They are two of the biggest cities in the world and both are set to grow significantly over the next decade, yet Mumbai and São Paulo are still unable to supply the clean and safe water residents need.



Precarious water supplies are nothing new in São Paulo but today Brazil’s largest city is suffering the worst drought since records began in 1930. This urban crisis is being replicated across the world, with one in four of the biggest cities experiencing water stress.

Much of the focus on water scarcity has been on agriculture, which accounts for the lion’s share of global water consumption – more than 90% of the total on average. Yet the fundamental reason for the precipitous drop in water supplies is the explosive expansion of cities and their growing demand for high quality water.

From grey to green

The conventional response is to invest in grey infrastructure such as treatment plants, reservoirs and distribution systems. These remain an important part of the solution.

But there is also a growing recognition of the role natural infrastructure plays in addressing water stress. Upstream approaches such as restoring river banks and terracing hillsides to reduce nutrient and sediment runoffs are investments in a better functioning environment that can improve water quality and lower treatment costs.

According to research conducted by The Nature Conservancy (TNC), reducing sediment and nutrients in water supplies by 10% could lead to a reduction of about 5% in treatment costs. Furthermore, we calculate that if every conservation method – forest protection, reforestation, agricultural best management practices, riparian restoration and forest fuel reduction – was applied to cities around the world, total savings would amount to $890m (£579m) per year.

This makes a compelling case for local governments and water utilities to invest in this kind of conservation, enabling utilities to reduce their capital expenditures over time by using cheaper treatment technologies rather than upgrading to more complex, expensive ones.

The river Tansa, north of Mumbai, supplies water to the city, which has been identified as one where water quality could be improved most by restoring river banks and working with farmers to reduce runoff. Photograph: David Levene/Guardian

Take India. More than 40% of the population lives in cities – this is set to double to 814 million by 2050. Prime Minister Narendra Modi recently announced a set of initiatives, among them the Smart Cities Mission, to make Indian cities globally competitive growth centres. This will involve a modernisation programme in which providing clean water will be high on the agenda.



Our research has identified 17 Indian cities where conservation strategies could help the country meet its development goals. We believe that Mumbai is one of the five cities in the world where water quality could be improved most by restoring river banks and working with farmers upstream to reduce runoff.

Scaling up

In all, a quarter of the more than 500 cities TNC studied could make a positive return on investing in watershed conservation. Among the leading cities that could benefit is São Paulo. Improving farming techniques, protecting forests, reforestation and restoring riverbanks in the watersheds of the Cantareira water system on which the city depends together have the potential to cut sediment and pollutants in the city’s water supply by 10% and reduce treatment costs by 5%.

In 2005, a small community in the Cantareira watershed, Extrema, launched an effort to counter the effects of deforestation and the subsequent increase in runoff and sediment. The local government, in collaboration with the São Paulo watershed committee and Brazil’s federal government, set up a programme that pays landowners $120 per hectare to reforest land. So far, 3,500 hectares have been reforested. Analysis completed by colleagues in Brazil suggests that restoring an additional 14,200 hectares of deforested areas and preventing erosion on just over 2,000 hectares within certain river basins could cut the concentration of sediment of the system in half. While results won’t be seen overnight, this restoration has the potential to improve water supplies and benefit more than 13 million people who live in the São Paulo metropolitan region and Extrema.

The potential for upstream nature conservation to protect water quality in cities is not limited to developing countries. The US cities of Santa Fe and Albuquerque, both in New Mexico, have suffered from forest fires, increasing the chances that subsequent rains will carry topsoil, debris and ash into rivers and reservoirs. One answer has been to ecologically restore the watershed forests to reduce the amount of combustible material and lower the risk of fires.

To realise the potential benefits of conservation, it is essential to forge effective partnerships operating on the ground at the local level between official bodies, NGOs, community representatives and businesses. Cities are powerful, but they must also be stewards of the hinterlands from which their water flows.