As cities around the world struggle to address water shortages, Dhaka, Bangladesh, is poised to become one of most populous cities in the world that requires new buildings to collect rainwater on their roofs. Rainwater harvesting is an old idea, but laws requiring the practice in urban areas are only starting to become mainstream. Bangalore, India already requires privately owned buildings to collect rainwater, and in western states in the U.S. are beginning to relax rules that make it illegal. In Dhaka, the city is planning on modifying its building code by the end of this year to make the change.
Why it’s a good idea. Growing cities are straining water resources and droughts can shut off water across a city, yet rainwater in urban areas can be a burden, rather than a boon. Floods of storm water run-off can overwhelm sewer systems, change the flow patterns of surface water, and impact animals and plants in the surrounding area. Meanwhile, Dhaka's population—more than 15 million people—requires 2.4 billion liters of water a day, but the city can only produce 2.1 billion liters. Stored rainwater can provide an alternative to polluted rivers and dwindling groundwater supplies for drinking.
Storing water on rooftops can also help cool a city: dry, hard surfaces absorb heat and release it slowly, creating a bubble of hot air around a city and increasing air conditioner use. And provide water to urban households from a centralized system requires about five times more energy than implementing rainwater harvesting, according to several studies. Climate change could make both droughts and floods more common: in both cases, rainwater harvesting can help mitigate the impact while drawing down carbon emissions.
How it’s working elsewhere. The Caribbean island of St. Thomas has required all residential buildings to collect rainwater for years through storage systems positioned under roofs. Although as a rule rainwater is clean when it falls, it can pick up contaminants as it passes over roofs and through pipes, and as it sits in collection barrels. In St. Thomas, testing has found contaminants that exceed water standards set by the Environmental Protection Agency, and the water, if not treated, has can only be used for non-drinking purposes like landscaping or washing cars.
In Australia, droughts have pushed some states to require that new houses include additions like rainwater tanks for toilet flushing or to offer incentives for installing them. In the United States, Sante Fe County has required new buildings to include rainwater collection systems. But despite the water shortages and droughts that have struck the American West, New Mexico is one of the few states that has allowed its residents to take advantage of these systems.
Why it’s complicated. Rainwater harvesting was illegal in Utah until 2010 and in Colorado until 2009. Colorado now allows harvesting only when landowners already own a well or have the right to use one on their property. Earlier this year, a bill in the Arizona state assembly to allow rainwater harvesting projects was reduced to a proposal that would have formed a commission to study the idea, and even that was killed.
The problem is the system of water rights that guarantees downstream residents a portion of the water that originates further up. In this system, individuals harvesting rainwater are stealing water from its rightful owners simply by preventing it from entering the watershed. But a 2007 study found that most water—97 percent of it—was absorbed by plants or evaporated before it could reach residents downstream. Now, Colorado is conducting a pilot program that could convince state lawmakers to show a bit more flexibility about who own the rain.