Tropical rainforests and the water cycle (2)

[Students often have to study rainforests, and all A Level students in the UK have to study the water cycle. This is the second of two Substacks that link these two topics together.]

The water cycle in tropical rainforests affected by human activities.

Where land has been deforested or degraded, the natural recycling of water is disrupted. When rain falls in areas where forest vegetation has been removed or replaced with crops, much less of the water is intercepted, as there are fewer leaves to catch the raindrops. Therefore, more rainfall reaches the ground.

The removal of forest vegetation means that the litter and humus layers are reduced. Hence, it is less easy for water to infiltrate. This is a particular issue when rainfall intensities are high, with the convectional rainfall events that are common in these environments. This reduced infiltration means that more water will move over the land surface as overland flow. Overland flow reaches streams and rivers much more rapidly than water which travels through the soil via throughflow, particularly when surfaces don’t have much surface vegetation to slow down the flow. Consequently, the risk of flash flooding is increased.

If crops have replaced the forest vegetation, the water which infiltrates will be taken up by the roots and will enter the atmosphere as water vapour through evapotranspiration. However, rates of evapotranspiration from crops are much lower than rates from forests. Not only are crops less dense than forest vegetation, but there are also fewer plants to transpire, the roots tend to be less expansive, so they are unable to access water deeper in the soil. Therefore, instead of re-entering the atmosphere as water vapour, soil water will move towards the river as throughflow, further exacerbating flood risk.

The reduction in water evaporated from leaves after being intercepted, together with a reduction in transpiration, means that there is much less water vapour being returned to the atmosphere after a rainfall event. This leads to a reduction in humidity. As air masses move inland, they become increasingly arid as they pass over unvegetated areas.

This atmospheric aridity leads to a reduction in rainfall as there is less water vapour in the air to condense. This has serious implications for the health of forest vegetation. The reduction in rainfall means that less water will infiltrate into the soil, so there is less moisture available for plants to take up through their roots. This leads to water stress, whereby the supply of water is not enough to meet demand from the vegetation. Ultimately, this leads to vegetation dieback, as some plants cannot tolerate the dry conditions.

These disruptions to the ecology and hydrology of tropical forests have further implications for these environments and the processes operating within them. Dry soils and vegetation are prone to wildfires, as the likelihood of combustion increases as moisture content decreases. Forest vegetation is also sometimes burnt deliberately, to clear the land for other uses. Whether accidental or intentional, these fires release small soot particles into the atmosphere, which then impact raindrop and cloud characteristics, which further inhibits rainfall.

[You can read more about fire and climate change in the Amazon at:

Drought Fuels Wildfires in the Amazon (nasa.gov) ]

A lack of available moisture also has implications for temperature. As less water falls as rain, the resulting reduction in evaporation and transpiration means that the cooling effect of latent heat transfer is reduced. This leads to further locally intensified warming of the environment, already subject to high temperatures and sensitive to global changes in climate.

Finally, the drying and heating resulting from vegetation loss leads to positive feedback mechanisms. The removal of vegetation reduces atmospheric moisture and increases temperatures as evaporation and transpiration are reduced. Warmer and drier conditions mean that many forest plants are unable to survive, leading to further vegetation loss through dieback, increasing the risk of fire, both of which cause further drying and heating, amplifying the initial effects.

[Advance notice: In June 2023, one of the UK examination boards set an essay question on the impact of changes in the carbon cycle on a named tropical rainforest. Another exam board also set a theoretical question on the carbon cycle. I shall provide a sample answer to these questions in subsequent Substacks.]