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Prepare for Water Day Zero

Earlier this year alarming headlines blared that Cape Town, South Africa, was headed for Water Day Zero — the date when the city's taps would go dry because its reservoirs would become dangerously low on water. That day — originally expected in mid-April 2018 — has been postponed until at least 2019, thanks to water rationing and a welcome rainy season. But the conditions that led to this desperate situation will inevitably occur again, hitting cities all over the planet.

As the climate warms, extreme droughts and vanishing water supplies will likely become more common. But even without the added impact of climate change, normal rainfall variation plays an enormous role in year-to-year water availability. These ordinary patterns now have extraordinary effects because urban populations have had tremendous growth: by 2050 the United Nations projects that two thirds of the world's people will live in cities. Urban planners and engineers need to learn from past rainfall variability to improve their predictions and take future demand into account to build more resilient infrastructure.

How did Cape Town get into a Water Day Zero situation? The city gets its water from six reservoirs in Western Cape province, which usually fill up during the rainy season, from May through August. But since 2015 the region has been suffering from the worst drought in a century, and the water in those reservoirs dwindled (减少) dangerously. Compounding the problem, Cape Town's population has grown substantially, increasing demand. The city actually did a pretty good job of keeping demand low by reducing leaks in the system, a major cause of water waste, and has even won awards for its conservation policies. But the government of South Africa was slow to declare a national disaster in the areas hit hardest by the drought, paving the way for the recent crisis.

Fortunately, steps can be taken to avoid urban water crises. In general, a combination of approaches that relies on multiple water sources is probably most effective. Cape Town has already begun implementing a number of projects, including tapping groundwater and building water-recycling plants.

The global community has an opportunity right now to take action to prevent a series of Water Day Zero crises. If we don't act, many cities may soon face a time when there isn't a drop to drink.

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