Earth and Moon

Aridity on planet Earth

Aridity on planet Earth

More than 600 million human beings live in arid areas, characterized by their poverty and desertification. In 1977 UNESCO carried out a project focused on the delimitation of this type of territories, qualifying them according to the degree of aridity they show. As a result, a world map was produced that has served as a reference for ecological studies. An extension of the extension of these regions is detected as time progresses.

The passage of water in a liquid to gas state (water vapor), having acquired enough energy to overcome the surface tension that bound the water molecules together is called evaporation. In living things, for example, in plants, this process is called perspiration.

Well, it is considered that an area is arid when it suffers periods of drought because the evaporation set + potential perspiration (the union of both is called potential evapotranspiration) exceeds rainfall or rainfall. Therefore, aridity is considered a climate concept.

Types of arid zones

Basically, the sterile lands are divided into three types or degrees of aridity, by using the PP / ETP index (that is, the annual medial precipitation of the area with respect to the annual potential evapotranspiration). They are the following:

1.-Hyper-arid zone: index (PP / ETP) less than 0.03. Corresponds to deserts proper, with vegetation limited to very specific areas (oasis). The average annual rainfall does not exceed 50 mm. They represent approximately 5% of the surface of the continents.

2.-Arid zone: index between 0.03 and 0.2. They are the semi-deserts and sub-deserts. The vegetation continues to be little dense (thickets, small woody plants and cacti). Nomadic grazing and sometimes cereal crops. Average annual rainfall (pp): 50-126 mm (rains are usually concentrated in a single season of the year). They occupy 15% of the surface of the continents and are distributed around the hyper-arid zones, while constituting the central part of the less absolute deserts.

3.-Semi-arid zone: index between 0.2 and 0.5. They correspond to steppes, meadows and certain tropical savannas and much of the Mediterranean areas. 350-400 mm of pp spread over six months per year (the winter period in the Mediterranean and tropical latitudes). They are usually livestock regions and areas of plantation of rainfed crops. They cover 14% of the surface of the continents.

In areas marked by aridity, extreme climatic conditions involve the accentuated and repetitive action of external geological agents (water and wind). Therefore, the main process that occurs is erosion so that inevitably the situation will lead to subsequent desertification.

Therefore, we find that an area without extreme dry conditions, but with strongly erosive natural or anthropic conditions (that is, generated by human action) can end up becoming a desert.

Precisely this is happening in many places of the earth's crust, but limiting ourselves to Spain we can mention the Monegros (Aragon), the Bardenas Reales (Navarra) or the province of Almería (Andalusia).

The overexploitation of ecosystems mainly due to inappropriate behaviors by human beings usually leads to the progressive extension of vacant land. When the misuse of the land adds to the installation of dry climatic phases, the area inevitably ends up degenerating into sub-deserts and semi-arid zones.

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Deserts and arid areasDesertification and desertification