In September, the reservoirs in China’s Henan province dried up, leaving crops to shrivel and forcing some residents to drink from puddles on the ground. In late October, the city of Hyderabad, India, discovered that its water supply might be diverted next year for agricultural uses upstream, leaving some eight million people to wonder where they will find the 190 million gallons of water they need every day.

City officials usually respond to such supply crises by upgrading their water infrastructure, namely, drilling, damming, and laying pipes. Every day, the world’s largest 100 cities move 3.2 million cubic meters of water more than 5,700 kilometers to address local water shortages or problems with pollution. But this is an expensive solution, one that only the wealthiest cities can afford. It also puts city managers at odds with environmentalists, who campaign for restrictions on development to ease pressure on forests and watersheds. Fortunately, it is not the only option.

Nature, it turns out, can play an important – and so far largely untapped – role in water delivery and treatment. Protecting water at its source can be cheaper and more efficient than treating it after it has already been polluted. In a new report, my colleagues at The Nature Conservancy, the C40 Climate Leadership Group, and the International Water Association show that investing in forest protection, reforestation, stream bank restoration, improved agricultural practices, and forest-fire management can reduce the amount of pollutants flowing into supplies of drinking water.

The report, "The Urban Water Blueprint,” analyzes the state of water supplies in 534 cities and 2,000 watersheds to provide a comprehensive overview of the potential natural solutions that can be integrated with traditional infrastructure. The results are compelling. Water quality for more than 700 million people could be significantly improved by adopting conservation practices in watersheds. And at least one in four cities examined would find such interventions financially viable, based solely on savings from avoided water-treatment costs.

In some places, such measures have already been introduced. Farmers near Beijing, for example, have been paid to convert croplands from rice to corn. Rice paddies need to be constantly flooded, and, because they are often located on steep slopes, this leads to significant runoff of fertilizers and sediment. Shifting to corn not only reduces water consumption; it also cuts the amount of pollution that reaches city residents downstream. The program costs about $1,330 per hectare of farmland to implement, but produces $2,020 per hectare of benefits.

In Brazil, a water-conservation fund is working to restore the Cantareira watershed, the source of 50% of Sao Paulo’s water. The area has lost 70% of its original forest cover, and sediment from eroding hillsides has clogged the city’s reservoir, jeopardizing the water supply of Brazil’s largest city. Under the new program, farmers and ranchers are paid $120 per hectare to reforest or terrace their fields. So far, about 3,500 hectares have been planted with trees or put under improved soil-management practices. Reforesting another 14,200 hectares could cut the concentration of sediment in the watershed by half.

The savings produced by these programs should be viewed in the context of the $90 billion per year that cities spend to build treatment plants, pipes, and other components of water infrastructure. The authors of "The Urban Water Blueprint” have calculated that more than $18 billion could be productively directed toward conservation activities, saving cities money and creating a new market comparable in size to the market for the water sector’s existing technologies.

But if these solutions are to be adopted at the necessary scale, environmentalists and city officials alike will have to expand the scope of their traditional activities. Environmentalists will have to embrace the idea that conservation does not only mean protecting pristine landscapes. It also sometimes requires the improvement of lands under production. Indeed, these are the areas where some of the most cost-effective solutions are to be found. Water quality would be improved for more than 600 million people if the farms and ranches operating within their watersheds limited runoff and restored stream banks.

City officials, meanwhile, need to think beyond their municipalities’ boundaries. The 100 largest cities occupy much less than 1% of the planet’s land, but the watersheds on which they depend account for a full 12%. Because many cities share water resources, cross-jurisdictional financing mechanisms and a shared sense of commitment will be needed in order to protect and restore natural sources of clean water. These partnerships will require the cooperation of a broad variety of interest groups, all of which will have to be persuaded to support efforts to improve the water supply. Farmers and ranchers should be at the top of the engagement list.

Giulio Boccaletti is the Global Managing Director for Water at The Nature Conservancy.