Water, more or less: Do more forests mean more water?

The linkages between forests and water are complex and often debated. In the lead up to International Day of Forests 2016 and World Water Day 2016, IUCN's Becca Andrasko explores how to optimize these connections for sustainable ecosystems and healthy communities now and into the future. 

Indonesia has almost 6 per cent of the world's water resources and about 21 per cent within the Asia Pacific region.

“It frequently happens that in spots where forests have been felled, springs of water make their appearance, the supply of which was previously expended in the nutriment of the trees…Very often too, after removing the wood which has covered an elevated spot and so served to attract and consume the rains, devastating torrents are formed by the concentration of the waters.” – Pliny the Elder, Natural History

Although Pliny’s knowledge of how forests affect water supply was limited, his observations were correct. More importantly, he was one of the first to bring attention to a debate that has persisted over the millennia since his writing: Does the presence of a forest increase or decrease the available water supply?  

On the one hand, adding more forests – afforestation – can be seen to decrease the amount of available water in a region. Soon after planting forests in an area, more water is absorbed into biomass growth and soils, resulting in less water runoff into streams and rivers and decreased water levels. On the other hand, afforestation can cause more water to be evaporated from the landscape into the atmosphere, leading to higher amounts of rainfall and more water overall.

The debate, then, is about scale: large or small spatial scale, and long term or short term. In a recent literature review (Filoso & Weiss, A Systematic Review to Assess the State of the Art on the Relationship between Forest Restoration and Water Yield, in press), the authors found that roughly 50 percent of papers arguing that forests have a positive response on water yield were focused on large-scale areas; while a majority of papers focused on small-scale or local areas found a negative result on water yield. And the authors conclude that the expansion of forests is likely to negatively affect water quantity for at least the short term, and within a relatively small spatial scale.  

But when making the link between forests and water supply, there are other considerations that must be weighed. Afforestation greatly reduces annual rainwater runoff, leading to less soil erosion and less unexpected flooding. In addition, more forest cover can result in lower surface temperatures in the region, as trees provide a cooling effect on soils and landscapes. This can have long-term regional or even global implications and contribute to climate change mitigation. Forests also provide many important ecosystem goods and services for communities, including marketable timber, biofuels and food.

Forests improve water quality. Riparian forests in particular play an important role in filtering sediments and pollutants from rainfall and urban runoff, which has been linked to reduction of sediment in water bodies and is especially advantageous when these water bodies supply drinking water. Forested lands can dramatically decrease the amount of filtering and pumping that municipal treatment centers have to do, and since nearly one-third of the world’s largest cities get their water from forested protected areas, forested watersheds can save cities millions of dollars every year.

Conversely, deforestation has well-demonstrated negative consequences. In the Amazon rainforest, for example, between 35 and 50 per cent of mean annual rainfall is recycled through evapotranspiration. Large-scale deforestation in the Amazon has the potential to decrease rainfall severely, which would jeopardize local people who rely on the Amazon as the largest source of freshwater in the world. The Amazon is so influential on meteorological and hydrological events that widespread deforestation and degradation of the forest has the potential to cause massive problems throughout the world, such as disrupting atmospheric moisture patterns throughout the South American continent and contributing to global warming trends.

Whether forests increase or decrease available water supply is of particular importance to practitioners of forest landscape restoration (FLR). From a large-scale, long-term perspective, improving the quality, scale, and type of forests (one of many possible FLR interventions) is likely to lead to better access to water across a broad region. And on a small-scale, shorter-term perspective, effective FLR considers the need for downstream communities’ access to reliable sources of water and water-related services.

For example, FLR is a demonstrated tool for slowing those negative effects of deforestation in the Amazon. IUCN and partners are working in the Brazilian Amazon to restore forests and enhance ecosystem services, and the increased or restored forest area is expected to bring increased water yield throughout the region. Futhermore, Brazil’s Atlantic Forest Restoration Pact has committed to restoring 1 million hectares of forest land by 2020 and 15 million hectares by 2050 through a Bonn Challenge pledge.

In China, IUCN and partners have been using FLR to improve water quality and availability since 2007. The Miyun Watershed Forest Landscape Restoration project is working to address Beijing’s acute need for a more reliable, clean water supply to sustain the city’s 21 million residents. Project managers have identified priority restoration sites within the Miyun watershed, many of which are located on the mountains surrounding the Miyun Reservoir.

Restoring the degraded forests in these mountains “can mean the difference between taps that run and taps that don’t,” because mountain forests are able to trap sediment and soil with their roots that would otherwise erode into the reservoir below. This results in higher quality water, as the water contains fewer pollutants, and more available water, as mountain forests tend to have a higher rate of of evapotranspiration than forests elsewhere. And this restoration provides other numerous benefits for the region’s ecosystems and communities.         

In the end, there is no debate; forests are good for landscapes, people, and water. The question is how to value and measure these ecosystem services. Should we value ecosystems for what they can do and what they can give us in the immediate future? Or should ecosystems be valued for the long-term benefits they provide? Answers change depending on who is doing the asking. For some villagers, forests’ immediate benefits, such as firewood for heating their homes and timber to sell to feed their children, outweigh uncertain future water yield.

Both the short-term and the long-term needs of communities and ecosystems must be balanced – and this is the FLR practitioner’s goal. Forest landscape restoration can improve both water quality and quantity on a large, relatively long-term scale, as well as providing other benefits to communities and the environment as a whole.

As for the debate persisting since Pliny’s time, the answer is about balance and perspective. And IUCN is working to bring these qualities to ecosystem analysis through its FLR projects around the world.

Work area: 
Forests
Water
Forests
Forest Landscape Restoration
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