Too Much, Too Little: Floods, Heavy Rain and Drought

Warm air can hold more water vapour: roughly 7% more per 1 °C of warming (the Clausius–Clapeyron relation). When that moisture rains out, downpours become more intense.

Swiss measurements confirm the physics:

• The heaviest daily rainfall of the year is about 11% more intense than in the early 20th century, and heavy-rain days that occur about four times a year have become about 25% more frequent (MeteoSwiss analysis, 1901–2023).
• Short bursts intensify fastest: since 1981, 10-minute rainfall intensity has risen by about 4.7% per decade, 6-hour intensity by about 2.4% per decade.
• The Climate CH2025 scenarios expect extreme precipitation to intensify in all seasons, scaling close to ~7% per degree of local warming — with Switzerland already about +2.9 °C above pre-industrial levels.

Why it matters in a city: short cloudbursts overwhelm sewers and sealed surfaces before rivers even rise. This is called surface runoff (Oberflächenabfluss). The national surface-runoff hazard map (BAFU, 2018) shows that about two-thirds of Swiss buildings are potentially exposed, and surface runoff causes up to half of all flood damage.

The Sihl flows from the Sihlsee through the Sihl valley and passes directly underneath Zurich main station (HB) before joining the Limmat.

Fact	Value
Potential damage of an extreme Sihl flood in Zurich	up to CHF 6.7 billion
Area of city centre that could have flooded in 2005	about 5 km², up to 0.5 m deep
Sihl peak in August 2005	290 m³/s
Sihl peak in July 2021	248 m³/s

A near miss in 2005

In August 2005, floods caused about CHF 3 billion of damage across Switzerland, but only about CHF 15 million in Kanton Zürich. The rain centre happened to sit over the Bernese Oberland. Had it sat over the Sihl catchment, large parts of the city centre — including the HB track field — would have been flooded.

Zurich has flooded before: in 1846, 1874 and 1910, when large parts of the city and the plain towards Schlieren were under water. Today far more buildings, infrastructure and people are concentrated in the same area, so the potential damage is much higher.

The solution: send the water into the lake

The canton is building the Sihl relief tunnel (Entlastungsstollen Sihl–Zürichsee):

• About 2 km long, from Langnau am Albis to Lake Zurich at Thalwil, 6.6 m inner diameter.
• Starts diverting at a Sihl flow of 250 m³/s, can divert up to about 330 m³/s, protecting against Sihl peaks of up to 600 m³/s.
• Cost about CHF 175 million; construction began in March 2022; operation planned from 2026/27.

It is part of a wider package: the Sihl riverbed under HB was lowered in 2007, a driftwood rack catches logs that could block bridges, and since 2018 a forecast system allows the Sihlsee reservoir to be lowered 3–4 days before an expected extreme storm.

Climate change does not only bring more water. Hot, dry summers such as 2003, 2015, 2018 and 2022 showed the opposite problem.

Rivers

• In August 2018, parts of the Rhine (bordering Kanton Zürich) exceeded 27 °C. About 1,000 kg of dead fish, mostly graylings (Äschen), had to be removed. In 2003, about 90% of the grayling stock in that stretch had died. Graylings are cold-water fish and suffer severe stress above about 23–25 °C.
• In 2015 and 2018, many small streams in the canton dried out completely, and thousands of fish had to be rescued and relocated.

Lake Zurich

• The lake's surface water in October has warmed by about 0.5 °C per decade — roughly +4 °C over 80 years of measurements (BAFU).
• A lake needs cold winters so that surface and deep water mix, bringing oxygen to the bottom. From 2009 until winter 2020/21, Lake Zurich never mixed completely. The deep water stayed oxygen-free, and phosphorus was released from the sediment.

Drinking water

Zurich is relatively well protected:

• The city's water comes from about 70% lake water, 15% groundwater and 15% spring water. Lake water is drawn from about 30 m depth, where heat and drought have little effect.
• In the canton, more than 95% of people are connected to supply networks with two or more sources.
• However, in hot summers such as 2026, spring yields drop, and farmers need emergency irrigation permits.

The national hydrological scenarios (Hydro-CH2018) project how runoff will change by the end of the century (2070–2099 vs 1981–2010):

Change	Without climate mitigation	With mitigation (Paris-compatible)
Winter runoff	+10 to +50%	0 to +20%
Snowmelt runoff	−30 to −60%	0 to −30%
Summer/autumn runoff	−30 to −50%	much smaller decrease

More precipitation falls as rain instead of snow, so water runs off in winter instead of being stored until summer. Glaciers, the second "water tower", are shrinking fast: Swiss glaciers lost about a quarter of their volume between 2015 and 2025, and more than 5% in 2026 alone — almost 20% in five years (GLAMOS). Over 1,000 small glaciers have already vanished.

For Zurich, this means: wetter winters with higher flood potential, and drier late summers with warmer, lower rivers.