Want clean water? Save your forests!

Cities can ensure they have a cheap and abundant supply of clean water by protecting and regenerating forests in their surrounding watersheds, according to a major analysis just undertaken in Malaysia.

Forests are a vital source of clean water -- and important for lots of other reasons too.

Jeffrey Vincent from Duke University, USA and his colleagues have just published the largest cost-benefit analysis ever conducted in the tropics, and they find that pristine and even selectively logged forests are very cost-effective ways to produce clean, drinkable water. 

Vincent and colleagues ran their analyses under a wide range of scenarios.  They used as a baseline the costs of standard water-treatment plants, which are often required to make polluted water safe to drink.

The authors found that the relative advantages of forests depend on local circumstances, with the financial benefits being greater in some situations than others. 

Also, if one factors in profits that can be made by exploiting the forests -- such as by converting them to agriculture -- then the numbers could change. 

The problem, of course, is that waters that drain off of agricultural lands are often polluted by fertilizers, pesticides, and organic wastes, making expensive water-treatment necessary.

The authors argue that, beyond water purification, intact forests have many other financial and non-financial values. 

For instance, they store large stocks of carbon, and thereby reduce greenhouse gas emissions. 

They also can harbor enormous biodiversity while helping to attract substantial income from ecotourism.  Remarkably, it's currently estimated that protected areas across the world attract some 8 billion visits annually, underscoring their financial value for local communities.

Furthermore, via the process of evapotranspiration, forests emit enormous quantities of water vapor.  For example, one-third to one-half all the rainfall that falls on a tropical forest is quickly recycled back to the atmosphere, as water vapor.

This water vapor (plus natural organic aerosols emitted from forests) help to form clouds, which in turn reflect solar radiation back into outer space, thereby reducing global warming. 

Forests are natural cloud-makers.

Such clouds also help to produce life-giving rainfall during the dry season -- when forests are most drought-stressed and prone to fires.

Finally, forests are very good at reducing destructive floods.  They tend to act like giant biological sponges, trapping water and releasing it slowly, thereby reducing downstream flooding.  Especially in areas where forests are denuded, flooding can cause billions of dollars in damage and costs thousands of lives each year.

The conclusion: It's increasingly becoming apparent that it's smart to conserve pristine and selectively logged forests -- even when one uses just hard, cold economic logic.