Desertification and Drought Day: 4 Reasons to Celebrate the Symbiosis Between Water and Forests

It is hard to imagine rainfall without healthy trees and forests. And rainfall is the reason behind the lush green forests around the world. 

The roots of the trees help absorb water from the soil. The water is then transported up a tree’s trunk all the way to the leaves, and out into the atmosphere. Forests help in creating rainfall by adding to the atmospheric moisture; they do this by giving off water vapor through its leaves, a phenomenon known as evapotranspiration. 

In addition to increasing atmospheric moisture, trees and forests release aerosols: organic molecules released by trees into the atmosphere. Moisture and aerosols come together to form clouds. 

Winds help circulate the atmospheric moisture and clouds across terrestrial surfaces. Arid lands and areas prone to desertification in downwind areas rely on this redistribution of moisture and rainfall through winds, also called precipitation recycling. 

It is estimated that evapotranspiration is responsible for at least 40% of all rainfall on all land (oceans being responsible for most of the rest). In many parts of the world, this rainfall serves as the lifeline for small-scale fisheries in ponds and rainfed agriculture: cultivation of crops on lands without irrigation, dependent solely on rainfall. 

On the other hand, areas prone to deforestation witness diminished rainfalls, in some instances, by over 20%. Besides reduced rainfall, deforestation and degradation of lands lead to reduced storage of biomass and increased drought-like conditions.

While trees bring rainfall, heavy rains gushing downstream can wash away topsoil. The topsoil consists of humus and minerals essential for the growth of crops. Lands without trees and vegetation are vulnerable to erosion caused by heavy rains and winds, thereby reducing the nutrient load in the existing soil as the critical topsoil layer is lost. 

Growing trees alongside other crops thus aids food production. The inclusion of trees in agricultural systems prevents the erosion of the fertile topsoil and makes moisture available during the dry seasons. 

The presence of trees on farmland can also address soil compaction: a phenomenon where the soil gets denser and less porous. In most cases, this is a result of increased mechanization and tillage. The roots of a tree can break up the soil structure and allow the rainfall and water to seep into the ground. 

In arid and semi-arid areas, repeated evaporation of water also results in the accumulation of soluble salts like sodium, magnesium, and calcium in the soil, also called soil salinization. Soil salinization decreases the fertility of the soil and makes the soil vulnerable to further degradation and desertification. 

The correct choice of trees can address the salinization of soils and turn barren lands back to productivity. In India, for example, growers have successfully planted a mix of trees and grasses to reduce the salinization of the soils. 

These principles find application in agroforestry systems. By retaining soil, organic matter, and moisture in dryland agriculture, agroforestry helps sustain agriculture in regions prone to aridity, compaction, and desertification.

Many of the world’s cities and millions of people depend on mountains and upstream forests for their drinking water. In the United States, major cities like New York, Boston, San Francisco, Seattle, and Portland, directly rely on forests in upland catchment areas for their water needs. 

Trees and vegetative cover in the catchment areas slow stormwater runoff after heavy downpours. The deep root systems of the trees and ground litter in the forests create a sponge effect that regulates stream flows, filters sediments and other pollutants, and slowly releases the water to the ground and streams. 

When compared to barren lands, the sponge effect helps cool temperatures and increase retention of organic matter – resulting in higher carbon sequestration and contributing to climate change mitigation. 

Forests’ water filtration and groundwater recharge services have dual benefits: increased availability of water and decreased cost of artificial water treatment. 

Cities like New York have secured their drinking water by actively preserving upland forests and watersheds that provide 90% of the city’s water. In economic terms, the city spent $1.5 billion USD conserving these ecosystems and saved roughly $6-8 billion USD that might have been otherwise spent on water treatment and filtration. 

As per experts, a 10% increase in forest cover reduces water treatment and filtration costs by 20%.

As water percolates and infiltrates into the soil, it performs another vital function: flood moderation. Trees, their network of root systems, and the resulting organic soil matter reduce the risk of heavy and sudden discharge of sediment and water into streams and rivers. 

Lands with healthy vegetation and forest cover thus help in minimizing and controlling flash floods in downstream areas. Even in the event of large-scale floods caused by powerful storms, trees and forests can reduce the impact of extreme weather events by acting as water breaks. This can in turn limit the extent and damage of severe flooding. 

The mitigating effect of extreme weather seems even more important in the context of coastal areas. For example, during the devastating 2004 tsunami in the Indian Ocean, soil erosion and damage to lands and houses in rural areas were greatly reduced in areas with mangroves and coastal forests. 

In forested areas, the roots of trees bind soils together and reduce the chances of land slipping, the flow of debris, and landslides during heavy rains. This prevents sedimentation downstream, safeguarding the ecosystem services provided by wetlands, river ecosystems, and reservoirs.

In deforested areas, water absorption is decreased, and the soil becomes more vulnerable to erosion from rain and wind. This turns into a vicious chain of losing the productive layers of soil with each flood. 

The city of Jakarta has been facing this tragedy firsthand. More than 50% of the Ciliwung watershed that provides water and flood mitigation has seen a complete loss of forest. The rest of the watershed has lost significant amounts of its original forest and remains degraded. 

Consequently, the biggest city in Southeast Asia finds itself in a routine tragedy of flooding and depletion of groundwater. The National Development Planning Agency of Indonesia estimates that the economic damage from flooding in Jakarta has already reached IDR 5.2 trillion ($364 million USD) in addition to human displacement and loss of lives. 

Initiatives on planting trees, growing forests, and restoration of the watershed in upstream areas could offer some hope of undoing this damage to human lives and the economy.