Modeling snowpack evolution and water discharge in the Po River basin at 1 km resolution: a retrospective analysis (1991-2020)

The Alps are a crucial water reservoir for over 170 million people in Europe, storing water as snow and ice during the cold season and releasing it during the warm season. However, climate warming is causing earlier snowmelt and glacier shrinkage, leading to a mismatch between water availability and peak demand, thereby increasing the risk of water shortages and conflicts among key users such as agriculture, energy, and tourism. In this framework, skillful seasonal climate predictions combined with snow-hydrological modeling might help in the early warning of water shortages. Recent studies show advancements in the seasonal prediction of mountain snow water equivalent (SWE); however, it remains uncertain whether these improvements translate into accurate predictions of streamflow and water availability.  

Within the PRIN-2022 SPHERE¹ project, we developed a novel modeling chain which takes advantage of the state-of-the-art bias-corrections methods, downscaling techniques, and snow-hydrological modeling tools to model snowpack evolution, river discharge, and water availability in Alpine river basins. The modeling chain comprises the S3M snow model and the HMC Continuum hydrological model. S3M is a spatially distributed cryospheric model that simulates snow and glacier mass balance, while HMC is a spatially distributed hydrologic model that solves the mass and energy balance of vegetation and soils. The modeling setup, validated in previous projects, operates on a regular grid with a 1 km spatial resolution and an hourly time step over the Po River basin. For this domain, ERA5 reanalysis meteorological variables were used to generate the forcing for the baseline run of the modeling chain. ERA5 inputs, originally at 0.25° spatial resolution, were appropriately bias-corrected and downscaled to 1 km. A 30-year baseline simulation was then generated to reconstruct the historical evolution of mountain snowpack (in terms of SWE and snow depth), meltwater runoff, and streamflow.

We present here an initial evaluation of the snow-hydrological modeling chain over the study area for the period 1991-2020. Simulations of snow depth, SWE and river discharge were compared against various available observations from snow gauges and Italian regional hydrological networks, in terms of bias, RMSE and correlation.  As a next step, the modeling chain will be extended to run using seasonal forecasts from the Copernicus seasonal prediction systems (ECMWF S51, MF S8, CMCC S35, DWD S21) to generate retrospective seasonal forecasts of snow and hydrological variables over the same study domain. The forecast skill of the modelling chain will be evaluated for starting dates November 1st and May 1st, to assess the possibility of anticipating water availability several months in advance for the winter and summer seasons, respectively.

¹Progetto di Ricerca di rilevante Interesse Nazionale (PRIN-2022): Seasonal Prediction of water-availability: enHancing watER sEcurity from high mountains to plains (SPHERE)