How Large Reforestation Projects Impact Water Cycles and Local Water Availability
The need to reforest portions of Earth has been well documented. The known benefits of reforestation and afforestation are unquestioned and far-reaching, particularly when it comes to impacts on water. Reforestation greatly reduces annual rainwater runoff, for instance, leading to less soil erosion and unexpected flooding. Ample forest cover lowers surface temperatures and cools soils, lakes, rivers and landscapes.
Forests also enhance water quality and reduce carbon dioxide in the atmosphere. Planting trees can increase carbon sinks that absorb and store carbon.
In addition to climate-related benefits, reforestation can potentially protect endangered species. A restored forest can undo habitat loss and improve species health.
Still, with all these benefits, Earth’s forested lands continue to shrink.
A Crippling Trend
According to the UN’s Food and Agriculture Organization (FAO), the total forested area of Earth is about four billion hectares—31% of the global land area—or about 50 x 100m per person. Of this area, only about one billion hectares are primary, native forests that are largely undisturbed.
The number of forested hectares is huge, but so is the number of hectares lost. According to the FAO’s 2020 edition of The State of the World’s Forests, the world lost about 420 million hectares (mha)— approximately 10% of its total forest area—to deforestation in the last thirty years.
The FAO report estimates that between 2015 and 2020, the rate of deforestation was ten million hectares per year. That is down from sixteen million hectares in the 1990s. Even so, the area of primary forest worldwide has decreased by over eighty million hectares since 1990.
Tree Planting has Side Effects
The world is planting trees in response. The FAO says that 7% of the global forest area is currently planted. The area of naturally regenerating forests has decreased since 1990—at a declining rate of loss—but the area of planted forests has increased by 123 million ha.
It is not easy, however, to successfully create—or recreate—a forest. According to an April 8, 2021 article in Yale 360, scientists and environmentalists have concerns about large-scale tree planting programs, as they can reduce water supplies and negatively impact agriculture and associated livelihoods.
How might planting trees dry up water supplies? It has to do with a tree’s ability to absorb and evaporate water at relatively high rates. According to Filoso et al., the authors of a 2017 study inPLOS One, "forests have relatively high evapotranspiration (ET) rates in comparison to most other land use and cover types, which is why water yields usually decrease upon the conversion of different land uses into forests." While it is true that large scale plantings may generate more evapotranspiration and lead to higher rainfall, this is common. To be effective, it needs to cover an area about as big as Switzerland. The major concern is that many plantation species can tap the groundwater and make the conditions less favorable for local native forest species or can lead to creeks and ponds drying up.
Water Impacts Can Be Felt Far Away
It takes relatively few trees to intensify the water cycle. According to a UN University report, "more than two square kilometers of forest expansion can increase the possibility of rainfall."
What's more, when trees, through evaporation, move water vapor into the atmosphere, it can travel far distances through "wind-driven circulation," thus increasing "the possibility of precipitation in another location," states the study’s author. This, the author writes, "indicates that, at a global scale, afforestation can indeed bring benefits to the water cycle." However, this does not take into account losses en route, such as if wind driven rain-bearing clouds pass over parched areas.
Dijke et al. (2022) observed that the effects of "directly enhanced ET and indirectly enhanced precipitation" can cause shifting patterns of water availability. They found that "large-scale tree-cover expansion can increase water availability by up to 6% in some regions, while decreasing it by up to 38% in others." Actual decreases have been more commonly reported in places such as India, Ethiopia, and China instead.
Large-scale tree-cover expansion can increase water availability by up to 6% in some regions, while decreasing it by up to 38% in others.
The effects of drying out local regions and increasing rainfall in other places can be extraordinarily far-reaching, write the authors. "Tree-cover change in the Amazon forest could impact precipitation in Canada, Northern Europe and all the way into Eastern Asia."
The study’s authors added that "several so-called hot spots for reforestation could lose water, including regions that are already facing water scarcity today." Such effects may not show up on trees themselves, but they could be impacting the water tables and small streams.
Local Winners and Losers
Dijke and colleagues predict local water-supply winners and losers (following reforestation), even though they see overall benefits for the planet. They write "that for half of Earth’s surface (47%), the indirect moisture recycling effects of large-scale tree restoration could offset the direct evaporation effects, thus resulting in slight increases in water availability rather than decreases."
Where do the authors think local water losses from reforestation will occur? The United Kingdom (UK) is one such place. They write that the UK "has a high tree-restoration potential and therefore a high increase in evaporation." This will result in "low evaporation recycling [rainfall] due to the dominant westerly moisture transport [aided by winds] from the country towards Eurasia."
Possible winners? "Low latitudes" enjoy an increase in water availability because local evaporation recycling is high. This is due to "strong convection above the tropical forest [and short] travel distances of the atmospheric moisture," according to Dijke et al.
Possible Effects on Rivers
Dijke and colleagues predict varying effects on streamflow (following reforestation) by combining the direct effects of reforestation "via increased evaporation" and indirect effects "through increased precipitation" for twenty-one large river basins from the Yangtze to the Mississippi. For all of them, "enhanced evaporation reduces streamflow (up to 9%)," they found.
Why the potentially different streamflow outcomes? Some river basins benefit "when evaporated water rains out within the same river basin," say the authors. Those same basins might also recycle rain from regions upwind. For river basins in the tropics that enjoy high local evaporation recycling, the gains could make up for losses via evaporation, they write.
What About Arid Regions?
River basins with limited water (arid regions} have a low tree-restoration potential because arid regions often lack enough groundwater to support tree growth. In such cases, state the authors, there is likely to be "a small absolute change in evaporation and precipitation" following reforestation.
The authors speculate that some arid regions might benefit from tree planting because trees can increase soil porosity and soil organic carbon, thus promoting the infiltration capacity and water storage capacity of local soil.
There is also a need to factor in seasonal impacts for arid areas that receive most of their precipitation on a few occasions per year.
Despite these factors, the authors affirm their hypothesis that post-reforestation "streamflow will decrease for most of the world’s important river basins despite the indirect effect of evaporation recycling."
In the past, afforestation has failed unless it is done with an aggressive weedy tree such as the Acacia nilotica or Prosopis juliflora, which was officially grown in the Thar Desert of India in the 1930s to afforest the desert wasteland.
India: Learning from Possible Miscalculations
Just because an area is without tree cover does not mean it is time to start planting. According to a 2015 study, The World Resources Institute (WRI) and the International Union for Conservation of Nature (IUCN) once "misidentified nine million square kilometers of ancient grassy biomes as providing ‘opportunities’ for forest restoration." In reality, establishing forests in such grasslands, savannas, and open-canopy woodlands would "devastate biodiversity and ecosystem services," according to the study’s authors.
Fortunately, missteps based on miscalculations can be avoided. In the case of India, an Expert Technical Committee constituted by the Madras High Court recently found the Uppanar backwater region unsuitable for mangrove reforestation because of the area’s steep slope and tidal conditions in which mangroves would not thrive. The investigation was ordered to address concerns over reforestation proposals for the area.
India’s Rajasthan State: Reforestation Success
India has seen hydrological benefits from reforestation, however, even in arid regions such as The Rajasthan State.
Rajasthan receives 16.05 billion cubic meters of water from rainfall annually but loses four billion cubic meters of that to runoff. Despite the heavy losses, work done under the Mukhyamantri Jal Swavlamban Abhiyan—Chief Minister’s Water Self-reliance Campaign (MJSA)—has raised average ground water levels in local villages by nearly five feet in twenty-one of Rajasthan’s non-desert districts. In addition, the need to supply locals with water via tankers has fallen to about 56% due to this project. Soil erosion has also declined and soil fertility has improved in the region, resulting in increased agricultural production.
MJSA, which was launched in 2016, has been linked to the "Van Kranti" (Afforestation Mission) and the planting of about 148 lakh (1 lakh=100,000) saplings across the state. Thousands of newly constructed or renovated water structures under MJSA are being covered or surrounded by saplings to retain groundwater levels, reduce soil erosion, and boost biodiversity through the protection of local wildlife.
According to the Chief Minister of Rajasthan: "It is a foregone conclusion that MJSA has been a huge success and a trendsetter in the country on [the] water management front. In many ways, MJSA is an important step towards ‘climate proofing’ the State."
Taking into account both the global benefits and possible negative local water impacts of reforestation, Dijke and colleagues call for more careful planning. "We emphasize that future tree-restoration strategies should consider these hydrological effects."
*Dr. Mahesh K. Gaur is Principal Scientist at the ICAR-Central Arid Zone Research Institute, Jodhpur, India. He specializes in aridlands geography and the application of satellite remote sensing, GIS and digital image processing for natural resources mapping, management and assessment and also researches drought, desertification, land degradation, indigenous knowledge systems, and the socio-economic milieu of the Thar Desert of India. He is author/editor of 8 books on Drylands, Desertification, Watershed, Food Security, Remote Sensing, etc. A member of the Association of American Geographers and the Society for Conservation Biology, and several editorial boards of journals, he has been awarded the Citizen Karamveer Award 2011 by iCONGO; the Millennium Award and recognitions by the UGC of India and Scientific Assembly of the International Committee on Space Research (COSPAR).
*Dr. Victor R. Squires is a Distinguished Guest Professor in the Institute of Desertification Studies, Beijing. An Australian with a PhD in Rangeland Science from Utah State University (US), he is a former (retired) Foundation Dean of the Faculty of Natural Resource Management at the University of Adelaide and author/editor of 22 books on Drylands, Desertification, and Ecological Restoration. He has been a consultant to World Bank, Asian Development and various UN agencies in Africa, China, Central Asia and the Middle East. He was awarded the 2008 International Award and Gold Medal for International Science and Technology Cooperation by the Government of China and in 2015 was honored by the Society for Range Management (USA) with an Outstanding Achievement Award and was recognized a member of the Order of Australia for services to ecology, especially rangelands.