Healthy watersheds protect water quality, because trees and other plants filter water, prevent erosion, and capture silt and sediment. But what about the quantity of water? It might seem that trees would soak up available water, leaving less for human use. However, a recent review of studies on forest restoration reveals that the story is more complex than that – and that there is a lot that we simply don't know yet about how restored forests may affect water yields.
The new report, “Impacts of forest restoration on water yield: A systematic review,” examined hundreds of academic studies on how reforestation (also called forest restoration) impacts the quantity of water in local streams and surface water systems. Its findings are surprising: although 80 percent of the relevant studies found evidence that forest restoration reduced water availability on an _annual _basis, the data tell a different story when looking at water supply during different seasons and considering overall benefits.
Of the studies reviewed:
40 percent found that forest restoration has a positive or negligible impact on water availability during the dry season (when it is often needed most). This is because forests can act like sponges, storing water during the wet season and releasing it slowly to streams, wetlands, and reservoirs when water is scarce.
83 percent found that forest restoration increased the amount of water entering and being absorbed by soil – which can support agriculture productivity and also increase the release of stored water into streams during in dry seasons.
82 percent found that forest restoration led to reduced flooding intensity or frequency, an important benefit to protect communities in a changing climate.
The survey shows that the current body of literature on forest restoration and water supply is incomplete. It omits key geographies, fails to evaluate reforestation efforts specifically aimed at improving water supply, and doesn’t consider the effects that healthy forests and sustainably managed landscapes can have years down the road. These research gaps make it difficult to extrapolate the water supply impacts of forest restoration in any given location.
Take Latin America, for example. Less than 8 percent of studies about water supply and forests were from Latin America, and less than 10 percent of studies were from the tropics and subtropics. Many forest restoration projects have been proposed over the next several decades in these areas, and they are arguably among the most important areas for proving the relationship between forests and water supply.
Could “green” natural infrastructure – like restored forests—be part of the solution? Facing significant research gaps, it is hard to know for sure what the impacts of forest restoration will be on water quantity in Latin America. But there are many reasons to believe that forest restoration could be a viable water management strategy in this region. Future water management strategies in Latin America should consider the following:
Forest Restoration Projects Can Be Designed and Amended to Enhance Water Supply
Not all forest restoration projects are created equal; they can be specifically designed to enhance water benefits. Research groups like those at the University of São Paulo are testing and developing forest restoration techniques intended to promote water recovery, such as preparing soil and prioritizing specific forest types. Water managers and their partners can draw on these techniques to help increase the likelihood that forest restoration will lead to water yield improvements in water-stressed areas.
Longer-term restoration studies have found that initial drops in annual water yield due to high levels of water use (transpiration) by trees gradually recover over time—suggesting that any water stress caused by restoration may be temporary, and level out after 15-20 years. For water managers planning decades into the future, the many benefits of forest restoration could outweigh any short-term losses.
Forest Restoration Benefits More Than Just Water Supply
Forests’ sponge effect and erosion control services help shield populations from other negative impacts of climate change beyond water scarcity. This is especially relevant in cities like Sao Paulo, which are predicted to experience a wetter climate, more intense rainfall events and more prolonged dry periods by 2050 as the global climate warms. In fact, some studies suggest that the primary water management challenge of the future for Southeast Brazil will be dealing with floods and heavy siltation due to heavy rains. If this is the case, healthy forests may be one of the most effective strategies for buffering the impacts of both more intense storms and droughts.
Ongoing Natural Infrastructure Projects Can Provide Insights Critical for Water Managers
“Impacts of Forest Restoration on Water Yield: A Systematic Review” calls for further research that focuses specifically on restoration sites that were designed with water supply in mind. Such studies could center on restoration efforts like TNC’s Water Funds, which restore forests upstream of large cities to protect urban water supplies, as well as projects within WRI’s Initiative 20x20.
Managers also need decision-relevant research that blends the best available science with business considerations. In cities in Brazil and Mexico, WRI’s Natural Infrastructure for Water team is partnering with TNC, IUCN, Boticario Group Foundation, FEMSA Foundation and IBIO to evaluate the business case of investing in forest restoration as a solution for water insecurity. This work aims to identify cost-effective and robust solutions that make use of natural infrastructure such as forests.
As water risks rise, forests need not be feared as competition for scarce water resources. Instead, water managers should take into account the wide variety of long-term economic and environmental benefits that forests provide, and seek out the latest research on projects that share their goals.