Securing Water for Urban Farms

With More than Half the World's Population Now Living in Cities, Getting Food to Where the People Are - and Getting Water to the Farmers Who Grow that Food - is an Increasingly Big Challenge

Surprisingly large share of the world's cropland is found not in rural areas, but within cities and their immediate surroundings. Some 456 million hectares (1.13 billion acres) of land is cultivated directly in cities or within 20 kilometers (12.4 miles) of an urban perimeter, according to a recentstudyconducted by researchers at the International Water Management Institute in Colombo, Sri Lanka, the University of California-Berkeley and Stanford University, and published in the journal Environmental Research Letters .

That's an area about the size of the European Union. And it doesn't include the small backyard or rooftop gardens from which a growing number of city dwellers harvest vegetables and other foods for their own consumption.

Even within the urban core, a large amount of farming is taking place. Some 67 million hectares (166 million acres) - including 11 percent of the world's irrigated land — are located right within cities. And these lands often produce more per acre than their cousins in the countryside.

As we attempt to reduce the environmental footprint of our food system, managing urban water supplies in ways that meet the needs of urban farmers could be key.

Some cities are awakening to the idea that capturing stormwater and recycling wastewater can boost local water supplies. Along with conservation measures, these local sources can help meet current and future demands without the need for costly long-distance water transfers. Los Angeles, for example, has set a goal ofhalving its relianceon imported water within a decade by implementing measures such as theseAs cities creatively re-balance their urban water budgets, the opportunity to deliver water supplies to urban farms is potentially a multiple win-win that shouldn't be overlooked.

Indeed, designing urban water and wastewater systems to sustainably supply these urban farmers can help solve some of our vexing water, food, energy and climate challenges.

Cities, for example, can direct stormwater runoff to urban farms and gardens for irrigation use (with or without treatment, depending on conditions) and simultaneously reduce the sewer overload and pollution problems that often come with uncontrolled runoff.

The city of Cleveland, Ohio, for example, has established a separatezoning code for urban agriculturefor purposes that include facilitating local food production and improving stormwater management. And New York City has providedgrants to four urban agriculture projectsthat aim to improve stormwater management.

Recycling partially treated wastewater to urban farms can also make good sense. Most of the "pollutants" in wastewater are nutrients that came from soils and can beneficially be returned to soils. Farmers spend heavily on chemical fertilizers to give their crops the nitrogen, phosphorus, and potassium that household wastewater contains in significant amounts.

By using urban water supplies twice, once for household use and again for irrigation, would-be pollutants can become valuable fertilizers.

Decentralized wastewater treatment and recycling, whether through onsite treatment units or so-calledsewer mining, could allow farms to get a local, renewable supply of water while reducing energy use and wastewater treatment costs.

In poorer parts of the world that lack adequate sanitation, however, the use of untreated wastewater for farm irrigation (while a valuable supply of irrigation water) can pose health risks.

In cities of the West African nation of Ghana, for example, some 2,000 farmers supply about 800,000 people with vegetables that are often irrigated with polluted water. According toPay Drechsel, a scientist with IWMI and a co-author of the urban agriculture study, as much as10 percent of household wastewaterin the capital city of Accra is reused on urban vegetable farms, but without first receiving treatment.

Source: NatonalGeographic

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