Drought-heat conditions in Swiss rivers: Operational forecasts and future projections

The services provided by streams and rivers are conditioned by water quantity as well as quality. Streamflow and water temperature are important for aquatic biodiversity, drinking water provision, electricity production, agriculture, and recreation. We inform users about the ongoing hydro-meteorological conditions in Switzerland through the platform www.drought.ch/de, with a focus on drought-related hazards. Here we provide operational probabilistic forecasts of daily runoff anomalies and maximum water temperature for the next 32 days. These forecasts are generated twice per week using ensemble meteorological forecasts as input to the process-based PREVAH hydrological model (runoff) and to a Deep Learning Temporal Fusion Transformer (TFT) model (water temperature).

In part one, we present the setup of our TFT model and assess its predictive skill. The continuous rank probability score (CRPS) is 0.70 °C averaged over all 32 lead times, 54 stations, and 90 forecasts distributed over 1 year. It degrades from 0.38 °C at a lead time of 1 day to 0.90 °C at a lead time of 32 days, largely driven by the uncertainty of the meteorological ensemble forecasts.

In part two, we use Swiss climate projections to obtain future scenarios of streamflow (PREVAH model) and stream water temperature (TFT model). Our results highlight a projected intensification of combined drought-heat conditions under further warming. Events with an average occurrence probaility of 2% over the last 30 years are expected with 26% probability under an additional 2 ºC of warming in Switzerland. This steep increase illustrates the challenges that lay ahead to maintain the services that rivers provide today.