the Water Cycle
How it works and how our human land use and global warming are changing it.
The good news: we can also have a positive, balancing influence on the water cycle.
the natural water cycle
The water on our blue planet moves in a perpetual, diverse, complex cycle. Nature creates site-adapted vegetation on almost any soil. These ecosystems, especially forests, store large amounts of water and carbon. Through evapotranspiration, forests provide an average of 40% of precipitation globally, in many regions - especially inland, often a larger proportion themselves (science.org, 2020; UNEP Foresight, 2021).
Forests thus play an important role in cloud and rain formation, and are thought to also play an important role in transporting moisture from the oceans inland - forests acting as a 'biotic pump' (science.org, 2020).
Diverse, site-adapted vegetation also ensures that the rain falls on cool, well-covered, shady, humus-rich and well-rooted soil. In this way, the water can seep in well, be stored in the ground or enrich the groundwater in deeper layers (Paul et al., 2003).
the disturbed water cycle
Human land use changes the natural water cycle. Deforestation and farming often expose soil to direct sunlight, causing it to dry out and heat up more quickly and then absorb less water when it rains (UNEP Foresight, 2021). In addition, floors are sealed and groundwater is pumped out. The landscape is actively dried out by drainage ditches and pipes. With the consequences: drought and flood. On closer inspection, it becomes clear that these are two symptoms of the same problem.
the regenerated water cycle
Human intervention can also be positive. By creating decentralized water retention measures, a large part of the rainwater can be collected and seeped away, made available for irrigation or used as a body of water to cool the landscape. The simultaneous development of permanent, site-adapted vegetation has a balancing effect on ground-level temperatures and humidity. Soils are protected from heat and erosion.
If this happens on a large scale, the additional evaporation can have a balancing effect on precipitation dynamics.
the effect of global warming
Temperature plays an important role in the water cycle as warmer air can hold more moisture (7% more for every 1°C increase in temperature). One consequence of this is that in a warmer climate, more water evaporates, but at the same time more moisture accumulates in the air until it starts to rain (IPCC, 2021). This results in more erratic but heavier rainfall, thunderstorms and storms. Generally, dry places become drier and wetter places become wetter. However, the models used by the IPCC also calculate that there will be an increase in extreme precipitation events globally.
