Do trees really bring rain?

I am sure we are all very familiar with this notion that trees make it rain. Actually, I am sure that most of us have heard or participated in tree planting campaigns to believe that more trees mean more rain. But is it true that trees bring rain? Is it scientifically proven? If you seek to reveal the truth, hand in here, we will scientifically find these answers soon.

There is enough scientific evidence backing up the theory that vegetation is a fundamental factor in precipitation patterns (Shukla & Mintz, 1982). Rains come from clouds. Clouds are formed when water vapor condenses on tiny particles (aerosols). One of the sources of these particles is plants. Vegetation releases microorganisms (microbes) and other microparticles, which act as a platform for water to condense and form clouds.

The water vapor, which leads to clouds formation, is mainly from evaporation and transpiration. Unlike bare grounds, vegetated surfaces store large volumes of water, increasing both infiltration and percolation. As a result, a vegetated area will generate less runoff as compared to a bare land surface. Less runoff means large volumes of water are retained in our watershed for plants' transpiration, leaf surface evaporation, and evaporation from the soils. This leads to enhanced cloud formation leading to increased rainfall and minimum temperatures within the area (Kravcik et al., 2008).

Over 40% of rainfall falling overland comes from evaporation and transpiration(Ellison et al., 2017). When we cut down trees for settlement, agriculture, pasture, etc., we reduce our water storage, which means less evaporation and transpiration and eventually less rainfall. Converting a forested area into a residential area leads to increased runoff generation and less infiltration, significantly decreasing soil moisture content. Also, rangelands and pasture zones hold less water as compared to a forested area. All these factors lead to minimal soil evaporation (Cabello et al., 2012). Deforestation is severe and can decrease rainfall amounts close to 30% (Spracklen et al., 2012).

Increasing vegetation cover over a large area will significantly lead to increased rainfall. According to Makarieva & Gorshkov (2009), afforestation and reafforestation increase rainfall and enhance overland transport. It is scientifically proven that preserving relatively intact and incessant vegetation cover from the coast will result in consistent rain within the mainland interiors of Australia, Africa, and elsewhere (Makarieva & Gorshkov, 2009).

A perfect example and evidence can be withdrawn from Australia. In southwest Australia, a fence of about 750 km long was built to prohibit rabbits. Initially, the western part of the wall experienced vast amounts of rainfall compared to the eastern side. However, today the eastern part of the fence experiences more rain than the western side. This is because most vegetation on the west side was cleared, and the land was converted to different land use land cover types (Rosa & Luca, 2012).

The following diagram represents the relationship between deforestation and rainfall patterns within the tropics.

The rainforest’s water pump

Figure 1: Relationship between deforestation and rainfall patterns within the tropics source: (Rosa & Luca, 2012)

According to the diagram, the section marked with "a" depicts that considerable rainfall amounts experienced around the tropical forests originate from water vapor carried within the atmosphere from a different location. However, the most significant rainfall component is recycled rainfall. Recycled rainfall means that water is pumped from the soil to the atmosphere through evapotranspiration. Within the tropical rainforests, water only exits as runoff into the streams or through evapotranspiration. The water vapor atmospheric transport in the forest is well-adjusted by the departure of water in runoff and vapor.

Deforestation minimizes evapotranspiration and constrains water recycling (Spracklen et al., 2012). Portion "b" of the diagram above represents a scenario of deforestation. Deforestation minimizes moisture content carried away within the atmosphere, thus decreasing rainfall amounts in the regions the moisture is being transported. Besides, deforestation is likely to lead to increased flood levels due to increased runoff generation.

Therefore, there is enough scientific evidence suggesting that trees do truly bring rain. The next time you cut down a tree, think. Before we complain again that the rains have reduced, temperatures have increased, let us ask ourselves what we have done to suffer such.

Let us increase forests cover

References

  1. Cabello, J., Alcaraz-Segura, D., Ferrero, R., Castro, A. J., & Liras, E. (2012). The role of vegetation and lithology in the spatial and inter-annual response of EVI to climate in drylands of Southeastern Spain. Journal of Arid Environments, 79, 76–83. https://doi.org/10.1016/j.jaridenv.2011.12.006
  2. Ellison, D., Morris, C. E., Locatelli, B., Sheil, D., Cohen, J., Murdiyarso, D., Gutierrez, V., Noordwijk, M. van, Creed, I. F., Pokorny, J., Gaveau, D., Spracklen, D. V., Tobella, A. B., Ilstedt, U., Teuling, A. J., Gebrehiwot, S. G., Sands, D. C., Muys, B., Verbist, B., … Sullivan, C. A. (2017). Trees, forests and water: Cool insights for a hot world. Global Environmental Change, 43, 51–61. https://doi.org/10.1016/j.gloenvcha.2017.01.002
  3. Kravcik, M., Pokorný, J., Kohutiar, J., Kovác, M., & Tóth, E. (2008). Water for the Recovery of the Climate - A New Water Paradigm. 1–94.
  4. Makarieva, A. M., & Gorshkov, V. G. (2009). Reply to A. G. C. A. Meesters et al.’s comment on “biotic pump of atmospheric moisture as driver of the hydrological cycle on land.” Hydrology and Earth System Sciences, 13(7), 1307–1311. https://doi.org/10.5194/hess-13-1307-2009
  5. Rosa, L. De, & Luca, M. De. (2012). The rainforest ’ s water pump. 8–9.
  6. Shukla, J., & Mintz, Y. (1982). Influence of Land-Surface Evapotranspiration on the Earth’s Climate. 7.
  7. Spracklen, D. V., Arnold, S. R., & Taylor, C. M. (2012). Observations of increased tropical rainfall preceded by air passage over forests. Nature, 489(7415), 282–285. https://doi.org/10.1038/nature11390