Added: 04/24/2006 |
Characteristics of arid environmentA peculiar feature of the arid environment is some low annual precipitation (0 to 800 millimeters), which is infrequent and irregular. Arid zones are vegetated by drought resistant flora. The arid environment in common insight is a desert. Although the most familiar image of a desert involves hot sand, the Arctic North and Antarctica are also placed among deserts, as they also receive little moisture, generally in the form of snow.
Arid environments are extremely diverse in terms of their land forms, soils, fauna, flora, water balances and human activities. They can be divided into three types of zones: hyper-arid, arid and semi-arid. As a result of this complex characteristics' diversity, no practical definition of arid environments can be derived. However, they have one common feature- a binding element to all arid regions is aridity. Out of the total land area of the world, the hyper-arid zone covers four point two percent; the arid zone comprises fourteen point six percent and the semiarid zone twelve point two percent. Thus, almost one-third of the total area of the globe has attributes of an arid land.
The rainfall in arid climates is occasional, and when it finally occurs, normally it has a form of a thunderstorm. A dry, compact soil and infertile plants cannot absorb such a great amount of water quickly enough to capture the rain. There often comes out a danger of flash floods. Nevertheless, usually streams of water swell for several hours and then dry up again until the next cloudburst.
Contrary to the state of affairs in temperate regions, the rainfall distribution in arid zones varies between summer and winter. For instance, Rabat, Morocco, receives most of the rainfall in winter, while the summer precipitation is very insignificant. On the other hand, Sennar, Sudan, has a long dry season during the winter and a considerable rainfall during the summer months. Although Rabat and Sennar receive about the same amount of the rainfall, the benefit from it varies to a great extend. Winter rains can go through the soil to underground storage, while the summer rains, falling on a hot soil, are lost to the evaporation. For the most part, it happens with light showers.
Flora, growing in an arid climate, has learned how to deal with the drought. Some plants, so called non-succulents, can remain dormant (inactive) most of the time, only growing and reproducing, when water is available. They can endure the stress of the waterless environment, since the cycle of activity and inactivity is linked with the availability of water (and sometimes to other factors, like temperature). This way to acclimatize allows these hardy plants to carry on living for years.
Other desert plants have developed different forms of adaptation to severe weather conditions. Ephemeral annuals quickly go through the life cycle from a seed to a seed-producing plant in one year. After having distributed their seeds in a few wet days, following a heavy rainfall, these plants die, and the seeds lie, waiting for the next big rainfall. Succulents can store water in leaves and use it during the drought. The arid zone vegetation is classified as a desert (less than 100 millimeters of precipitation annually), semi desert (100 to 300 millimeters of annual rainfall), low rainfall woodland savanna (300 to 600 millimeters), and evergreen scrub (in excess of 500 millimeters).
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