Better Forests, Better Cities

Chapter 3


Conserving, restoring and sustainably managing forests in and around cities can provide cleaner water, help reduce flooding, and protect water supplies. The world’s large, intact forests also contribute to the maintenance of global hydrological cycles, moving water thousands of kilometers around the world and producing rainfall in important agricultural and urban areas.



Cities around the world face multiple challenges in providing residents with adequate, clean water (Juno and Pool 2020). Extensive urbanization and population growth, land conversion, and water diversion have put pressure on local and regional water supplies in and around cities. Ongoing climate change threatens to disrupt water cycles even further, increasing the incidences of both drought and flooding. Water-related challenges have an impact on the economy, human health, and overall city resident well-being. These are the main water-related challenges for cities:

  • Too dirty. Many cities are unable to provide their residents with an essential and basic service: a reliable supply of clean drinking water. Contaminated or unclean drinking water causes severe health issues in many regions. Water treatment facilities and infrastructure are costly to establish and maintain.
  • Too much. Flooding threatens the lives and safety of 15 million people as a result of coastal flooding and 132 million people as a result of riverine flooding, putting at risk $177 billion and $535 billion in urban property, respectively (Ward et al. 2020). Safeguarding water and preventing flooding are essential services to urban residents and a key agenda item for many city leaders.
  • Too little. Water scarcity can be caused by extreme drought, depletion of groundwater, or reduced river flows. Many cities around the world—especially in arid regions—face seasonal or year-round issues with water supply, which are likely to increase as populations grow.
  • Too erratic. City residents—like much of the world—are also vulnerable to the increasingly erratic weather patterns, including longer and more intense droughts and heavy rainfall (Seneviratne et al. 2012). Variability and unpredictability in precipitation and water supply create additional challenges for urban leaders. Shifting precipitation patterns at the global scale—driven by climate change and land-use change—may exacerbate existing issues related to flooding or water scarcity.

To face these water challenges, leaders—from China to the United States to Brazil—are embracing NBS such as forests to complement traditional “gray” city water infrastructure solutions. Increasing areas of forest and green space in strategic locations in cities can reduce burdens on aging or overburdened water infrastructure while delivering key cobenefits. And in contrast to gray infrastructure, green infrastructure can appreciate in value over time.

Scientific evidence suggests that forests and trees can help address all four of these challenges. In this section, we outline the forest benefits:

  • Provide cleaner water. Forests help to filter sediments and pollutants from the water upon which cities rely. Forested watersheds, in particular, provide higher-quality water than other land uses because forests filter pollutants and prevent erosion and sedimentation of streams and rivers (Brauman et al. 2007; Calder 2007).
  • Help reduce flooding. Within cities, forests intercept rainwater and reduce the burden of stormwater on built water infrastructure, reducing the risk of combined sewer overflows and flooding (Figure 8). Trees and other vegetation in bioretention areas, green roofs, and bioswales can complement traditional “gray” water infrastructure (i.e., engineered systems using materials such as concrete and steel) solutions for stormwater management in urban areas (Berland et al. 2017). Forested watersheds regulate flows and help prevent flooding and landslides.
  • Protect water supply. Forests in and around cities can replenish groundwater supplies and help sustain rainfall patterns and regulate quantity across seasons (Neary et al. 2009; Ellison et al. 2017).
  • Support healthy hydrological cycles. Forests play a vital role in global regulation of water cycling, which supports sufficient precipitation in cities and in agricultural regions key to supplying cities with food. Forests (especially large, intact rain forests) can affect hydrological cycles and rainfall patterns hundreds to thousands of miles away by “recharging” atmospheric moisture supplies (Ellison et al. 2017). This linkage between faraway forests and climate hundreds or thousands of miles away has substantial implications for urban residents and global food security (see Box 1 in Section 4). And because of the key role forests play in the global carbon cycle, climate change mitigation by forests may also help to prevent further shifts towards extremes.

Why context matters

Forest benefits related to water will vary with the local context. The effects of watershed deforestation on water quantity, including seasonal flow regulation, depend on the local geographical context, climate, and the type of forest. The impacts of reforestation on water also depend on the context, such as species used and other elements of planting design. Reforestation may even negatively affect water availability in the near term (Filoso et al. 2019). Additionally, the effects of climate change may alter forest-water interactions (Jones et al. 2020).

Figure 8 | The Role of Trees and Forests in the Water Cycle

Notes: Simplified hydrological cycle, showing (1) the role of forests and other vegetation in intercepting and recycling precipitation, which is then transported across landscapes, and (2) the role that forests play in promoting infiltration and deep infiltration into groundwater sources.

Source: Authors. Adapted from Ellison et al. (2017) and Bonnesoeur et al. (2019).

Water Challenge 1: Too Dirty


Most cities draw water from local watersheds. Development, industrialization, land-use change, and deforestation in catchment areas above cities can lead to problems related to sedimentation, eutrophication, and contamination by pollutants—including sewage, microplastics, hormones, and other pharmaceutical products and chemicals from mining processes—which all translate to reductions in water quality (McGriff 1972). Combined, these issues may increase burdens on water filtration and treatment facilities, passing on a slew of added considerations and unexpected costs to cities and water utilities.

What roles can forests play?

The protection of water quality—a measure of the pollutants, nutrients, microbes, and sediment present—is an important benefit of both inner and nearby forests (Brauman et al. 2007). Watersheds covered by forests tend to have superior water quality and less sedimentation than degraded watersheds and can also reduce water treatment costs. In fact, 33 of the world’s 105 largest cities currently rely on forested protected areas to supply their water (Dudley and Stolton 2003). City source watershed degradation is widespread globally, with 9 in 10 watersheds having lost significant amounts of natural land to agriculture and urbanization, resulting in increased water treatment costs for 1 in 3 large cities globally (McDonald et al. 2016). This close interconnection means that clearing and degrading forests in watersheds can affect municipal water supply.

Forests support water quality in urban areas and watersheds by the following actions:

  • Increasing infiltration and reducing rates of erosion, thereby preventing sedimentation of streams and rivers (Neary et al. 2009; Carvalho-Santos et al. 2014; Tellman et al. 2018), reducing treatment costs
  • Absorbing or transforming contaminants from both soil and water (e.g., heavy metals, hydrocarbons, pesticides; Brauman et al. 2007; Luqman et al. 2013)
  • Tightly cycling nutrients (Neary et al. 2009), reducing the likelihood of eutrophication (i.e., excessive concentrations of nutrients in a body of water) that can contribute to algal blooms and “dead zones” in water bodies
  • Stabilizing riverbanks and steep slopes (Brauman et al. 2007), which also reduces soil erosion and sedimentation of waterways
  • Shading streams and rivers, which maintains conditions for aquatic biodiversity (Richardson and Béraud 2014)

Forest loss, however, can impair water quality. Compared with forests, industrial or agricultural land uses generate pollution and reduce the landscape’s capacity to intercept pollution and sediment (Postel and Thompson 2005). Grazing, logging, and the construction of roads may all further degrade forest ecosystems and can dramatically increase runoff (Brauman et al. 2007; Bonnesour et al. 2019).

Inner forests

Trees can be incorporated into various “green infrastructure” or hybrid “green-gray” systems designed to capture and filter stormwater. In doing so, they can reduce sedimentation and pollution carried in surface runoff (Browder et al. 2019). For example, in Portland, Oregon, urban green infrastructure reduced flooding in the city by up to 94 percent and filtered 90 percent of pollutants, leading to $224 million in water infrastructure savings (City Parks Alliance n.d.). Trees also played an important role in Philadelphia’s Green City, Clean Waters initiative (Case Study 2).

Targeted tree plantings may offer benefits in highly contaminated urban areas. In a process called phytoremediation, targeted plantings on contaminated urban lands may offer a low-cost method to reduce soil and water contamination while providing other cobenefits, such as increased aesthetic appeal (Song et al. 2019).

Case Study 2 | Philadelphia’s Green City, Clean Waters Action Plan

Philadelphia is one of the largest cities in the United States. After significant population decline during the 20th century, the city is once again growing. To accommodate a larger population and reduce burdens on water infrastructure, Philadelphia is leveraging its urban forest.

Philadelphia’s contracting budget and aging infrastructure—including one of the oldest sewer systems in the nation—posed a particularly formidable challenge to the Philadelphia Water Department (PWD).a With increasing impervious surface cover, the city especially struggles with combined sewer overflows.

The Green City, Clean Waters Action Plan

Rather than invest in a tunnel below the Delaware River at a cost of up to US$10 billion to manage its stormwater,b the city chose to focus its efforts on restoring its waterways and on large-scale implementation of green infrastructure to reduce pressure on its stormwater systems and reduce risks related to its combined sewer overflows, reaching an agreement with the U.S. Environmental Protection Agency to do so in 2011. Green City, Clean Waters is a 25-year implementation plan to realize its sustainable water management vision. PWD plans to invest $2.4 billion—an avoided cost of up to $6.5 billion—in these efforts to reduce stormwater pollution by 85 percent.c After 45 years, the value of benefits to residents will exceed the cost of investment, for example, by increasing property values of homes near restored parks and green spaces by up to $390 million and by sequestering CO2 emissions equivalent to removing 3,400 vehicles from the road every year.d

To ensure adequate buy-in, PWD has partnered with community members and a suite of different municipal organizations, including its parks and recreation department, school district, and planning commission. It also has invested extensively in educational outreach programs and offered technical assistance to private property owners.e More than eight years into this program, PWD has already exceeded the targets it set by installing more than 1,000 “green acres,” which capture runoff from impervious surfaces.

What Roles Do Trees Play in Green City, Clean Waters?

Tree planting and green space restoration play an important role in the program, including being used to

  • improve appearance and manage stormwater around city streets;
  • restore habitat around streams and rivers; and
  • preserve and revitalize open spaces.

Unified Efforts to Expand the Urban Forest

Philadelphia also operates an innovative urban forestry program called TreePhilly,f which aims to achieve a canopy cover of 30 percent in all neighborhoods, a target set by the city in 2009. An interdisciplinary team of researchers found that if this 30 percent canopy cover was achieved, the city could avoid 403 deaths annually, including 244 avoided deaths per year in low-income areas.g With benefits for water quality, property values, jobs for marginalized populations, and human health, Philadelphia’s urban forest management is helping to transition one of the United States’ oldest cities to a green and sustainable future.

Note: For more information about TreePhilly, see

Sources: a. PWD 2011; b. Luntz 2009; c, d. PWD 2011; e. APA 2015; f. TreePhilly n.d.; g. Kondo et al. 2020.

Nearby forests

Water from forested watersheds is typically higher quality than water from watersheds with other land uses, such as agriculture or industry (Brauman et al. 2007; Calder 2007). Protection of forested watersheds can lower the costs of a clean water supply for cities downstream:

  • Forests can act as buffers for bodies of water. The protection or planting of trees between agricultural fields and streams or rivers can reduce surface runoff of contaminants and nutrients into these water supplies (Brauman et al. 2007; Luqman et al. 2013). To provide and conserve habitat for biodiversity and maintain temperatures, riparian buffers of at least 30 m or more are recommended (Sweeney and Newbold 2014).
  • Ecologically sensitive forest management can reduce unwanted inputs into waterways. In forest plantations on former agricultural lands, the adoption of management practices to conserve soil and reduce erosion can reduce sedimentation and nutrient loading into local rivers and streams (van Dijk and Keenan 2007). In Colorado, following two catastrophic wildfires in 1996 and 2002, it was estimated that the inflow of debris and sediment into Denver Water’s main reservoir resulted in $26 million of city expenses to repair damaged water infrastructure and even more investment to restore its source watersheds in an effort to reduce future water treatment costs (Gartner et al. 2013).
  • Upstream forest protection and restoration can reduce costs for water utilities. A cross-city analysis found that upstream forest protection and restoration can reduce costs for water utilities in the world’s 534 largest cities by $890 million per year collectively (McDonald and Shemie 2014). Analysis from Rio de Janeiro suggests that restoring nearby forests could save the city up to $79 million in water treatment costs alone over 30 years (Feltran-Barbieri et al. 2018) and that restoring 4,000 ha of forest in São Paulo’s Cantareira watershed could reduce sediment pollution by 36 percent in 30 years and reduce water turbidity by almost half, resulting in a 28 percent return on investment for the region’s water utility, Sabesp (Ozment et al. 2018). And yet, globally, these cost-saving nearby forests are being rapidly lost. In the world’s major city watersheds, tree cover has fallen from a historical average of 68 percent to 29 percent, according to data from Global Forest Watch Water (Springgay et al. 2019).

Caveats and considerations

Water quality benefits differ with context, and each individual effort to improve water quality that uses forests needs to consider site-based conditions and be designed based on observed, empirical data:

  • Trees can contribute organic matter such as leaves and branches to waterways, which may contribute to excessive nutrient levels in local waterways, possibly decreasing the safety of drinking water (Pataki et al. 2011; Decina et al. 2020).
  • In arid and semiarid regions where ecosystems have been degraded, reforestation of watersheds can slow or reverse salinization, increasing the quality of water resources and potentially making nonpotable water potable (Brauman et al. 2007).
  • Sedimentation of surface waters may increase in poorly managed plantation forests, where erosion is significant due to the lack of forest understory and the presence of roads and heavy machinery (Calder 2007).
  • Benefits can change over time and differ with the age of the forest; for example, younger forests do not provide the same water quality benefits as older, more mature ones (van Dijk and Keenan 2007).
  • Seasonal considerations affect benefits, where the water purification services from trees that go dormant during the dry or winter seasons are not as high as those from evergreen forests or where there is no seasonal change (van Dijk and Keenan 2007).

Water Challenge 2: Too Much


In and around urban areas, flooding—including pluvial, fluvial, and coastal—presents enormous risks to economic security and human safety. Poorly planned urbanization can compound these risks (Jha et al. 2012). Inhabitants of informal settlements are particularly vulnerable to the effects of flooding, both directly and in terms of structural damage (Jha et al. 2012; De Risi et al. 2013). Flooding can also lead to death, injury, water- and animal-borne diseases, and psychological trauma (Ahern et al. 2005). From 1995 to 2015, floods affected more than 2.3 billion people, mostly in Asia (CRED 2015).

The dangers posed by flooding continue to grow. The number of people impacted by river flooding over the next decade is expected to double to 132 million people annually (Ward et al. 2020). It is estimated that flood losses may total $1 trillion annually by 2050 under business-as-usual scenarios and approach $60 billion annually by 2050, even with significant adaptation measures. The most devastating impacts are projected to be in low-lying cities such as Guangzhou, China; and New Orleans (Hallegatte et al. 2013).

The urban environment is prone to problems with flooding:

  • Urban areas contain high amounts of impervious (sealed) surfaces, which means rain events create runoff more quickly and peak runoff (flood) levels are higher (Figure 9; Douglas 2008). These sealed surfaces also decrease groundwater recharge, which in turn reduces regular flow of water in streams (McGriff 1972). Even soils and lawns in urban areas can become so compact that they act as impervious surfaces (Douglas 2008). These sealed surfaces also increase the likelihood of flooding: a 1 percent increase in impervious surfaces can increase the annual flood magnitude by 3.3 percent (Blum et al. 2020). Surface or street ponding and overflow (pluvial flooding) driven by rainfall may contribute to combined sewage overflows or residential flooding (Rosenzweig et al. 2018).
  • Urban areas tend to rely on extensive built infrastructure with a finite capacity. Old or deteriorated infrastructure may be insufficient to manage the increasing variability in precipitation associated with climate change.
  • Urbanization transforms the characteristics of rivers and streams as floodplains in urban areas are typically cleared of vegetation and streams may be channelized, redirected, or buried (Douglas 2008). These highly modified streams surrounded by impervious surfaces often flood quickly.

Figure 9 | Water Infiltration in Natural Areas versus Urban Areas

Source: Authors. Adapted from PWD (n.d.).

What roles can forests play?

Inner forests

The urban forest can reduce stormwater runoff and associated flooding (Figure 10) by

  • intercepting and delaying water during rain events, reducing the amount of stormwater runoff (Berland et al. 2017; Kuehler et al. 2017), potentially reestablishing a hydrological cycle more closely resembling local historical conditions;
  • altering soil water-holding capacities to promote infiltration (entry) and percolation (downward movement) of rainwater in the soil (Berland et al. 2017; Kuehler et al. 2017; Zhang and Chui 2019); and
  • increasing the amount of water returned to the atmosphere by evapotranspiration (Berland et al. 2017; Kuehler et al. 2017; Jones et al. 2020). In doing so, trees free up space in soil, making room for stormwater storage in the future (Berland et al. 2017; Kuehler et al. 2017; Berland et al. 2017). Evapotranspiration also has a cooling effect and helps to mitigate the urban heat island effect (Berland et al. 2017).

As a result, cities with healthy urban forests often enjoy a number of benefits, including the following:

  • Reduced burden on sewer systems. In combined stormwater and sewage systems, heavy rainfall can lead to a combined sewage overflow of thousands or millions of gallons of untreated sewage into nearby lakes and streams, potentially compromising the safety of drinking water supplies and the quality of aquatic habitats. By reducing the total amount of surface runoff during a storm event, the urban forest can help to reduce the likelihood of combined sewage overflows (Berland et al. 2017).
  • Reduced stormwater volume on surfaces. By increasing infiltration, trees and other elements of green infrastructure provide space for rainwater storage, reduce surface runoff and pollutant inputs to local waters, and promote groundwater recharge and increased baseflow (Case Study 3; Zhang and Chui 2019).

Case Study 3 | Integrating Trees and Forests into Infrastructure in Singapore to Catch Rainfall and Reduce Flooding

Singapore has one of the highest population densities in the world.a The city-state lacks the natural water resources to meet the demand of its residents. Since 1961, Singapore has imported up to 250 million gallons of water per day from the Johor River in Malaysia to meet about half of its 430 million gallons per day water demand.b Singapore also has two monsoon seasons per year that pose flood risks but also provide opportunities to harvest and collect rainwater.c

Singapore’s national water agency, Public Utilities Board, has addressed these issues by managing water resources using blue-green infrastructure—which combines vegetation and natural waterflows—to reduce pollutant runoff into waterways, improve sanitation, and create new city green space, transforming the island into an urban water catchment area (Figure CS3.1). This has also helped reduce flood risk and increase water supply.d

Using blue-green infrastructure provides many socioeconomic benefits to Singapore, including the following:

  • Operational and maintenance costs of the naturalized river were 75 percent lower than the concrete canal as stormwater infrastructure.e
  • The naturalized river has a stronger ability to withstand extreme weather events.
  • The surrounding vegetation absorbs rainfall and reduces runoff by about one-third compared to concrete alone, decreasing the flood risk of surrounding residential areas.f
  • The total value of ecosystem services provided by the park and value of recreational space to residents is estimated at US$73 million per year, which is over twice as much as the value of the park estimated without blue-green infrastructure at only $34.5 million per year.g

To date, 28 blue-green infrastructure projects have been implemented around the city. Collectively, they save Singapore $390 million per year in water costs by reducing the need to import water, reducing flood and water treatment costs, and increasing water supply.h

Figure CS3.1 | Before and After Blue-Green Infrastructure Pilot in Bishan-Ang Mo Kio Park, Singapore

Source: ASLA 2016.

Sources: a. Urban Green-Blue Grids for Resilient Cities n.d.; b. PUB 2022; c. Goh et al. 2017; d. Urban Green-Blue Grids for Resilient Cities n.d.; e. Dreiseitl et al. 2015; f. Yau et al. 2017; g. Dreiseitl et al. 2015; h. Kapos et al. 2019.

Figure 10 | Urban Trees and Rainfall

Source: Authors. Adapted from Marritz (2013).

Nearby forests

Outside city boundaries, natural forests can offer protection from small- and medium-sized floods, moderating the severity of impacts on both people and property (Lele 2009; Carvalho-Santos et al. 2014). This is true particularly along rivers and coasts (Lele 2009; Bhattacharjee and Behera 2018). Protection and restoration of forests in upstream areas can help to reduce peak streamflow and thus reduce the risk of river flooding. Trees lower flood risk by increasing infiltration in the soil, storing excess runoff, and slowing the release of water (Gunnell et al. 2019). In this sense, healthy forest ecosystems often act as “sponges” (Laurance 2007). Conversely, some research has linked forest loss to increased flooding frequency (Bradshaw et al. 2007), but more research is needed to better substantiate the link. Forested riparian zones (the areas immediately adjacent to rivers) and floodplain forests can also help to stabilize banks and restore natural river flow, further reducing the chance of flooding (González et al. 2017). Mangroves play an especially important role near coastal cities (Case Study 4).

Caveats and considerations

Urban leaders seeking to maximize the stormwater benefits of their urban forests should consider the following:

  • Built environment modifications. To address urban flooding, it may also be necessary to increase the area of permeable surfaces and to promote connectivity between stormwater retention areas (Phillips et al. 2019).
  • Synergies with other NBS. Strategically incorporating trees into other green infrastructure can make these interventions more effective. For example, trees planted in bioswales, green roofs, and bioretention basins may help to address pluvial flooding and restore predevelopment hydrological function (McDonald and Shemie 2014).
  • Limitations on capacity. Although green infrastructure and urban forests can relieve some of the pressure on traditional infrastructure, cities will still need to consider multiple options to handle excess water—especially related to coastal and river flooding from climate change.

Flood prevention benefits of nearby forests are limited. Although existing forests can attenuate some flood-related risks, forest restoration is unlikely to have a significant impact on large-scale floods or extreme weather events, which can quickly overwhelm a forest’s absorption capacity (Calder 2007; Ellison et al. 2017).

In some contexts, afforestation (planting trees where forests historically did not occur) with plantation forests can also reduce peak flows and the risk of flash floods following medium- to large-scale rain events in small catchments (van Dijk and Keenan 2007). However, the effect of plantation forests on large landslides is probably negligible and is currently poorly understood (van Dijk and Keenan 2007). Overall, the outcomes of afforestation on water may be unpredictable, and other interventions are generally better suited to address larger floods, including discouraging human settlement in floodplains (FAO and CIFOR 2005; van Dijk and Keenan 2007).

Case Study 4 | The Value of Mangroves

On the heavily populated coasts—and major urban areas—of more than 100 nations in the subtropics and tropics, mangrove forests serve an especially integral role.a For cities, mangroves shield coastal developments from the force of waves and wind.b They can reduce global flooding losses by US$65 billion and protect 15 million people from exposure annually, including people and property in major cities such as Lagos, Mumbai, Karachi, Wenzhou (China), and Miami.c Indonesia is home to most of the world’s mangroves (nearly 25 percent of remaining mangrove forests), followed by Brazil, Malaysia, and Papua New Guinea.d Mangroves can play a particularly important role in densely populated, socially vulnerable communities—including those in cities—where other stormwater infrastructure investment may be inadequate or absent.e

In the past fifty years, these underappreciated but immensely valuable forest ecosystems have been rapidly cleared and degraded.f Today, mangrove deforestation rates (0.16–0.39 percent annually) are decreasing after peaking in the 1980s and 1990s, but some regions are experiencing significantly higher rates (e.g., in Southeast Asia, where deforestation rates are estimated to be still around 3.5–8.0 percent).g Despite their importance to coastal resilience and economic activity, mangroves are being lost at a rate three to five times the average rate of forest loss globally.h Production of goods consumed in cities is a major driver of mangrove loss. For example, mangroves are cleared to make way for shrimp aquaculture ponds.i

Beyond attenuating flooding, mangroves provide other benefits:

  • Climate. Mangroves trap sediment in their roots, locking away massive amounts of carbonj—approximately 907 metric tons per hectare. In comparison, intact tropical montane forests in Africa are estimated to store around 150 metric tons of carbon per hectare.k Mangrove clearing is responsible for a fifth of all carbon emissions from deforestation.l
  • Biodiversity and livelihoods. Along coasts and in estuaries, mangrove forests provide habitat and serve as nurseries for juvenile fish.m The loss or degradation of coastal forests causes local decline and loss of species.n Local fishermen in Mexico, the Philippines, Kenya, and many other locations demonstrate extensive traditional ecological knowledge of these forests.o
  • Coastal protection. Mangroves mitigate annual flood damages due to tropical cyclones and flooding under regular conditions by an estimated $65 billion and protect 15 million people around the world annually.p
  • Economy. When harvested sustainably, mangroves provide fuel, charcoal, medicines, and other raw materials and products that supplement people’s incomes.q In total, the ecosystem services mangrove forests provide are valued at an estimated $33,000–$57,000 per hectare annually to the economies of developing countries.r

Sources: a. Hamilton and Casey 2016; b, c. Menéndez et al. 2020; d. Hamilton and Casey 2016; e. Menéndez et al. 2020; f. Walters et al. 2008; Valiela et al. 2001; g. Hamilton and Casey 2016; h. UNEP 2014; i. Valiela et al. 2001; j. Walters et al. 2008; k. Cuni-Sanchez et al. 2021; l. UNEP 2014; m. Whitfield 2017; n, o. Walters et al. 2008; p. Menéndez et al. 2020; q. Walters et al. 2008; r. UNEP 2014.

Water Challenge 3: Too Little


About 25 percent of the world’s population faces extreme water stress (Hofste et al. 2019). Climate change and a growing population are projected to ramp up pressure on finite water supplies. Flörke et al. (2018) find that one in six large cities will face water deficits in the future. With more than 50 percent of the world’s population concentrated in cities, urban water scarcity can have dire economic and health impacts. For example, the “Day Zero” drought-induced water crisis in Cape Town of 2017–18 resulted in the loss of thousands of jobs, dramatically reduced regional agricultural production, and put many more people at risk for diseases due to inadequate sanitation (Parks et al. 2019).

What roles can forests play?

Forests around cities play a vital role in protecting and sustaining water supplies (Figure 11). Maintaining forests in watersheds is generally positive for regulating water supply across the year. Deforestation and forest degradation in watersheds jeopardize water availability by altering local hydrology and reducing interseasonal stability in water supplies.

Nearby forests

Forests drive downwind and interior precipitation. Forested areas often “use” more water than other ecosystems on a local scale because they have high evapotranspiration rates (Brauman et al. 2007). But from recent theoretical research, scientists have begun to posit that this water, rather than being “lost,” serves as an important source for precipitation in downwind regions (Ellison et al. 2012; Ellison 2018). By capturing and recycling precipitation, evapotranspiration sends water into the atmosphere, where it can fall as rain in downwind regions (Ellison et al. 2017). For example, coastal forests can capture and transpire fog and humidity from oceanic winds, carrying it deeper into continental interiors than it might otherwise travel (Ellison et al. 2017). Thus, even forests beyond watershed boundaries can influence water availability and provide key water regulation services to agricultural and urban areas (Melo et al. 2021).

Figure 11 | How Forested Watersheds Protect Water Supplies

Source: Authors. Adapted from Qin and Gartner 2016.

Intact forests in watersheds can reduce seasonal fluctuations in water availability. Forests generally use more water than other land uses (with the exception of irrigated crops), which means that forestation can reduce flows in catchments, and forested watersheds may have similar or slightly lower streamflow than nonforested catchments (Calder 2007; Carvalho-Santos et al. 2014). But forests play a key role in water regulation throughout the year: they reduce the impact of floods in wet periods and help to mitigate the impacts of droughts in dry periods (Neary et al. 2009). Consequently, forest conversion and degradation may lead to increased water flows in the wet season but reduced dry season flows in many contexts (Lele 2009). Forests increase the infiltration capacity of the soil, which promotes groundwater recharge (and reduces flood peaks), helping to stabilize flows throughout the year and benefiting areas far downstream. Soil penetration by tree roots improves the capacity of soil to store water, as does the presence of organic material from decaying plants and chemicals released by plants and other forest organisms in the soil (Brauman et al. 2007; Neary et al. 2009).

Montane cloud forests, for example, can be vital sources for water contribution during the dry season. Foliage in cloud forests in mountainous regions captures the water vapor of clouds, which run down the stems and leaves of plants and thereafter into soils and streams. Cloud forests have especially pronounced effects in drier regions (Postel and Thompson 2005). Fog capture can provide as much water as rain in many of these regions, which can be especially important for maintaining stream flow in the dry season (Ellison et al. 2017).

Avoiding deforestation is key for regulating water quantity in ecosystems. Deforestation can increase local water availability (Brauman et al. 2007; van Dijk and Keenan 2007; Ellison et al. 2012; Ellison et al. 2017; Filoso et al. 2017; Zhang et al. 2017), which, in water-scarce regions, may seem like a benefit. But deforestation often also increases runoff and short-term streamflow and can thus increase soil degradation and flood frequency (Brauman et al. 2007). The increase in surface water is only part of the picture: deforestation often decreases infiltration rates, leading to higher peak flows but reduced groundwater recharge and reduced dry season flows—two factors extremely relevant to land-reliant livelihoods in the tropics (Ellison et al. 2017). Short-term gains in water availability following deforestation do not make up for other important lost services, such as the regulation of water availability throughout the year between seasons, prevention of sedimentation, and reduced water treatment costs. For example, in Malawi, analysis of satellite data and household surveys showed that a 1 percent increase in local deforestation decreased household access to clean drinking water by 0.93 percent, suggesting that the deforestation Malawi has experienced in the last decade may have effectively had the same impact on water access as a 9 percent reduction in rainfall in the region (Mapulanga and Naito 2019).

Caveats and considerations

In contrast with the benefits of forests conservation or avoided deforestation, however, reforestation or restoration of degraded forests may reduce year-round water availability, at least in the short run, and particularly in arid or semiarid regions (Filoso et al. 2017). This is because forests generally increase water infiltration into soils and evapotranspiration, reducing the availability of water for other uses (Ilstedt et al. 2007; Filoso et al. 2017; Lozano-Baez et al. 2019). Trees planted in former shrublands or grasslands may reduce both flood peaks and low flows (Brauman et al. 2007). In some cases, infiltration rates can exceed evapotranspiration rates, with net positive effects on groundwater recharge. But in general, evidence suggests planting new forests typically has negative or neutral effects on annual catchment (surface water) flows (Calder 2007).

Furthermore, not all forest types provide equal benefits. Compared to native forests, plantation forests reduce water runoff, with negative effects on water supply (Case Study 5; Lele 2009; Alvarez-Garreton et al. 2019; Yu et al. 2019). Unlike native forests, plantations can put pressure on surface water quality and supply.18 In particular, planting native grasslands or other native ecosystems with non-native trees, or planting non-native trees that use water inefficiently, can reduce dry season water availability (Postel and Thompson 2005). Fast-growing plantation species can reduce flows in catchments, especially while the trees are young (Brauman et al. 2007; Calder 2007; van Dijk and Keenan 2007; Bonnesoeur et al. 2019; Jones et al. 2020). For example, plantations of water-intensive eucalyptus have been shown to dramatically reduce surface runoff and streamflow in multiple climates, with effects persisting for decades (Case Study 5; Farley et al. 2005). Although these reductions may be desirable in certain watersheds (e.g., those struggling with salinization issues or highly degraded former agricultural lands), they could further endanger water supplies in many areas (Farley et al. 2005; Brauman et al. 2007; van Dijk and Keenan 2007).

Case Study 5 | Monoculture Eucalyptus Plantations and Decreased Water Availability

Eucalyptus trees are native to Australia and Southeast Asia but have been planted around the world because of their adaptability to different climates and fast growth rates.a The wood from eucalyptus trees can be used for paper, charcoal, firewood, construction lumber, and biofuel. However, eucalyptus requires more water to grow than most other types of trees because of its extensive root system, rapid growth, and high rate of evapotranspiration. The high water requirement of these trees has led to water shortages in some countries that have increased eucalyptus plantations. b

South Africa and Uruguay are two countries that experience more droughts because of extensive non-native eucalyptus plantations. South Africa has established over 515,000 hectares (ha) of eucalyptus plantations to meet high demand for timber. Replacing the native vegetation with eucalyptus has reduced streamflow in affected watersheds by 90–100 percent.c Similarly, in Uruguay, eucalyptus plantations cover over 1 million ha established in the 1970s.d In watershed regions, hydrologic yield was reduced by 50 percent when native vegetation was replaced with eucalyptus, and 13 percent of streams near eucalyptus plantations have dried up.e

In both countries, afforestation with eucalyptus resulted in less available drinking water and more severe droughts. Also, monoculture tree plantations tend to use more water than diverse, natural vegetation.f In South Africa, policymakers addressed this challenge with legislation that requires timber plantations to apply for permits to plant non-native trees in order to preserve water.g Policymakers should consider the impact of the type of tree on water supply and prioritize planting native trees. Legislation and economic incentives can be used to require monoculture plantations to pay for the amount of water they use and to prevent drought.

Sources: a. Albaugh et al. 2013; b. Cespedes-Payret et al. 2009; c. Albaugh et al. 2013; d. Pozo and Säumel 2018; e. Cespedes-Payret et al. 2009; f. Albaugh et al. 2013; Hubbard et al. 2010; g. Albaugh et al. 2013.

Water Challenge 4: Too Erratic


Erratic and unpredictable weather—including extreme drought, torrential rainfall, and violent storms—continues to make global headlines and affects millions of people annually. Although some variability and extremes of climate are natural, evidence suggests that anthropogenic climate change has increased the frequency, intensity, and duration of many of these events (Seneviratne et al. 2012). Precipitation and drought are both expected to increase during the 21st century (Seneviratne et al. 2012). In addition to the effects of changing atmospheric GHG levels, large-scale changes in land cover affect climate locally, regionally, and globally by altering surface temperature, humidity, evaporation, cloud formation, precipitation, and more (Mahmood et al. 2014).

Cities are vulnerable to these types of changes. Increases in flooding, drought, or heavy rainfall within the immediate vicinity of cities presents obvious challenges to urban leaders. Additionally, as net importers of water, food, and materials, cities rely on the functioning of productive ecosystems outside their boundaries. The vast majority of urban demand is met by goods imported from far beyond city boundaries—from rice to milk to timber. This means that cities both rely on food produced in the world’s major agricultural areas and have significant effects on the world’s forests via city consumption patterns. Thus, disturbances in supply chains due to increasingly erratic weather patterns around the world may have indirect impacts on cities too.

What roles do forests play in global precipitation patterns?

The world’s large and intact forests play an important role in cycling and transporting water, thereby shaping global and regional weather patterns. Forests—especially intact tropical forests—affect large-scale weather patterns. Forests regulate regional and global precipitation patterns, which impact both city water supply and the production of food to city residents in key agricultural regions (Lawrence and Vandecar 2015). Because land cover change is associated with changes in precipitation and climate (Mahmood et al. 2014), the persistence of forests can be a stabilizing factor.

Evapotranspiration by forests cycles large amounts of precipitation into the atmosphere, creating “flying rivers” (atmospheric currents laden with moisture capable of driving weather patterns in distant locales). Forests effectively recycle rain by returning moisture to the atmosphere, which contributes to future rain events (Ellison et al. 2012). This recycled “green water” released during evapotranspiration affects local areas, as described in the previous section, and also composes a large portion of the world’s rainfall in locations far away. The emerging scientific literature on tropical forests and their impact on global air circulation, also known as teleconnections, highlights how these processes change when tropical forests are cleared, and how this affects global precipitation patterns.

Forests move water from the earth’s surface to the atmosphere at large scales. Tropical forests contain air that is warmer and more humid than in surrounding areas, creating a low-pressure zone that drives wind patterns (Makarieva and Gorshkov 2010). It is estimated that the moisture moving over the Amazon rain forest is recycled five to six times as it moves westward across the continent (Lovejoy and Nobre 2018). Deforestation drastically reduces evapotranspiration rates and, with them, the movement of water.

Current forest practices and risks for the hydrological cycle

Three major tropical forests disproportionately affect global water cycles:

  • Amazon rain forest. The largest tropical forest in the world exerts great influence over regional (Davidson et al. 2012) and global precipitation and air circulation (Lawrence and Vandecar 2015).
  • Congo Basin (Figure 12). This forest region in Central Africa drives precipitation patterns around Africa (Gebrehiwot 2019) and beyond (Lawrence and Vandecar 2015).
  • Southeast Asian rain forest. The smallest of all three tropical forest basins—but the most humid and with the highest rainfall—the Southeast Asian rain forest influences the climate within and beyond the region through impacts to the Asian monsoon circulation, potentially resulting in higher rainfall in some areas and lower rainfall in others (Lawrence and Vandecar 2015).

Figure 12 | Forest Cover, Water Risk, and Water Movement

Notes: Because forests require more water than ecosystems such as grasslands, it is no surprise that forest cover (Box A) often coincides with areas of lower water risk (Box B). But as Box C shows, water also moves throughout the atmosphere, directed by winds. The loss of forests in Central Africa could have cascading impacts on those who depend upon the Nile River, for example, which is fed by precipitation from both areas to the south.

Source: Creed et al. (2019).

Deforestation of tropical forest could impact wind patterns and circulation of precipitation regionally and around the globe, with potential for impacts on water availability for agriculture (Figure 13) (Lawrence and Vandecar 2015; Avissar and Werth 2005; Werth and Avissar 2005; Medvigy et al. 2013; Lawrence and Vandecar 2015). Continental interiors hold some of the world’s most important agricultural areas. Without contributions to rainfall from forests, research suggests these downwind interiors may experience far less rain (Ellison et al. 2017). Likewise, deforestation also could affect precipitation in some of the world’s continental interior cities, which could present significant challenges for their water supply. Water-scarce regions, such as the dry regions of northeastern Africa where the Nile River is the main water source, are likely to be most affected by deforestation (Ellison 2018).

In particular, changing weather patterns due to deforestation of tropical forests could have a large impact on water availability for agriculture (Lawrence and Vandecar 2015). Fluctuations in climate can have a huge impact on agricultural productivity (Liang et al. 2016). Figure 13 describes the impacts of deforestation of tropical forests in different regions. Evidence also suggests that climate and precipitation are already more variable because of deforestation. Research typically models impacts using simulations:

  • Within the Amazon Basin, forest loss is associated with reductions in rainfall (an average of 8 percent by 2050) in model simulations, according to a 2015 meta-analysis (Spracklen and Garcia-Carreras 2015).
  • Modeling suggests that forest loss in the Amazon (Medvigy et al. 2013) and Central Africa (Akkermans et al. 2014) could reduce precipitation in the U.S. Midwest (Werth and Avissar 2005), with potential negative consequences on food production.
  • Modeled changes resulting from deforestation in Southeast Asia appear to be smaller in magnitude but could reduce local precipitation in Southeast Asia as well as in Hawaii and the U.S. Pacific Northwest and even southern Europe (Avissar and Werth 2005).

Figure 13 | Simulated Total Deforestation of Major Forest Regions and the Effects on Global Precipitation Patterns

Note: Despite global rates of deforestation, total deforestation of the world’s major forest regions as simulated in this scenario is highly unlikely in reality.

Source: Authors. Adapted from Lawrence and Vandecar (2015).

These forests are in jeopardy. Since 2001, tree cover in the Congo Basin has decreased by more than 20 percent, mainly due to clearing for agriculture (Tyukavina et al. 2018). Southeast Asia is a global deforestation hotspot, with particularly high rates of habitat and biodiversity loss (Estoque et al. 2019). And, alarmingly, climate feedbacks between the Amazon Basin and Brazil’s nearby Cerrado forest and grassland ecosystems—currently subject to high rates of deforestation and agricultural conversion—may further threaten the precipitation patterns of both of these valuable regions (Spera et al. 2016).

Forest loss may accelerate the unpredictability of changes related to global warming. It is unlikely that forest protection or restoration can prevent or undo the changes to climate seen in recent decades, but it is worth stating that forest restoration is a good way to sequester more carbon in the landscape and offset other emissions. Yet because of their essential contributions to global climate as well as climate change mitigation efforts, the persistence of large forests may prevent further acceleration of global changes in climate and weather patterns. Conversely, increasing land cover change—including forest loss—may create greater uncertainty (Mahmood et al. 2014).

Caveats and considerations

Tropical forests may soon approach a tipping point where deforestation is occurring at such a high rate that the forests lose their capacity to recycle water effectively (Nobre and Borma 2009; Mahmood et al. 2014; Lovejoy and Nobre 2018). For example, if deforestation continues at present rates in the Amazon, 8 percent mean annual reductions in rainfall are expected by 2050 (Spracklen and Garcia-Carreras 2015). Consequently, its capacity to maintain continental and global water flows may erode.

Conserving forest is important for global water regulation, but it is less certain what the effects of large-scale reforestation or afforestation would be on regional evapotranspiration, precipitation patterns, and teleconnections (Mahmood et al. 2014). Large-scale afforestation of midlatitude regions, in particular, could have unintended consequences related to regional water availability and temperatures, resulting from changes in albedo (Swann et al. 2012). Protection of existing forests and restoration of previously forested lands should thus be prioritized over forest expansion.

To better understand the effects of forest changes to water availability and movement, more research is needed. Although the field of research relating to forests and water continues to grow rapidly, little research has been conducted on the effects of forest loss and degradation on hydrology in many parts of the world, including many low- and middle-income countries (Jones et al. 2020).

Concluding Thoughts

Although growing urban populations and shifting global climate patterns complicate efforts to provide clean and affordable drinking water, harnessing the power of forests in and around cities can help. Urban forests can reduce the burden of stormwater management on built infrastructure and insulate urban watersheds from the effects of pollution. In the watersheds surrounding cities, forests can protect water quality, buffer the risk of flooding, and ensure water availability from season to season. Vast tracts of intact forests—especially tropical rain forests—play an integral role in regional and global water movement and weather patterns. Yet global discourse and decision-making on protecting tropical forests does not yet sufficiently address their role in local, regional, and global hydrologic cycles (Ellison et al. 2017).

Cross-boundary water management is key to manage the interdependency of catchments and watersheds at multiple scales (Melo et al. 2021). Global cooperation is warranted to protect tropical forests because deforestation impacts agriculture, flood risk, and water availability around the world. To ensure that key agricultural regions—vital for urban sustenance—have adequate water supplies, cities will need to act to protect forests far beyond their boundaries.