The water cycle

There are some alarming examples of the influence of the grow of the population in some countries in relation to the use of the area of the land. For example the city of Bangalore (Source: Indian Institute of Science). The image below shows the grow of the population in the city during the last 43 years (1973 - 2016).

 

There are always some fast expanding cities to be find. Nevertheless, what is – in general – the consequence of the transformation of “unspoilt nature” into “concrete cities”?

The question is a bit vague because a dessert and mountains are unspoilt nature too. It is more explicit when I describe it as “intact biological area that is suitable for humans to live”. So there is fertile soil, enough water available, the temperature is not too cold or too hot so agriculture is possible and the local surface area is not too steep.

In such a region of “unspoilt nature” there are mostly a lot of trees. Without any graze by cattle or other animals trees become dominant in the area. In other words: “What are the differences between unspoilt nature and built up area?

The debate about the climate change is “captured” by the concentration of greenhouse gasses in the atmosphere (mainly CO2). However, when we look to the absorption of infra red radiation – the figure below – we notice that water vapour (H2O) is the most influential of all the greenhouse gasses in our atmosphere.


The raise of the average temperature of the atmosphere will increase the ability of the atmosphere to contain water vapour so we may conclude that the raise of the concentration of CO2 – as a non-condensible gas – is one of the “triggers” for an accumulation of greenhouse effects.

Nevertheless, the evaporation of water by plants is the most important mechanism to cool the air. Because the transformation of water into water vapour needs a lot of energy from the environment. Besides that, the leaves of the trees will prevent the air to heat up by solar radiation because they create shadow. Moreover, the leaves of the trees have a far lower heat accumulation in relation to the mass of buildings and roads.

 

The image above shows in a schematic way the difference of the cooling effect by large trees and grass land. Hight A determines the creation of a cooled layer of air by the evaporation of water (and shadow) and hight B determines the depth of the water absorption by the plant. It is easy to understand the reduce of thermal heat by woods and forests in relation to grass lands.

 

However, trees adsorb solar radiation too like buildings and roads. But a part of the radiation in the visible wave length is transformed into biomass. So a part of the solar radiation is chemical bond and not available for thermal radiation during the night. The image above shows the spectra of the Chlorophyll (a and b) that exists in the leaves of plants.

 

Satellites measure constantly the reflected radiation by the surface of the area and the image above (The Netherlands) shows the reflected thermal radiation on 16 July 2006 at 12:07 o'clock. It was a very hot sunny day (> 30 degrees Celsius). If you know the areas you can notice differences of about 5 to 10 degrees Celsius between the type and hight of trees (needle leaved trees and deciduous trees), mostly in relation to the soil of the area (sand, clay, marsh, etc.). That's why I conclude that humidity is a very important condition in relation to the local climate.

It is difficult to estimate the influence of the greenhouse gasses on the global climate when the use of the surface area on the continents by man has changed so fast because of the grow of the world population (houses, buildings) and the grow of public services like roads, sewerage, the canalizing of rivers, the transformation of woods into farmland, etc, etc. All these changes influence the water cycle of the distinct areas.


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