Water scarcity is considered one of humanity’s greatest challenges, preventing us from achieving the sustainable future we’re pursuing. The situation is getting even worse as we’re suffering a lot of negative effects caused by climate change. This is one of the main reasons leading to the current decline in water resources.
Groundwater resources have long played an irreplaceable role not only in sustaining ecosystems but also in addressing global water shortages. All the agricultural production, economic activities, and human life of any country are highly dependent on groundwater. With such importance, the task of improving groundwater levels has become a priority for all scientists for years, and the crux of the matter revolves around one key factor: trees.
The Relationship Between Trees and Groundwater
Groundwater resources provide half of the world’s drinking water. Groundwater is formed by rainwater seeping through the soil before evaporating. Rainwater stores up over time, forming underground lakes and rivers that provide water for different purposes.
It sounds easy theoretically, but much more difficult in practice. In nature, large amounts of rainwater usually evaporate or go to lakes/streams before it can seep through the ground. This situation is even more evident in places that lack the existence of trees, since water infiltration can only take place efficiently with the help of landscape conditions and organic materials provided by trees.
Trees, especially those with complex roots, can help retain moisture and allow water to enter the ground more easily. Tree roots play a vital role in creating and maintaining underground pores, creating favorable conditions for water to infiltrate and replenish groundwater. Without trees, these pores would become sparser, making them less efficient in absorbing water. You can see that clearly in areas where there are no trees and the ground is compacted or paved. Water doesn’t even have a chance to get into the ground, eventually getting stuck on the surface and evaporating.
However, the mechanism of action of tree roots and leaves also requires continuous water absorption and transpiration. This, ironically, can lead to the depletion of groundwater resources. If this situation persists for a long time, areas with limited groundwater resources can experience many undesirable hydrological impacts.
The complex relationship between trees and groundwater has aroused mixed opinions among researchers in making plans to improve groundwater resources. This has formed 2 different perceptions called “trade-off theory” and “sponge theory”, and we need to analyze both in order to find the best solution.
“Trade-off Theory” and “Sponge Theory”
According to the old scientific point of view, the density of trees is inversely proportional to the groundwater level. In other words, many scientists believe that planting trees will significantly reduce groundwater resources. Trees absorb and evaporate more water than they contribute to the ground. This has actually been proven in groundwater-dependent or arid regions, especially in South Africa. The locals have even encouraged tree uprooting as a solution to improve groundwater availability. Scientists call this the “trade-off theory”, and it has been the dominant paradigm in scientific circles for a long time.
However, a lot of new findings have shown that the dominant “trade-off theory” is not absolute, as it is most relevant in arid tropical regions. Many scientists have also conducted studies on the impact of trees on groundwater in humid temperate regions, and the results are completely opposite. In these regions, the water regime depends greatly on the rainfall regime as well as on the thaw. Trees, in this case, serve as sponges to store water and facilitate rapid infiltration of water into the ground, which is very important in maintaining groundwater and controlling water flow during the dry season. This is the premise for the emergence of “sponge theory” – a new perception that is becoming more and more popular with scientists and environmentalists.
From the first part of the article, I have highlighted that groundwater resources can benefit a lot from trees, but can also be negatively affected by them. These influences, whether positive or negative, do contribute to groundwater recharge to some extent. Thus, the balance between them is the key to improving groundwater resources.
Tree Cover – The Core Element In Improving Groundwater Resources
The existence of trees is always necessary, no matter what geographical conditions we talk about. According to a report from Nature.com, an area can achieve maximum groundwater recharge at a specific tree cover level, known as “optimal tree cover theory”. The optimal level of tree cover is decided by regional factors such as tree species, soil quality, and climatic conditions, but in all cases, it should be non-zero.
This optimal tree density represents a balance between the benefits and negative influences that trees have on groundwater recharge. More specifically, an area will receive more hydrological benefits from trees when tree density is suboptimal. This means the ability of trees to increase groundwater recharge outweighs their water use. When the tree density exceeds the optimal, the opposite will happen.
The “optimal tree cover theory” doesn’t completely deny the other two theories, as they are true to some extent. It mainly emphasizes the importance of tree modeling and management under specific landscape conditions. Optimizing tree cover not only improves access to groundwater resources in water-scarce areas, but also contributes greatly to combating desertification.
Even when hidden underground, groundwater resources still bring undeniable benefits. For that reason, UN-Water has launched many activities for this year’s World Water Day in order to raise awareness about the importance of taking care of groundwater resources.
At Tenere, we’re also doing the same thing by strengthening our partnerships with global organizations to plant trees wherever they’re needed. We believe that these activities will help hundreds of millions of people around the world improve their lives with all the benefits that trees provide.