved flow velocity better catches the discharge peaks after extreme rainfalls. The new routing model is not able to improve the volumes of simulated river discharge, whose magnitude depends on the ability of the ESM land surface scheme to generate correct surface and sub-surface runoff. Once implemented in coupled mode, HYDROS will guarantee a plausible amount and timing of freshwater discharge into the global ocean, unveiling possible unresolved feedback mechanisms occurring in proximity of river mouths.